Damp Proof Course in Outer Leaf of Cavity Wall

SolarQuark

Hi all.

New here and first post. I am often puzzled by what I see around me and this is one of those situations. Pehaps some of you can reduce my level of bewilderment a tad.
In a cavity wall construction method there is a layer of plastic sheeting called a Damp Proof Course (DPC) placed near the base of the outer walls both in the inner and outer leaves.
In the inner leaf this is, typically (if there is ever such a thing as 'typical'?), over the Damp Proof Membrane (DPM) where the DPM crosses over the inner leaf into the cavity.
In the outer leaf this is placed perhaps 9" (225mm) above the (outer) ground level (and is often level with the DPC in the inner leaf).
Does anyone know what purpose this DPC is supposed to serve in a cavity wall construction?

mad m

Damp Proof Course is to stop Rising Damp going up the block work..

Mellor

As mentioned, it is to stop damp issues. First course could often he sitting in a puddle of water, depending on drainage and weather, so without the DPC this would rise up the wall quite quickly.

SolarQuark wrote: »

In the inner leaf this is, typically (if there is ever such a thing as 'typical'?), over the Damp Proof Membrane (DPM) where the DPM crosses over the inner leaf into the cavity.

While that does happen. It's technically wrong. There is little point in putting a DPC above a DPM, as the DPM is supposed to be a barrier to water ingress. There is no real harm (some raise concerns of slip palnes) but it also raises the question, what else are they doing wrong.

In the outer leaf this is placed perhaps 9" (225mm) above the (outer) ground level (and is often level with the DPC in the inner leaf).

225mm is common (1 block or, 3 bricks), the minimun is 150mm. Below this level it is incorrectly installed.

SolarQuark

mad m wrote: »

Damp Proof Course is to stop Rising Damp going up the block work..

What's the point in the outer leaf of a cavity wall?
This leaf is subject to rain, sometimes very heavy wind driven rain and so the blockwork above the DPC will become wet (sometimes very, very wet) anyway.
The primary purpose of the cavity is to prevent water crossing from the outer to the inner leaf. The outer leaf can, therefore, become wet while the inner leaf stays dry.
There doesn't seem to be any point is preventing rising damp creeping up the outer leaf when that leaf will become wet anyway.

I could also make a similar argument for the inner leaf, but the DPM here is often, but not always, a light plastic sheet that does allow a certain amount of mositure penetration and so, perhaps, there is an argument for the DPC in the inner leaf.

Bewildered still.

Mike

Poor Uncle Tom

SolarQuark wrote: »

What's the point in the outer leaf of a cavity wall?

To prevent unnecessary penetration of dampness. Say one corner of the sub-surface outer leaf is immersed in water all year round, if there is no DPC in the outer leaf, then even on a dry summer you would have an unsightly damp section on your wall to whatever height the water will travel through capillary action. The DPC stops this risk.

SolarQuark wrote: »

This leaf is subject to rain, sometimes very heavy wind driven rain and so the blockwork above the DPC will become wet (sometimes very, very wet) anyway.

Yes, and it will dry out again.

SolarQuark wrote: »

The primary purpose of the cavity is to prevent water crossing from the outer to the inner leaf. The outer leaf can, therefore, become wet while the inner leaf stays dry.
There doesn't seem to be any point is preventing rising damp creeping up the outer leaf when that leaf will become wet anyway.

The climate here may seem excessively wet but it does dry out for extended periods during any year.

SolarQuark wrote: »

I could also make a similar argument for the inner leaf,

It doesn't need any explanation why it is vitally important to keep the inner leaf dry.

SolarQuark wrote: »

but the DPM here is often, but not always, a light plastic sheet that does allow a certain amount of mositure penetration and so,

Not the case. A DAMP PROOF MEMBRANE should do exactly what it's name says. A DPM is NON-PERMEABLE and should not allow any moisture penetration.

SolarQuark wrote: »

perhaps, there is an argument for the DPC in the inner leaf.

Bewildered still.

