New blockwork detail - No cavity closer

mickdw

What do people think of the idea of not having a cavity closer at the top of a double leaf block wall. The latest details show the insulation running from the attic space straight into the wall cavity. I dont see how this will work when 2 100mm walls are being used. How do you get the stability at the top of the wall to support roof loads

RKQ

I think a cavity closer is important to "close" the cavity to prevent fire spread.
The cavity closer also supports the wallplate, helping to spread the load and allowing slight changes to the wallplate position to ensure the roof timbers are square.
Interesting topic mickdw

Jimbo

I think closing the cavity with slate is a good compromise.
Low risk of fire spread and cold bridging is minimal

sinnerboy

And cills no longer to rest on inner leaf .

And when dry lining - studs not to be fixed direct to external wall blockwork

I doubt there was any structural engineering input into these details

RKQ

jimbo78 wrote: »

I think closing the cavity with slate is a good compromise.
Low risk of fire spread and cold bridging is minimal

I agree but its yet another detail to be checked on site and no doubt the Blocklayers will not want to do it, so it will only lead to confusion and possible difficulties on site.

mickdw

I guess the block closer can still be used if an insulated slab is being used internally.

Slig

mickdw wrote: »

I guess the block closer can still be used if an insulated slab is being used internally.

The main purpose, as I see it, of the concrete cavity closer is to support the wallplate and therefor the roof on BOTH 100mm leafs. I'm sorry but without the concrete cavity closer then you are supporting the entire weight of the roof on the inner leaf and a 100mm blockwork wall is not sufficent to support that weight.
If you want to use this detail then the inner leaf should be structural.

mickdw

Slig wrote: »

The main purpose, as I see it, of the concrete cavity closer is to support the wallplate and therefor the roof on BOTH 100mm leafs. I'm sorry but without the concrete cavity closer then you are supporting the entire weight of the roof on the inner leaf and a 100mm blockwork wall is not sufficent to support that weight.
If you want to use this detail then the inner leaf should be structural.

I dont want to use this detail but its the detail being suggested by dept of environment under new regs to get rid of thermal bridge at that location. Now as I said in my original post, you could never get the required stability using this detail when both walls are 100mm and I was just looking for opinions on this. Your response is bordering on giving me a lecture.

Builderfromhell

I always understood that the wallplate rests on the inner leaf of a cavity wall. The load is taken by both leafs of the cavity wall as they are connected with wall ties.
The installation of wallplates is not an exact science. Walls can be off square so wallplate may be at edge of inner leaf in some cases and closer to middle of wall in others.
Interesting topic though.
In my experience of supervising work in Ireland - Cavities are not always closes especially at gables (where blockwork follows slope of roof).

muffler

mickdw wrote: »

Your response is bordering on giving me a lecture.

I dont see it that way. Im actually amazed that you read that into an opinion that you yourself sought.

Peace brethren

mickdw

muffler wrote: »

I dont see it that way. Im actually amazed that you read that into an opinion that you yourself sought.

Peace brethren

Sorry but just to clarify, In my original post I clearly expressed the view that I though this detail was odd and not suitable for use with 2 100mm walls. This Slig says "Im sorry but ....." as if I had constucted something in this manner and had made a mistake.

Maybe its just my reading of it when he used "you" at all times

muffler

I think "you" was meant in the general sense and not user specific.

sinnerboy

http://uk.youtube.com/watch?v=qgDKtLPp46s

Quazzie

sinnerboy wrote: »

http://uk.youtube.com/watch?v=qgDKtLPp46s

I can't believe I nearly clicked on that link..

I'd prefer a rickroll!

Slig

mickdw wrote: »

I dont want to use this detail but its the detail being suggested by dept of environment under new regs to get rid of thermal bridge at that location. Now as I said in my original post, you could never get the required stability using this detail when both walls are 100mm and I was just looking for opinions on this. Your response is bordering on giving me a lecture.

