200mm cavity some questions!

[Moggaman]

Hi all.

Any of you experienced putting in windows in a 200mm cavity.?

I have a few questions regarding 200 cavity that ye may help me with;

My engineer is detailing block on flat for inner leaf for 2 or 3 sections of my external walls..says it’s to do with the opes rather than the wide cavity. I have precast detailed also the sit on the inner leaf.

I think I might as well do block on flat all the way round?..At least the precast will have a strurdy unit to sit on.???

 

So its block on flat , 200 cavity, blown in beads,100mm block outer leaf, render.

some quite big windows?.. can I ask what ye did fixing them ?

Also, is there any requirement for me to change from standard concrete lintels over each leaf rather than the lintels that span across the cavity( of course , if the span is too much I realise I will need steel) but for smaller ones I’m wondering about.

Can I reduce the number of ties across the cavity to standard due to the inner leaf being block on flat?

thanks all

 

 

[joe90]

I had a 200mm cavity but brick outer skin, I also had “check” reveals. This means the inner skin has a bigger width than the outer skin and the windows and doors are fitted to the inside of the outside wall. Mine was held in place by stainless brackets and expanding foam strip between the window and outer skin. (I also had angled reveals giving better light into the rooms and looked cottagey.)

[Matt60]

10 hours ago, joe90 said:

I had a 200mm cavity but brick outer skin, I also had “check” reveals. This means the inner skin has a bigger width than the outer skin and the windows and doors are fitted to the inside of the outside wall. Mine was held in place by stainless brackets and expanding foam strip between the window and outer skin. (I also had angled reveals giving better light into the rooms and looked cottagey.)

 

That's interesting, I like the idea of check reveals, does this work best with UPVC windows as they have a thicker frame to take in to account the difference in aperture width? My build is specced with aluminium windows and I assume they have a narrower frame and so the check difference would be more critical? What was the check difference on yours? The angled reveals, was that achieved by just cutting the blocks? 

 

Sorry for all the questions but you've raised some interesting points that I hadn't even considered.

[joe90]

1 hour ago, Matt60 said:

Sorry for all the questions but you've raised some interesting points that I hadn't even considered.

Dont be, that’s what the forum is for. Being a bit OCD, I asked the bricklayer to make my windows openings to a whole brick or half brick as I hate slivers of bricks, the wooden windows were hand made to fit the openings. To show off the windows and ease fitting, also to put the windows in the insulation layer check reveals work well IMO. I cut the blocks inner skin at an angle, fitted angle timbers to window sides and fitted plasterboard screwed to these timbers and plasterboard adhesive to the blocks. The window was foamed to the brickwork before topping up the insulation and fitting sides etc. 

[Matt60]

I see, thanks for the info - looks like a really neat job and the widow width to suit whole/half bricks is a really good idea. The more I know, the more I know I need to know...?

[Iceverge]

19 hours ago, Moggaman said:

think I might as well do block on flat all the way round?..At least the precast will have a strurdy unit to sit on.???

Ask the structural engineer or phone the engineer at the precast slab manafacturer.  

 

Beware the cost of blocklaying. You'll double the cost of your inner leaf. 

 

19 hours ago, Moggaman said:

can I ask what ye did fixing them ?

 

Resting on precast window sills and concrete screwed via brackets into the outer leaf. 

 

19 hours ago, Moggaman said:

Also, is there any requirement for me to change from standard concrete lintels over each leaf rather than the lintels that span across the cavity( of course , if the span is too much I realise I will need steel) but for smaller ones I’m wondering about.

Can I reduce the number of ties across the cavity to standard due to the inner leaf being block on flat?

Avoid lintels that span the cavity. Thermal bridging is an issue. Use concrete in preference to steel. It's cheaper and easier to finish. 

 

Again your structural engineer has the last word here. 

[joe90]

19 hours ago, Moggaman said:

My engineer is detailing block on flat for inner leaf for 2 or 3 sections of my external walls

This will (I think) be because of wide openings near the ends of walls, but why on earth do it all the way round?? ££££££.

[Iceverge]

7 hours ago, Matt60 said:

That's interesting, I like the idea of check reveals, does this work best with UPVC windows as they have a thicker frame to take in to account the difference in aperture width? My build is specced with aluminium windows and I assume they have a narrower frame and so the check difference would be more critical? What was the check difference on yours? The angled reveals, was that achieved by just cutting the blocks? 

 

Worked fine with PVC windows for us. 

 

I made angled 18mm OSB boxes after the windows were fitted and concrete screwed them to the inner leaf. No need to cut blocks just set back the inner leaf by 100-150mm depending on the angle you desire. In hindsight it would have been easier to build the boxes in as the walls went up. 

[Moggaman]

6 hours ago, joe90 said:

This will (I think) be because of wide openings near the ends of walls, but why on earth do it all the way round?? ££££££.

Well I thought it was to ensure that the 1st floor 150mm thick precast would have stable walls to sit on but yes the opes were mentioned.  

My proposed house is 1.5 stories with only 4 to 5 courses above the first floor . See attached.Opes aren’t crazy big ... do people have to thicken inner leaf at sliding doors generally?