Mike

I think Mellor adressed that point above.

mickdw

There is a big difference in allowing the outer wall to be wet through with rainwater compared to having a constant soaking effect of dirty water from the ground moving up through the wall. You would get all sorts of issues without this outer dpc, stuff like discoluration of the wall finish which painting would not cure as the complete wall would be permanently saturated with contaminated water.

SolarQuark

Mellor wrote: »

While that does happen. It's technically wrong. There is little point in putting a DPC above a DPM, as the DPM is supposed to be a barrier to water ingress. There is no real harm (some raise concerns of slip palnes) but it also raises the question, what else are they doing wrong.

But yet the HomeBond Manual has very many examples of where (and actually insists) this should happen. For instance, their section on DPMs (page 37 of the 4th Edition, elsewhere other editions) has nice diagrams and states 'Dpm should lap under dpc for full thickness of inner leaf'.

I agree that this must create a slip plane. And its purpose continues to puzzle me (particularly in the outer leaf but also in the inner).

Perhaps a stint in a home for the bewildered would sort me out?

Mike

mad m

Yes but the outer leaf will/should be finished in a render/dash finish. This will prevent dampness getting to the rest of the outer leaf, while the DPC is preventing rising dampness.

Rising dampness is one thing you do not want...Nightmare, but treatable. Rising dampness only goes up a meter. From what I read this is why the victorians had a dado rail up a meter. It was to keep their furniture from the wall and damage.......

SolarQuark

Poor Uncle Tom wrote: »

To prevent unnecessary penetration of dampness. Say one corner of the sub-surface outer leaf is immersed in water all year round, if there is no DPC in the outer leaf, then even on a dry summer you would have an unsightly damp section on your wall to whatever height the water will travel through capillary action. The DPC stops this risk.

But, as the DPC is 9" (ish) above ground level, then that section of that first row of block/brickwork would become discoloured. I haven't really seen this problem, but haven't been looking for same to be honest.
Also, if (localised) discolouration caused by ground water penetration were to be such a problem then surely a DPC would be specified for use in garden walls and the like?

Poor Uncle Tom wrote: »

It doesn't need any explanation why it is vitally important to keep the inner leaf dry.

Nope.

Poor Uncle Tom wrote: »

Not the case. A DAMP PROOF MEMBRANE should do exactly what it's name says. A DPM is NON-PERMEABLE and should not allow any moisture penetration.

Then why the need for the DPC?

Actually platic sheeting is permeable both to gases and fluids (of course a gas is a fluid in engineering terms, but you know what I mean). You really shouldn't put platic wrapped raw meats into the same carrier bag as cooked meats or other produce, for instance. The spec. sheet for each dpm will state the level of moisture penetration that can be expected. The heavier (if I may use that term) the membrane the less moisture penetration that can be expected. Some of the DPMs are heavier than the a typical DPC.
So, why is a DPC required above a DPM of reasonable quality?

The way my brain feels at the moment, I am sure I am qualified to be in government. If it melts any more I'd be suitable for a senior civil service role. Anyone able to save me?

Mike

Mellor

SolarQuark, some of the "But why" posts you keep making are wrong, are you even reading replies. A lot of the points you question, show a basic lacking of understanding on your behalf, which is in turn leading to the confusion.

SolarQuark wrote: »

What's the point in the outer leaf of a cavity wall?
This leaf is subject to rain, sometimes very heavy wind driven rain and so the blockwork above the DPC will become wet (sometimes very, very wet) anyway.

True, but the frequency of rain is small portion of total time. Seeing as the foundation sits 1000mm below the surface, there is potential for permanant dampness to the rising wall, and the turn the above cavity wall.
The volume of water in an average shower, is no where near the volume underground.

I could also make a similar argument for the inner leaf, but the DPM here is often, but not always, a light plastic sheet that does allow a certain amount of mositure penetration and so, perhaps, there is an argument for the DPC in the inner leaf.

That is wrong. There is no point question the correct detail by refering to an incorrect one.