I missed all that exchane completely. Sorry if I gave the impression that I was lecturing you, I wasent. I was trying to express my disgust that such a detail would be published using my everyday expressions to my fellow piers. I dont set myself as above anyone on this forum and I appologise if it ever appears that way.

sydthebeat

my view would be that the requirement to close a cavity was more from a point of view to reduce draughts, reduce fire risks and to restrict rodent movement, thus the common use of slate as a cavity closer when cavity construction began.

However use of a block cavity closer is a very good measure of acting against the twisting motion put on wallplates, especially on storey and a half type dwellings. I wouldnt even dream about suggesting no cavity closer on a storey and a half type. I would detail out the thermal bridge in other ways, either by use of deeper eaves with quilted insulation, use aac blocks, and maybe internal composite plasterboard.

Heatherview

Hi Mickdw
I would agree with you, it's where the roof starts to get it's structural requirements cavity wall - cavity closer - wallplate. Fire protection - vermin stopper - draughts from cavty below - final leveller for wall plate

Good topic worth investigation ?

jonilz

I've been looking into this and need a good solution soon. From an engineering perspective force bears down and is spread at an angle of 45 degs from eash outer edge of say the wallplate. So when claculate the outer 100mm block does not actually contribute so much, however i'll be getting exact figures. To elominate the colde bridge formed by the cavity closing block is vertuall impossible unless you go with exterior insulating slab or a type of low thermal conductive block. Anyone with update on this topic since it was originally opened please update this interesting thread.

Graaaaa

Slig wrote: »

The main purpose, as I see it, of the concrete cavity closer is to support the wallplate and therefor the roof on BOTH 100mm leafs. I'm sorry but without the concrete cavity closer then you are supporting the entire weight of the roof on the inner leaf and a 100mm blockwork wall is not sufficent to support that weight.
If you want to use this detail then the inner leaf should be structural.

Slig
I'm sorry but that is completely misinformation - as clearly stated in Part A of the Building Regulations, the inner leaf may be 100mm thick solid concrete blocks.
If the guidelines in the rest of Part A are kept to i.e. limiting dimensions of rooms, floor to floor heights etc. then the inner leaf is adequate to take the loading in most domestic 2-storey situations at 100mm thick. Anything more is an awful waste of material and money.
The outer leaf is not intended to be taking any loading through the closer - the wall plate should in theory be centred over the inner leaf so the closer cannot transfer the loads as there is no lever action going on. The fact that is does take some light loading from off-centre roofing is generally no harm but may explain cracks in your external renders.

However when assessing the resistance to wind loads, this is when the outer leaf is providing half your structural capacity. This is why it's important to use the correct type and quantity of wall ties to allow the two leafs to act together.

mickdw

Graaaaa wrote: »

Slig
I'm sorry but that is completely misinformation - as clearly stated in Part A of the Building Regulations, the inner leaf may be 100mm thick solid concrete blocks.
If the guidelines in the rest of Part A are kept to i.e. limiting dimensions of rooms, floor to floor heights etc. then the inner leaf is adequate to take the loading in most domestic 2-storey situations at 100mm thick. Anything more is an awful waste of material and money.
The outer leaf is not intended to be taking any loading through the closer - the wall plate should in theory be centred over the inner leaf so the closer cannot transfer the loads as there is no lever action going on. The fact that is does take some light loading from off-centre roofing is generally no harm but may explain cracks in your external renders.

However when assessing the resistance to wind loads, this is when the outer leaf is providing half your structural capacity. This is why it's important to use the correct type and quantity of wall ties to allow the two leafs to act together.

I think you are picking holes in the way the Slig stated his point. I agree that a 100mm block has the compressive strength to support the roof but this was not the original question asked by myself here. It is obvious that a single 100m block will not provide the stability to support the roof structure. Wind loading is a major part of this but also from a buildability point of view, a top course of 100mm block will take very little pressure before it is loosened/dislodged and really it would be an absolute disaster to consider building as such.
When Slig said the purpose of the closer was to support the roof on both leaf, it is pretty clear to me what he meant, i.e. that the outer leaf gives the additional stability to the support point.
It is irrelevent whether the wall plate in on the inner leaf or alittle toward the cavity imo, it is the stability we are questioning, not the actual compressive strength of the wall.

onq

In relation to loads, a two storey house will be okay with a cavity wall with sufficient wall ties.
The numer and location of these went up when the returns were done away with.
Nobody seems to mind that most look like they only work in tension.
I mean, what if the leaves try to move towards one another?

