DPC and Part M

Victorian House

Hi,

Could somebody clarify regarding the location of the DPC in the external leaf and ground levels?

My knowledge might not be up to scratch on this but my previous understanding was:

• DPC in external leaf min 150mm above ground
• DPC and DPM on internal leaf at same level
• Floor level approximately at same level as DPM/DPC so ~150-225mm above external ground level

However, with Part M level access, I can't figure out how the internal floor level will be the same as the external ground level and also keep the DPC at the right height.

Any help appreciated!

sydthebeat

Victorian House wrote: »

Hi,

Could somebody clarify regarding the location of the DPC in the external leaf and ground levels?

My knowledge might not be up to scratch on this but my previous understanding was:

• DPC in external leaf min 150mm above ground
• DPC and DPM on internal leaf at same level
• Floor level approximately at same level as DPM/DPC so ~150-225mm above external ground level

However, with Part M level access, I can't figure out how the internal floor level will be the same as the external ground level and also keep the DPC at the right height.

Any help appreciated!

Drainage channels

Victorian House

I understand that you should use a drainage channel in front of a door but surely you don't need to run one around the whole perimeter of a house?



In the image, I've explained it a bit better. On the left, you have what I would consider to be the norm for DPC - external level 150mm below DPC. As you ramp up to the level access door, you will have a zone that is all below 150mm.

sydthebeat

i suppose it depends on your understanding of why a DPC is required in the first place.
Its there to stop rising damp coming up through capillary action from below.

The drainage channel is required at the door becuase thats generally a flat area where rainwater could pool.... but on the sloped access rainwater shouldnt pool and should run off quite quickly.

There may be an increased risk of the outer blockwork wall getting extra wet from splashing, but that would be down to the failure of the weather proof render in the blockwork, and even at that, the outer leaf is generally expected to be a bit wet anyway.

also note that you are required to use extra strong concrete blocks (13 Newton) above and below the DPC level in the outer leaf to protect against freeze / thaw action.

Victorian House

sydthebeat wrote: »

i suppose it depends on your understanding of why a DPC is required in the first place.
Its there to stop rising damp coming up through capillary action from below./QUOTE]



Could you have the DPC up 150mm above the floor level for the external leaf so? And lower for the internal leaf?

This would seem against Part C of the TGD manual but this was written about 13 years before Part M:

Victorian House

Just to follow on from the previous message, I've seen a few house extensions online and can't quite figure out the detailing of the DPC, e.g:
from https://www.dezeen.com/2018/08/20/al-jawad-pike-peckham-house-extension-london-architecture/

Other examples of external and internal level being very similar:

dusteeroads

You can't assume that the images posted are compliant. I would have installed continuous channels to all parts of such elevations

Victorian House

dusteeroads wrote: »

You can't assume that the images posted are compliant. I would have installed continuous channels to all parts of such elevations

I just have noticed with a lot of new extensions that have access directly into the garden that they don't have the typical step that I used to see before.

Could compliance be found by assuming that the 'ground' element in Part C is actually the base of the patio rather than the top? If they had grass running up to the building, they would be non-compliant but a paved surface allows them to keep the 'ground level' ~150-200mm below the finished level.

sydthebeat

Victorian House wrote: »

I just have noticed with a lot of new extensions that have access directly into the garden that they don't have the typical step that I used to see before.

Could compliance be found by assuming that the 'ground' element in Part C is actually the base of the patio rather than the top? If they had grass running up to the building, they would be non-compliant but a paved surface allows them to keep the 'ground level' ~150-200mm below the finished level.

youre getting caught up in the prima facia compliance of the TGDs

The regulation C4 states

"The floors, walls and roof of a building shall be so designed and constructed as to prevent the passage of moisture to the inside of the building or damage to the fabric of the building."

and yes, theres nothing stopping you putting another DPC 150mm above the outside level if you feel it necessary.

google 'hidden aco drainage channel' for plenty of examples of how those details in teh pics above can be done.

Victorian House

sydthebeat wrote: »

youre getting caught up in the prima facia compliance of the TGDs

For things like a house extension, I wouldn't have thought you needed to go outside the TGDs. I thought Part M would have sorted it out and it seems like such a common detail that I'm surprised you would need a professional to sign-off on the compliance.

sydthebeat wrote: »

google 'hidden aco drainage channel' for plenty of examples of how those details in teh pics above can be done.

Yes, this is a neat solution (at a price!). I can't see the slot in the above pictures but maybe they have this in place.