SolarQuark wrote: »

But yet the HomeBond Manual has very many examples of where (and actually insists) this should happen. For instance, their section on DPMs (page 37 of the 4th Edition, elsewhere other editions) has nice diagrams and states 'Dpm should lap under dpc for full thickness of inner leaf'.

I beleive the detail you refer to shows a DPM, that laps partially under a DPC. The DPC (which covers the whole leaf) is the barrier here. This detail is obsolete, as materials have improved and the membranes are more plyable and better sealed at junctions.

SolarQuark wrote: »

But, as the DPC is 9" (ish) above ground level, then that section of that first row of block/brickwork would become discoloured. I haven't really seen this problem, but haven't been looking for same to be honest.

One of the reasons the plinth is commonly used.

Also, if (localised) discolouration caused by ground water penetration were to be such a problem then surely a DPC would be specified for use in garden walls and the like?

External walls are less likely to be painted, so discolouration is less of a concern.

Then why the need for the DPC?

There isn't a need for a DPC???
How can you not understand that? It was pointed out in post 3, and reinforced by other posters.

Actually platic sheeting is permeable both to gases and fluids (of course a gas is a fluid in engineering terms, but you know what I mean). You really shouldn't put platic wrapped raw meats into the same carrier bag as cooked meats or other produce, for instance. The spec. sheet for each dpm will state the level of moisture penetration that can be expected. The heavier (if I may use that term) the membrane the less moisture penetration that can be expected. Some of the DPMs are heavier than the a typical DPC.
So, why is a DPC required above a DPM of reasonable quality?

Most modern DPMs double as radon barriers, they are therefore expected to be reasonable gas tight (total gas-tightnes is not possible or required), this leads to very high water tightness.

Once again, a DPC is not needed. This was stated previously. Please read posts carefully. This really is a simple issue.

Mellor

mad m wrote: »

From what I read this is why the victorians had a dado rail up a meter. It was to keep their furniture from the wall and damage.......

Yeah, sort of.
The dado, was to conceal staining from rising damp, which is why timber panelling or TnG boardings was common.
The dado rail, did nothing for damp, but protected the wall from the backs of chairs, (not to protect furniture) which is why the 900-1000mm height was used as it was the typical height of a chair.

SolarQuark

Mellor wrote: »

SolarQuark, some of the "But why" posts you keep making are wrong, are you even reading replies.

Yep. But I can't say that I understand some of the points being made.

Mellor wrote: »

A lot of the points you question, show a basic lacking of understanding on your behalf, which is in turn leading to the confusion.

Sure now if I understood the matter I wouldn't be asking the questions

Mellor wrote: »

That is wrong. There is no point question the correct detail by refering to an incorrect one.

I don't understand what is wrong.

Mellor wrote: »

I beleive the detail you refer to shows a DPM, that laps partially under a DPC. The DPC (which covers the whole leaf) is the barrier here. This detail is obsolete, as materials have improved and the membranes are more plyable and better sealed at junctions.

No. The detail shows a continuous DPM crossing into the cavity and out over the outer leaf. The DPC is placed on the inner leaf on top of the DPM where it crosses that inner leaf. The DPM/DPC is at floor level. The outer DPC in placed level with the inner DPC. This is supposed to be a minimum of 150mm (6") above the outer ground level but is normally 225mm (or one block height) above the outer ground level.

Mellor wrote: »

One of the reasons the plinth is commonly used.

But not always. Stone-faced (is that the correct term?), stone built or 'Hallmark Walling' type outer leaves often don't have a plint. With a random-brought-to-course bonded or random bonded 'Hallmark Walling' it would be very, very difficult to incorporate a DPC in the external leaf as far as I can see. The same might well apply to stone-built or stone-faced outer walls.

Mellor wrote: »

External walls are less likely to be painted, so discolouration is less of a concern.

Looking out my back window here at the moment, all the external walls I can see are painted.

Mellor wrote: »

There isn't a need for a DPC???
How can you not understand that? It was pointed out in post 3, and reinforced by other posters.