Anyway...

I'd like to broaden the discussion a little if I may.

Fire can propagate along a cavity.
Unless your windows and cills can themselves be 30FR !closers you need something else to do it.
With a 100mm gap and I think you'll end up relying on the new 30 Min FR cavity closers.

However you could consider a 215mm hollow or solid block wall and insulate internally.
Stairs require to be offset to allow space for insulation on the gable.
Floor timbers may need to be offset also but timber is also an insulator.
The tricky bit is working in the vapour check, but no more difficult than trying to get the sealer detail on the DOEHLG website to work with an internal block wall T-ing an inner block leaf - anyone sorted that yet?

Anyway insulating internally gives significantly quicker warm up times than feeding heat into the heat sink of the inner leaf masonry.
Taking 38mm warmboard around the reveals sorts the cold bridge there.
Finally running the warmboard up to the ceiling means the tricky eaves detail is avoided.

For single storey buildings it seems fine - for two stories it needs a bit of work - sealing to a steel angle and supporting floors off that might do it.

FWIW

ONQ.

Graaaaa

mickdw wrote: »

I think you are picking holes in the way the Slig stated his point. I agree that a 100mm block has the compressive strength to support the roof but this was not the original question asked by myself here. It is obvious that a single 100m block will not provide the stability to support the roof structure. Wind loading is a major part of this but also from a buildability point of view, a top course of 100mm block will take very little pressure before it is loosened/dislodged and really it would be an absolute disaster to consider building as such.
When Slig said the purpose of the closer was to support the roof on both leaf, it is pretty clear to me what he meant, i.e. that the outer leaf gives the additional stability to the support point.
It is irrelevent whether the wall plate in on the inner leaf or alittle toward the cavity imo, it is the stability we are questioning, not the actual compressive strength of the wall.

If that's what was meant then fair enough, I didn't read it that way, and while it may be difficult to achieve in practice, a properly tied, braced and strapped roof should be stable on the inner leaf only.

I can see that a 1.5 storey roof is going to be a different situation and needs a ring beam or a more rigid wall plate. You could design-out the spreading forces at the eaves by providing a load-bearing support to the ridge - by which I mean a beam spanning from gable to gable or a loadbearing wall up through the attic and fix your rafters to each side of it.

My apologies to Slig et al if it appeared I was picking holes and pulling off topic. Hope the comments above from a structural point of view help, I'll leave others to comment on the fire spread issue.

Splinter Cell

This certainly is an interesting old thread .....

sydthebeat wrote: »

However use of a block cavity closer is a very good measure of acting against the twisting motion put on wallplates, especially on storey and a half type dwellings. I wouldnt even dream about suggesting no cavity closer on a storey and a half type. I would detail out the thermal bridge in other ways, either by use of deeper eaves with quilted insulation, use aac blocks, and maybe internal composite plasterboard.

What is your opinion about no block cavity closer on a story and a half with block on flat inner leaf supporting the roof structure? Would an insulated firestop cavity closer be sufficient?

I am still wondering about a question that I asked some time back - can I put on the roof before the outer leaf is built up? I only recently realised that this is how TF homes are built - roof first then outer leaf with firestop cavity closer attached to the inner leaf. If it can be done for TF, why not cavity wall with block on flat inner, 100 KS in 140 cavity and block outer?

Mellor

onq wrote: »

Anyway insulating internally gives significantly quicker warm up times than feeding heat into the heat sink of the inner leaf masonry.

Quicker warm up times also mean quicker cool down times.


i'm not sure how or why this was resurrected, pretty old thread, check dates guys.
but seeing as its on the front page, I feel I should clarify a mistake that people have repeated again and again on this page.