BryanF

Victorian House wrote: »

For things like a house extension, I wouldn't have thought you needed to go outside the TGDs. I thought Part M would have sorted it out and it seems like such a common detail that I'm surprised you would need a professional to sign-off on the compliance.



Yes, this is a neat solution (at a price!). I can't see the slot in the above pictures but maybe they have this in place.

Lots of factors at play.

Drainage

Orientation

Detailing..

Is the cost of professional opinion/certification the issue?

Lumen

Those discreet drainage slot solutions look like a blockage waiting to happen.

I'd want something that didn't require pulling up paving to clear out, something like a simple box channel with a removable grille.

Metric Tensor

The channel below the slot needs to be roddable for maintenance. Otherwise you are correct about it being a clog risk.

But if it can be accessed for rodding or jetting it's not really much different to a buried sewer pipe.

In my line of work I see the practical and functional being ignored in favour of the aesthetic and form much too often. There's no reason why the two cannot go hand in hand. In my opinion this is one of the key aspects of being a good designer.

Gumbo

Lumen wrote: »

Those discreet drainage slot solutions look like a blockage waiting to happen.

I'd want something that didn't require pulling up paving to clear out, something like a simple box channel with a removable grille.

They come with an end piece that you can remove and rod or put s power washer into. I was looking into them when I done my garden recently.

Victorian House

BryanF wrote: »

Lots of factors at play.
Is the cost of professional opinion/certification the issue?

No - just that level thresholds (Part M) seem to bring a lot of complication to a building (waterproofing and thermal break) and I thought I would find a lot more information online.

Poor Uncle Tom

Victorian House wrote: »

No - just that level thresholds (Part M) seem to bring a lot of complication to a building (waterproofing and thermal break) and I thought I would find a lot more information online.

No real complication, drain across the entrance at the DPC / footpath edge.

Victorian House wrote: »

In the image, I've explained it a bit better. On the left, you have what I would consider to be the norm for DPC - external level 150mm below DPC. As you ramp up to the level access door, you will have a zone that is all below 150mm.

It's all down to design, who says the ramp has to be in contact with the wall?

Victorian House

Poor Uncle Tom wrote: »

It's all down to design, who says the ramp has to be in contact with the wall?

I just made the diagram to explain the area I was having trouble understanding - the zone between the external path being 150mm below finished floor level and the level threshold. I included images of the house extensions as this is another area where the 150mm DPC could be an issue.

In summary, I had a good understanding of how the DPC system worked in external leafs before we had level thresholds. Now, there seems to be a grey area in projects with flush access to patios that don't appear to have drainage at the base of the wall.

rayjdav

The "level" areas outside a building are not supposed to be flat. 1:50 (which is deemed to be "level") gradient to patios/paths will allow for run off away from the building thereby eliminating ponding at structure and need for drainage channels.

Victorian House

rayjdav wrote: »

The "level" areas outside a building are not supposed to be flat. 1:50 (which is deemed to be "level") gradient to patios/paths will allow for run off away from the building thereby eliminating ponding at structure and need for drainage channels.

But is the DPC at finished floor level in the external leaf (+0mm to the external ground level) or at +150mm to the external ground level? This is the bit I'm trying to understand.

So far in this thread, my understanding is that the DPC would remain at low level if you have a drainage channel, i.e. top of drainage channel and DPC would be at approximately the same level.

dusteeroads

It amuses me always to see such images published like you have VH ( I am not poking fun at you in ANY way ). You never see published shots of these design after 5 years of weathering when green slimy moss and prematurely degraded pointing will take their toll on "the concept". Even the glazed screen level with the the external grade will be hard to maintain as pictured - will be forever covered with a waterline of gritty rain splash. DPC at 150mm above GL plus render plinth level to it is not trendy. But is has it's simple virtue - of working.

Metric Tensor

dusteeroads wrote: »

It amuses me always to see such images published like you have VH ( I am not poking fun at you in ANY way ). You never see published shots of these design after 5 years of weathering when green slimy moss and prematurely degraded pointing will take their toll on "the concept". Even the glazed screen level with the the external grade will be hard to maintain as pictured - will be forever covered with a waterline of gritty rain splash. DPC at 150mm above GL plus render plinth level to it is not trendy. But is has it's simple virtue - of working.

If I could thank this post 10 times I would!

Victorian House

dusteeroads wrote: »

You never see published shots of these design after 5 years of weathering when green slimy moss and prematurely degraded pointing will take their toll on "the concept"..

I'd love if I could find pictures of these buildings after five years!