Most modern DPMs double as radon barriers, they are therefore expected to be reasonable gas tight (total gas-tightnes is not possible or required), this leads to very high water tightness.

Once again, a DPC is not needed. This was stated previously. Please read posts carefully. This really is a simple issue.

Just to be absolutely clear.
Are you saying there is NO NEED FOR A DPC?
Sorry for the emphasis. Just trying the make sure we are not talking at cross-purposes.

Take care.

Mike

mickdw

SolarQuark wrote: »

Also, if (localised) discolouration caused by ground water penetration were to be such a problem then surely a DPC would be specified for use in garden walls and the like?

IMO dpc in a garden wall would be an excellent idea if it could be incorporated without causing the wall to fail. It would prevent alot of surface problems wit paint flaking etc Id say. Some people recommend 225mm of Engineering brick above ground level in garden walls as they absorb less water. I dont know how successful this is.

Mellor

SolarQuark wrote: »

I don't understand what is wrong.

You said that a DPM is light plastic and allows some moisture penetration.
It doesn't, its quite heavy, and allow no moisture penetration.
In practise, a high grade is used so that it serves as a radon barrier also. So it is quite gas tight.

You you understand the role of a DPM now?
And see why a DPC here is not needed.

No. The detail shows a continuous DPM crossing into the cavity and out over the outer leaf. The DPC is placed on the inner leaf on top of the DPM where it crosses that inner leaf. The DPM/DPC is at floor level. The outer DPC in placed level with the inner DPC. This is supposed to be a minimum of 150mm (6") above the outer ground level but is normally 225mm (or one block height) above the outer ground level.

Regardless, it is still obsolete.
Previously, it may have applied as DPM's needed to be light so as to be pliable enough to fold around slabs, blocks etc. So a heavier DPC was used as a primary protector, this was heavier, and stiffer and protected against tears leaking.
As I said, new materials, leave this obsolete. A single DPM is better.
All clear?

But not always. Stone-faced (is that the correct term?), stone built or 'Hallmark Walling' type outer leaves often don't have a plint.

True, but irrelevant, stone doesn't stain from water (however other things can and do stain stone, but that's not an issue)

With a random-brought-to-course bonded or random bonded 'Hallmark Walling' it would be very, very difficult to incorporate a DPC in the external leaf as far as I can see. The same might well apply to stone-built or stone-faced outer walls

Correct.
Which is why, when a DPC can't be applied a different solution for water proofing is used. Liquid of bitumen based damp proofing being common.

Looking out my back window here at the moment, all the external walls I can see are painted.

I don't see how this is relevant? I said less likely, i didn't say never.
A garden wall still works fine if wet, a externla wall of a house does not.

Plus, lacking the mass of a house to keep it in place, a garden wall with a dpc installed would fall over a lot easier.

Just to be absolutely clear.
Are you saying there is NO NEED FOR A DPC?
Sorry for the emphasis. Just trying the make sure we are not talking at cross-purposes.

Take care.

Where there is a suitable DPM/Radon barrier install over the inner leaf, a suitable height, there is no need for a DPC. Hopefully that's clear for you now.

Mellor

SolarQuark

mickdw wrote: »

IMO dpc in a garden wall would be an excellent idea if it could be incorporated without causing the wall to fail. It would prevent alot of surface problems wit paint flaking etc Id say. Some people recommend 225mm of Engineering brick above ground level in garden walls as they absorb less water. I dont know how successful this is.

At the risk of causing a digression in the discussion I have taken a look at 'external' (i.e. garden and other 'standalone' walls). The paintwork is mostly, annoyingly, in pretty good condition. Any walls that are showing a deterioration in the paintwork show this in the upper part of the wall. This would suggest that weather (wind, rain, sleet, hail, frost, etc), quality of paint, thickness of paint, age, etc, may play a greater role in the deterioration of paintwork than rising damp. Not scientific but an indication that may be useful.

Regards.