The cavity closer does not serve to transfer any loads to the outer leaf. The inner leaf is loaf bearing. Because of the position of the WP almost non of the load reaches the external. Proof of this is clear by the suitability of slate closers, which could never transfer a roof load. Restricting air, vermin and fire are its main functions imo

onq

Mellor wrote: »

Quicker warm up times also mean quicker cool down times.


i'm not sure how or why this was resurrected, pretty old thread, check dates guys.
but seeing as its on the front page, I feel I should clarify a mistake that people have repeated again and again on this page.


The cavity closer does not serve to transfer any loads to the outer leaf. The inner leaf is loaf bearing. Because of the position of the WP almost non of the load reaches the external. Proof of this is clear by the suitability of slate closers, which could never transfer a roof load. Restricting air, vermin and fire are its main functions imo

I don't want to be accused of starting an "argument" here, Mellor.
You raised this issue, and I've had a problem with it since Bolton Street.
So I'm stating the case as we were taught it - to spread the load of the roof using the cavity closer - and if quoting the relevant reference is being pedantic - I'm pedantic.
Youre comments, as ever, are invited and welcome.

Back in Bolton Street Des O'Dwyer used to be suspicious of cavity wall construction - he was old school guy.
He used say we still didn't really understand all the technology and it was developing as it didn't work as well as it might - how right he was.

Back in the day we were taught that both leaves "were working together" and the details showed a brick closer spreading the load.
Our reference work was Ivor H Seely and the Fifth edition (reprinted 1995) is still available.
Seeley's details used 250mm cavity walling which could be bridged by a 215mm brick and was in almost every detail.
The text mentioned using an 85mm cavity which always seemed very ropey to me.
I can still recall being given out to for using a brick in the centre of a 300mm cavity detail to equalize the load, as my fledgling technical ability sought to grapple with how best to span the 100 cavity were were told to use with a 215mm brick.
I think I ended up casting in a ring beam with a bevelled outer edge covered by insulation.

Anyway, even in the "current" 5th edition still states, under the section entitled Cavity Walls, P. 67:

"...The head of the wall is usually built solid to spread the roof loads over both leaves..."

Figure 7.2, P. 135 shows a monopitch roof at 7.2.1 which is supported both ends by 250mm cavity with a brick closer, resting 100mm on the inner leaf and 65mm on the outer

Figure 7.5, P. 140 shows a weird detail 7.5.1 for the top of a cavity wall and eaves detail, which I cannot recall from the old green cover edition:
A steeply pitched roof with brick outer and block inner leaf and what appears to be a humongous block spreader or concrete ring beam giving a massive cold bridge - odd because bevelling it would have allowed the attic insulation to cover it and significantly reduce head transfer.

Figure 7.6, P. 142 shows two different details for the top of a cavity wall and eaves detail:
7.6.1 Shows a brick spreader at the top of the inner leaf, again leaving a cold bridge by not extending the insulation over the closer. The cavity has fill fill insulation.
7.6.7 Show no spreader at the top of the inner leaf and no cold bridge, continuing the full fill cavity insulation [rockwoll cavity wall batts or similar] up to meet the attic insulation.

My "slate closers" were used as permanent shuttering for ring beams.

I accept in principle what you say is the current detail, but I want to point out that this arose partly from detailing to prevent cold bridging and as a consequence of having a wider cavity to accommodate more insulation, not from first principles of how the leaves in a cavity wall were intended to work originally.

In fact while all is "deemed to comply" with what is effectively a 100 inner leaf supporting the roof, and I have had several structural enginners confirm this to me when dealing with remedial work on other's jobs, it seems to cut against the minimum bearing requirement rule of thumb of 150mm mentioned in Part A.
"That's just for point loads for beams over opes", I hear you say - we'll see how things go long term.

On the other matters you raised:

Vermin will get in anywhere, especially where one uses timber in construction, and there are services - along the routes used for heating pipes, because HWPs are insulated and they just eat the insulation and get around - I suppose a cavity closer might make it harder to run up the cavity.

Closing the cavity to prevent fire propagation seems reasonable, but the opes and walls are supposed to prevent fire getting into the cavity in the first place, and it is intended that the roof has no FR and should burn away to release heat, according to F.O.'s I have spoken with over the years.