The precast manufacturer said that 100mm walls on the inner and outer leaf would be fine for precast. 
The perimeters of my building is 13m x 9m , a rectangle and I would only bring the block on flat to precast floor level, so whilst it may be more expensive , it’s not gonna be huge???. That’s why I said to myself ...ah just put block on flat all around.. 

i am interested did any of ye guys who have block built houses with precast first floor and average sized opes thicken the inner leaf?

thanks

 

[PeterW]

If you want a thicker wall then go for 140mm blockwork - 215mm is overkill and unnecessary. 

[joe90]

7 hours ago, Moggaman said:

The precast manufacturer said that 100mm walls on the inner and outer leaf would be fine for precast. 

I think  that says it all, your openings are not oversized (IMO) so I think it’s OTT.

[PeterW]

Going past anything other than 140mm will cause issues I think over openings as you will need custom lintels for all of those - 215mm lintels are designed to be installed vertically and not on the flat so you would have to potentially use steels on every opening as standard lintels will only support a max 150mm inner leaf. 

[Brickie]

2 hours ago, PeterW said:

Going past anything other than 140mm will cause issues I think over openings as you will need custom lintels for all of those - 215mm lintels are designed to be installed vertically and not on the flat so you would have to potentially use steels on every opening as standard lintels will only support a max 150mm inner leaf. 

...or two 215x100 lintols side by side. 
Agree block on flat is massive overkill on 1.5 storey house. 
Save your money-you’ll want it elsewhere on your build. 

[PeterW]

16 minutes ago, Brickie said:

...or two 215x100 lintols side by side. 
Agree block on flat is massive overkill on 1.5 storey house. 
Save your money-you’ll want it elsewhere on your build. 

I know BCOs who won’t accept that as there is a gap and they will only go with the load calcs of a single lintel. If we’ve needed to beef up an opening then using a concrete lintel on the inside flange of a standard cavity lintel is another option but they are heavy and need propping while they go off. 

[Brickie]

3 hours ago, PeterW said:

I know BCOs who won’t accept that as there is a gap and they will only go with the load calcs of a single lintel. If we’ve needed to beef up an opening then using a concrete lintel on the inside flange of a standard cavity lintel is another option but they are heavy and need propping while they go off. 

The gap issue would be easily overcome with a mortar joint at installation; not so the calcs issue. Easy to get annoyed when it’s my name on the sign off,even if your feeling is that it’s fine. 
Had to fit the additional concrete lintol on a cavity lintol many times when there’s a precast floor bearing on it-always seems to coincide with a batch of lintols which have been battered out of true in transit!

[Dave Jones]

21 hours ago, Iceverge said:

Ask the structural engineer or phone the engineer at the precast slab manafacturer.  

 

Beware the cost of blocklaying. You'll double the cost of your inner leaf. 

 

 

Resting on precast window sills and concrete screwed via brackets into the outer leaf. 

 

Avoid lintels that span the cavity. Thermal bridging is an issue. Use concrete in preference to steel. It's cheaper and easier to finish. 

 

Again your structural engineer has the last word here. 

 

you cant use concrete lintels on face brickwork it would look aweful.

 

There are thermally broken lintels available ... at a cost.

[Brickie]

4 minutes ago, Dave Jones said:

 

you cant use concrete lintels on face brickwork it would look aweful.

 

There are thermally broken lintels available ... at a cost.

He’s got 100mm blockwork outer leaf. 

[Dave Jones]

ahh render job. no brainer then.

[Mr Punter]

I suggest you get a quote for Hi-Therm lintels and if you go for them, make sure your SAP person includes the psi values in the calcs.  They are not a stock item, so leave yourself a good lead time.

[Mr Punter]

Precast floor is unusual.  Is it the hollowcore planks?  They have a fair bow in them so you need batten on underside a levelling screed on top.  Also, if you are having UFH at 1st floor you need insulation.  Look up the Tony tray as used by @tonyshouse.

[joe90]

37 minutes ago, Dave Jones said:

you cant use concrete lintels on face brickwork it would look aweful.

I did, but cut bricks into slips and glued them on afterwards,( lintel recessed enough.)

[Dave Jones]

Just now, joe90 said:

I did, but cut bricks into slips and glued them on afterwards,( lintel recessed enough.)

 

that's dedication to 10 pence of the annual fuel bill for sure!

[joe90]

3 minutes ago, Dave Jones said:

 

that's dedication to 10 pence of the annual fuel bill for sure!

Nothing to do with fuel savings, simply to look good!

[Iceverge]

@Moggaman get in contact with Killeshal precast in Offaly. 

 

I suspect your engineer will specify steel  lintels like they did for us for any walls bearing the slabs. With your engineers agreement you can substitute these for precast concrete lintels but they'll need to be specially ordered. We got our window sills from them too so I suspect it won't be an issue if you do it in time . We got the "Type H " as it has a very narrow top lip and allowed a layer of insulation between the inner and outer sills. 

 

Out of interest have you chosen your precast slab supplier? 

 

 

[Iceverge]

As you can see we used steel lintels for the wide openings (2m) on the external leaf.  All The narrower ones (1m) were standard 100x50 precast lintels.

 

This really is a question for your structural engineer though. 

 

If I had my time again I'd avoid precast floors. You end up with a very thick floor ~450mm, the added expense of including a suspended metal ceiling. They're a PITA to core through for services. Installing them without guys making ribbons of any airtightness tray is nearly impossible. Despite requests and being on site they still crowbar the slabs back and forward over the plastic membrane.