Mike

mickdw

SolarQuark wrote: »

At the risk of causing a digression in the discussion I have taken a look at 'external' (i.e. garden and other 'standalone' walls). The paintwork is mostly, annoyingly, in pretty good condition. Any walls that are showing a deterioration in the paintwork show this in the upper part of the wall. This would suggest that weather (wind, rain, sleet, hail, frost, etc), quality of paint, thickness of paint, age, etc, may play a greater role in the deterioration of paintwork than rising damp. Not scientific but an indication that may be useful.

Regards.

Mike

I dont agree with that. I know of many older garden walls that are destroyed near the base with bubbling of paint/crumbling plaster.

2 stroke

mad m wrote: »

Yes but the outer leaf will/should be finished in a render/dash finish. This will prevent dampness getting to the rest of the outer leaf, while the DPC is preventing rising dampness.

What I don't understand is. Render can have an additive to make it waterproof, why not use this additive in blocks and mortar?

Poor Uncle Tom

Generally additives weaken the mix.

SolarQuark

Mellor wrote: »

You said that a DPM is light plastic and allows some moisture penetration.
It doesn't, its quite heavy, and allow no moisture penetration.
In practise, a high grade is used so that it serves as a radon barrier also. So it is quite gas tight.

OK, I suppose I shouldn't have used a term like 'light' or 'heavy'. Such terms are relative (although often used by others when discussing DPMs). Membranes vary quite a bit in thickness. Commonly available membranes are as light as 0.2mm thick. Some are considerably heavier.
Is it possible you are misinterpreting the term 'proof'?
'Proof' should perhaps be best interpreted as 'tested'. So a DPM is a material that has been tested to some standard and has been found to meet that standard or be otherwise satisfactory.
Damp Proof should not be taken to mean 'Impenetrable to Damp', although I am sure this is a common misconception.

Mellor wrote: »

You you understand the role of a DPM now?

I have never had a problem understanding the need for a DPM.

Mellor wrote: »

And see why a DPC here is not needed.

I have never stated that a DPC is required. My original question(s) were about _why_ a DPC is required. That is, I couldn't quite understand the purpose of a DPC.
There have been quite a few responses to my query, all of which I appreciate.
As each person responded, I thought about the response and tried to establish, by research or investigation, any evidence to support (or otherwise) that response.
The responses have fallen into two categories:
1) A DPC IS required
2) A DPC is NOT required
Some people have presented reasonable arguments (if I might use that term) to support their point-of-view. Some seem to prefer the 'because I say so' rather than provide any supporting references or reasonable arguments.
One person seems to be holding both views simultaneously.

Mellor wrote: »

Regardless, it is still obsolete.

This is a bold statement. Can you provide any supporting documentation or references to this?

Mellor wrote: »

Previously, it may have applied as DPM's needed to be light so as to be pliable enough to fold around slabs, blocks etc. So a heavier DPC was used as a primary protector, this was heavier, and stiffer and protected against tears leaking.
As I said, new materials, leave this obsolete. A single DPM is better.
All clear?

Can you provide any references to support this point of view?

Mellor wrote: »

True, but irrelevant, stone doesn't stain from water (however other things can and do stain stone, but that's not an issue)

I am not sure how to quote the original quote, if you know what I mean. The above quote is a response to part of the text of an earlier message from me.
This related to the use of a DPC in the outer leaf of a cavity wall type construction.
An earlier respondent had suggested that the purpose of this DPC is to prevent the upwards penetration of dirty water from underneath the ground into the wall and the staining of the wall that would result.
I thought that this was a very interesting suggestion but posted some follow-up queries about the difficulties that would be encountered using such a DPC in a stone wall, stone-faced wall, or other similar constructions.

Would the mortar in the joints of stone walled type constructions not still soak up the dirty water and wouldn't there still be the risk of this spilling out and down over the face of the stonework during any future rainfall? The original suggestion was not that water would cause staining but that _dirty_ water would cause staining.