I've often wondered why there were no high level vents in masonry walling of older vintage as we began to understand migration of water vapour through a structure...

<musing>

Wasn't there once some thought about "sealing the cavity"? What was that about - preventing air circulation? Oh yes, Seeley again:
P. 66 "The cavity must not be ventilated as it will cease to act as a thermal insulator". There you go.

Masonry Cavities are effectively vented by the weep holes at the base of the walls and above lintols installed to allow moisture from a saturated outer leafe get away.

Or course in timber frame contruction they are deliberately vented top and bottom to take away moisture.

I'll stop now as I appear to be wandering off topic again.

ONQ.

Mellor

onq wrote: »

I don't want to be accused of starting an "argument" here, Mellor.
You raised this issue, and I've had a problem with it since Bolton Street.
So I'm stating the case as we were taught it - to spread the load of the roof using the cavity closer - and if quoting the relevant reference is being pedantic - I'm pedantic.

You raise some good points, and not pedantic at all.

Back in the day we were taught that both leaves "were working together" and the details showed a brick closer spreading the load.
Our reference work was Ivor H Seely and the Fifth edition (reprinted 1995) is still available.
Seeley's details used 250mm cavity walling which could be bridged by a 215mm brick and was in almost every detail.

Oh I agree, my comments refer specifically to the role of closers today. In a 50mm cavity situation, I perfectly accept that its role was to act as a load spreader. This was a natural carry over from 215 brick walls in my opinion. Header bricks used to transfer loads between leaves at top, and at various points all along the wall.
As you were in bolton street, you are no doubt familiar with the 215 brick walls of henrietta street, which are actually failing due to a lack of lateral tensile support between walls (the Georgians obviously didn't have wall ties)

My "slate closers" were used as permanent shuttering for ring beams.

A fine material for the job

I accept in principle what you say is the current detail, but I want to point out that this arose partly from detailing to prevent cold bridging and as a consequence of having a wider cavity to accommodate more insulation, not from first principles of how the leaves in a cavity wall were intended to work originally.

I agree complete, as I said, I refered to only current wide cavities.

In fact while all is "deemed to comply" with what is effectively a 100 inner leaf supporting the roof, and I have had several structural enginners confirm this to me when dealing with remedial work on other's jobs, it seems to cut against the minimum bearing requirement rule of thumb of 150mm mentioned in Part A.
"That's just for point loads for beams over opes", I hear you say - we'll see how things go long term.

to be honest, my retort wouldn't of been that it's just for beams over opes, as on a basic level how is a roof truss, rafter or joist ant different.
however, the reduced bearing is one of the reason that its vastly important to ensure adequate strapping and fixing of trusses to wall plates and walls. Not forgetting the fixing of wall plates themselves.

Vermin will get in anywhere, especially where one uses timber in construction, and there are services - along the routes used for heating pipes, because HWPs are insulated and they just eat the insulation and get around - I suppose a cavity closer might make it harder to run up the cavity.

I made sure to say restrict, not prevent, for the reasons you list.
Mice are one thing, a friend rang me the other day to let me know she had a huntsman spider in her house.

Closing the cavity to prevent fire propagation seems reasonable, but the opes and walls are supposed to prevent fire getting into the cavity in the first place, and it is intended that the roof has no FR and should burn away to release heat, according to F.O.'s I have spoken with over the years.

Nothing is supposed prevents fire from getting past, it it supposed to slow it for a reasonable period of time, hence 30/60/90/120 min fire rating. however, once the cavity is breached, the closer slows it getting into the roof space where it could spread to other room below.
When a roof burns away it collapses and releases heat, this "intention" applies to after the house is evacuated. And roofs are fire rated, 30/30/30 if its an escape route, and also there is an EU designation on fire spread based on roofing materials.

Wasn't there once some thought about "sealing the cavity"? What was that about - preventing air circulation? Oh yes, Seeley again:
P. 66 "The cavity must not be ventilated as it will cease to act as a thermal insulator". There you go.

No need to go to the trouble of refering to Seeley, you could also refer to Mellor

Mellor wrote:

Restricting air, vermin and fire are [the cavity closer's] main functions

I'm guessing you missed that the first time.