Mellor wrote: »

I don't see how this is relevant? I said less likely, i didn't say never.
A garden wall still works fine if wet, a externla wall of a house does not.

As I wrote earlier, when someone makes an interesting suggestion I try to find evidence that would support or discount that suggestion. This suggestion was made. I thought it was a perfectly reasonable suggestion. I investigated but the visible evidence does not support that suggestion.

Mellor wrote: »

Where there is a suitable DPM/Radon barrier install over the inner leaf, a suitable height, there is no need for a DPC. Hopefully that's clear for you now.

Mellor

Can you provide any supporting documentation or anything else that might support that point-of-view?

Thank you Mellor, and all other respondents. I really do appreciate the time and efforts you have made.

Regards.

Mike

Mellor

SolarQuark wrote: »

OK, I suppose I shouldn't have used a term like 'light' or 'heavy'. Such terms are relative (although often used by others when discussing DPMs). Membranes vary quite a bit in thickness. Commonly available membranes are as light as 0.2mm thick. Some are considerably heavier.
Is it possible you are misinterpreting the term 'proof'?
'Proof' should perhaps be best interpreted as 'tested'. So a DPM is a material that has been tested to some standard and has been found to meet that standard or be otherwise satisfactory.
Damp Proof should not be taken to mean 'Impenetrable to Damp', although I am sure this is a common misconception.

mike, don't be so pedantic. And lose the attitude (or the appearance of one, what ever the case may be) consider this an official warning.

I assure you i'm not misinterpreting the term 'proof'. Nobody has suggested that DPM impenetrable to damp, and you appear to be the one with a misconception.
To be clear, they don't need to be 'Impenetrable to Damp', the minute amounts that pass through (if any) have no effect.

I have never had a problem understanding the need for a DPM.

Clearly you did at the start or you wouldn't of started the thread.

This is a bold statement. Can you provide any supporting documentation or references to this?

I, like many of the others in this thread, have a degree in a related field and in my opinion it (like many homebond details) is obsolete.

Would the mortar in the joints of stone walled type constructions not still soak up the dirty water and wouldn't there still be the risk of this spilling out and down over the face of the stonework during any future rainfall? The original suggestion was not that water would cause staining but that _dirty_ water would cause staining.

Stone wall always present a risk of staining, metalic ores, organic growth and polution are all much bigger risk factors that "dirty water", which in my opinion won't travel throuh mortar in significant volume to cause major staining.
A bitumenous DPC will prevent water ingress, which is the main concern.

As I wrote earlier, when someone makes an interesting suggestion I try to find evidence that would support or discount that suggestion. This suggestion was made. I thought it was a perfectly reasonable suggestion. I investigated but the visible evidence does not support that suggestion.

Checking one wall is by no means investigation.
A common phrase applicable would be "too small a sample".
The main reason for not having one was also in my post, structural stability.

Can you provide any supporting documentation or anything else that might support that point-of-view?

The two materials are both adequetly resistant. The name you choose has no effect.




Mike, there realy is no need to anyone to provide evidence, the need for a DPC has already been pointed out, as have many reasons.
You may disagree with the reasons if you like. you have said that you didn't understand some of them, we have explained them to you. If you still can't then it must be beyond you.
There is no point in continuing this nonsense of "why", "provide evidence". It is futile. If you have some evidence that a DPC isn't required then by all means post it, otherwise continuing the same approach, raising issues already mentioned, that are quite simple, will be seen as trolling and dealt with as such.


Have a nice day,
Mellor

SolarQuark

mickdw wrote: »

I dont agree with that. I know of many older garden walls that are destroyed near the base with bubbling of paint/crumbling plaster.

Oops! I've just discovered the second page to this 'thread'. Perhaps us 'old-timers' should stick to cave drawings.

That's very interesting. That leaves open the suggestion that a DPC might be use in preventing rising water (rising damp) causing an unsightly appearance to an external wall. More evidence is needed to support or discount this theory, I guess.