ManFromAtlantis

Graaaaa wrote: »

If that's what was meant then fair enough, I didn't read it that way, and while it may be difficult to achieve in practice, a properly tied, braced and strapped roof should be stable on the inner leaf only.

I can see that a 1.5 storey roof is going to be a different situation and needs a ring beam or a more rigid wall plate. You could design-out the spreading forces at the eaves by providing a load-bearing support to the ridge - by which I mean a beam spanning from gable to gable or a loadbearing wall up through the attic and fix your rafters to each side of it.

My apologies to Slig et al if it appeared I was picking holes and pulling off topic. Hope the comments above from a structural point of view help, I'll leave others to comment on the fire spread issue.

spot on.
the idea of a ridge beam not used much in practice. unfortunately.
The rafter size could be increased depending on extended leg (storey and half construction) to reduce deflection/outward force but in my humble exp you more often than see a minor horz crack somewhere between floor and eaves in the block.

soldsold

Ive set a reminder on my iphone to post a comment about cavity closers and wallplates on this thread in about 9 months, it would be a shame to let this thread die peacefully

stylers

this thread is not dead yet !

I'm coming fairly close to the wall plate/roof stage on my 1.5 storey build. I've got a 6 inch solid inner leaf, 4 inch outer leaf for the front and back walls, and 140mm cavity. There is also steel spanning the length of the roof approx halfway down. I'm trying to avoid thermal bridging as much as possible, I figure whats the point in putting in so much insulation and then bridging it (I know the whole lot is bridged at the foundation but there's not much can be done about that ).
For the wall plate cavity closer, I've thought of aerated blocks, but I'm not sure how well mortar sticks to these. So I was thinking of ring beams for the closer, as ManFromAtlantis and others suggested. basically pour a ring on the top blocks, with a bridge at each end and at intermediate points, with reinforcement steel, and also set pins into the blocks below to make sure it is structurally joined to them. should be much stronger and rigid than just putting blocks across it. The steel will probably remove most of the roof spreading forces anyway. for the gaps between the bridge, close them off with something fireproof like fibre slate. fire resistant and relatively low thermal conductivity. Any thoughts ?

jonilz

hi, i recon that the 6" block will be fine to carry the load shared by the steel, but check with an open-minded engineer. Then use a cavity closer like (http://www.xtratherm.com/products/literature/thinrcloserirl.pdf). I did and worked with Xtratherm directly.
Best of luck and if you find a custom solution please let us know.
Rgds, Jon

sinnerboy

@ stylers and @ jonilz

You are sailing close to the wind with respect to this aspect of the forum charter

On construction related matters any specifying of structural components is prohibited. Any queries in this regard need to be dealt with by an architect/engineer privately.

Terms like "reckon" "open minded" "thinking of" and "probably" must play no part with respect to structural design .

Take note - such matters must be designed and inspected during works only by a Structural Engineer .

And take note with respect to your future contributions here

jonilz

Sinnerboy, you absolutely right and I could have given the wrong impression, so apologies. What I actually meant to say is that so many engineers, just like council planners have got stuck in a rut of only wanting to deal and see what they always see and do. Anything new means putting on the thinking cap. The irish building industry has been so slow to adapt and progress to new levels because the quickest way out is the easiest and visa versa! Finding a structural engineer that is prepared to put on his thinking cap will result in some great results. There are ways to get what you want and this cavity closer issue is no exception, but I agree it's NOT a DIY matter.
Regards
Jon

stylers

@ Sinnerboy, apologies, I agree completely and of course I'm consulting my engineer, I can only add to what jonilz said (which is bang on), but if we don't find out if these ideas are at least in some way possible or plausible, then probably no-one is ever going to ask or even try something different for fear of being dismissed or ridiculed, and things will carry on as usual. I'm sure everyone self-building nowadays is going to encounter these issues at some stage, well at least they will if they have any engineering interest in making something actually work with some level of certainty. that is all.

tinca

any further thoughts on this detail?

Poor Uncle Tom

:rolleyes: the grave robbers are out tonight........