Mike

SolarQuark

Mellor wrote: »

mike, don't be so pedantic. And lose the attitude (or the appearance of one, what ever the case may be) consider this an official warning.

I assure you i'm not misinterpreting the term 'proof'. Nobody has suggested that DPM impenetrable to damp, and you appear to be the one with a misconception.
To be clear, they don't need to be 'Impenetrable to Damp', the minute amounts that pass through (if any) have no effect.


Clearly you did at the start or you wouldn't of started the thread.



I, like many of the others in this thread, have a degree in a related field and in my opinion it (like many homebond details) is obsolete.




Stone wall always present a risk of staining, metalic ores, organic growth and polution are all much bigger risk factors that "dirty water", which in my opinion won't travel throuh mortar in significant volume to cause major staining.
A bitumenous DPC will prevent water ingress, which is the main concern.




Checking one wall is by no means investigation.
A common phrase applicable would be "too small a sample".
The main reason for not having one was also in my post, structural stability.



The two materials are both adequetly resistant. The name you choose has no effect.




Mike, there realy is no need to anyone to provide evidence, the need for a DPC has already been pointed out, as have many reasons.
You may disagree with the reasons if you like. you have said that you didn't understand some of them, we have explained them to you. If you still can't then it must be beyond you.
There is no point in continuing this nonsense of "why", "provide evidence". It is futile. If you have some evidence that a DPC isn't required then by all means post it, otherwise continuing the same approach, raising issues already mentioned, that are quite simple, will be seen as trolling and dealt with as such.


Have a nice day,
Mellor

I was going to respond to each and every point in this post but realised that this would, in turn, cause you to continue. And I don't want to be the cause of you wasting your time.
So please, Mr Mellor, I would appreciate it if you didn't respond to this post or any part of this thread again.

Take care.

Mike

SolarQuark

I'm going to make a suggestion why a DPC might be of use, indeed neccessary, in the inner leaf of a cavity wall construction. Please post if you believe this might be incorrect, and why, but also if you think this might be correct (and why if different even in part from the suggestion that follows).

I am not the best at explaining things, so please bear with me...

Anyone who has ever experienced a flooding incident will know that what we believe to be water proofing in walls of housing is anything but. Water pours through these almost as if the concrete wall was not there. So we can take from those experiences that where water is at a higher than ground level it can penetrate the outer leaf and 'flood' the cavity. This can happen even due to the normal downpour that can occur here in Ireland where a shallow 'river' of water can flow around a house (anything from a fraction of an inch upwards, for instance).
Examination of lots of the installation guides for DPMs would suggest that the DPM can be at a level lower than the finished floor level of the house. Without a DPC any water build-up in the cavity than reached a level above the DPM could, and would, penetrate the inner leaf and do its dirty deed. By placing a DPC at the same level as the floor and bonding this to the DPM this type of risk should be averted (assuming the job is done properly, of course) where the level of water does not rise above the DPC.
Could this be one reason for the use of a DPC in the inner leaf?

Take care.

Mike

ps
Theories are very wlecome. Without theories we would not have progressed very far as a species. Reasoned thought and supporting evidence likewise. Please do not submit the 'because I say so' line of argument. Such belong to poor parenting and religious organisations.

Poor Uncle Tom

SolarQuark wrote: »

I would appreciate it if you didn't respond to this post or any part of this thread again.

It may have skipped your attention but Mellor is a moderator of this forum and as such he is entitled to moderate any aspect of this thread he feels needs attention.

Also it is worth noting that anyone can start a thread but once posters contribute to that thread it is no longer the domain of the individual but rather belongs to the community of the forum and no one individual (this includes the Opening Poster) has any more rights over it than another.

SolarQuark wrote: »

I'm going to make a suggestion why a DPC might be of use, indeed neccessary, in the inner leaf of a cavity wall construction. Please post if you believe this might be incorrect, and why, but also if you think this might be correct (and why if different even in part from the suggestion that follows).

I am not the best at explaining things, so please bear with me...

Anyone who has ever experienced a flooding incident will know that what we believe to be water proofing in walls of housing is anything but. Water pours through these almost as if the concrete wall was not there. So we can take from those experiences that where water is at a higher than ground level it can penetrate the outer leaf and 'flood' the cavity. This can happen even due to the normal downpour that can occur here in Ireland where a shallow 'river' of water can flow around a house (anything from a fraction of an inch upwards, for instance).
Examination of lots of the installation guides for DPMs would suggest that the DPM can be at a level lower than the finished floor level of the house. Without a DPC any water build-up in the cavity than reached a level above the DPM could, and would, penetrate the inner leaf and do its dirty deed. By placing a DPC at the same level as the floor and bonding this to the DPM this type of risk should be averted (assuming the job is done properly, of course) where the level of water does not rise above the DPC.
Could this be one reason for the use of a DPC in the inner leaf?

Take care.

Mike

ps
Theories are very wlecome. Without theories we would not have progressed very far as a species. Reasoned thought and supporting evidence likewise. Please do not submit the 'because I say so' line of argument. Such belong to poor parenting and religious organisations.

This post is contradictory and bordering on preaching.

Poor Uncle Tom

SolarQuark wrote: »

I'm going to make a suggestion why a DPC might be of use, indeed neccessary, in the inner leaf of a cavity wall construction. Please post if you believe this might be incorrect, and why, but also if you think this might be correct (and why if different even in part from the suggestion that follows).

I am not the best at explaining things, so please bear with me...

Anyone who has ever experienced a flooding incident will know that what we believe to be water proofing in walls of housing is anything but. Water pours through these almost as if the concrete wall was not there. So we can take from those experiences that where water is at a higher than ground level it can penetrate the outer leaf and 'flood' the cavity. This can happen even due to the normal downpour that can occur here in Ireland where a shallow 'river' of water can flow around a house (anything from a fraction of an inch upwards, for instance).
Examination of lots of the installation guides for DPMs would suggest that the DPM can be at a level lower than the finished floor level of the house. Without a DPC any water build-up in the cavity than reached a level above the DPM could, and would, penetrate the inner leaf and do its dirty deed. By placing a DPC at the same level as the floor and bonding this to the DPM this type of risk should be averted (assuming the job is done properly, of course) where the level of water does not rise above the DPC.
Could this be one reason for the use of a DPC in the inner leaf?

Take care.

Mike

ps
Theories are very wlecome. Without theories we would not have progressed very far as a species. Reasoned thought and supporting evidence likewise. Please do not submit the 'because I say so' line of argument. Such belong to poor parenting and religious organisations.

As part of any site design it is usual to divert any rainfall away from the dwelling as well as any potential flood waters.

it can penetrate the outer leaf and 'flood' the cavity. This can happen even due to the normal downpour that can occur here in Ireland where a shallow 'river' of water can flow around a house

If a 'river' flows around a house or a cavity 'floods' as part of a normal downpour then I say the designer responsible for this element of the build did not do their job properly. A normal downpour should not result in any river or flood around or near a dwelling house.

A DPC and DPM are the last line of defence from the ingress of moisture to the internal building structure and should never be relied upon as the main defence in any design.

You state above

the DPM can be at a level lower than the finished floor level of the house

and then you say

By placing a DPC at the same level as the floor and bonding this to the DPM

How can you bond a DPC to a DPM if they occur at different levels?

Traditionally the DPM was laid on the blinded hardcore and lapped up the inner leaf and across the inner leaf beneath the DPC. With the introduction of Radon Barriers some of which are a much heavier gauge than the DPC's it was found unnecessary to have both, doing the same job at the same location. So provided the Radon Barrier is of a sufficient gauge and occurs at the location of the DPC I am happy to let the radon barrier substitute for the DPC at that location.

Mellor

SolarQuark wrote: »

So please, Mr Mellor, I would appreciate it if you didn't respond to this post or any part of this thread again.

I would appreciate it if you didn't try to tell any forum user, moderator or poster, where they can or cannot post.
Thanks