Andrew

Critique my foundation detail please

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Hi, 

 

Our structural engineer has come up with the attached details for our foundations. I'm getting a bit of feedback from the contractors I'm talking to about the double block (200mm wide) arrangement that the timber frame sits on, most would prefer to change this to 140mm wide single blocks. There's also a concern from the timber frame company that the current arrangement is exposing a significant thermal bridge. 

 

For info, the frame is filled with Frametherm insulation and there is 40mm PIR on the inside face followed by VCL, 25mm service void and then plasterboard. We are using an insulated block and beam flooring system called tetris. On most elevations we are using timber cladding fixed straight onto the frame, a couple of elevations have a brick outer skin. 

 

As I see it, one challenge moving to a 140mm block is that on the elevations without the brick outer skin (e.g. Wall Section 3-3 on the attachment) the blocks will be exposed at ground level. There is the option to build a small decorative plinth or perhaps even using brick slips. Is there a standard detail for this arrangement? 

 

Any thoughts, comments appreciated. Thanks, Andrew.

 

FoundationDetail.thumb.jpg.c6fee028170662a6c40cf3f74d213ff6.jpg

 

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In haste, I'm almost sure @TerryE has solved this (or similar) problem. He details it in his blog.  

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Not related to your question but I would be very uncomfortable with a sub floor level lower than the finished ground level.  I am not even sure building regs (at least here in Scotland) allow that.  In our case the water table can rise almost up to ground level and unless perfectly tanked that would fill with water, and may then sit there for a very long time before it drains away.

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Sorry off topic again, when you say cladding straight to the frame, do you mean via battens. 

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if you have the external ground level that high, why not do a solid floor construction rather than a suspended one?

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Have you looked at Foamglas Perinsul or similar around the edges to help with cold bridges? I think they do 140mm wide. Line it up with the bottom of the floor beams and you will still have enough room for a proper 140mm block for the sole plate to sit on (unless the sole plate can sit on the Foamglas).

 

Incorporate some UFH pipework.

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26 minutes ago, Danny68 said:

Sorry off topic again, when you say cladding straight to the frame, do you mean via battens. 

 

Yes, I do, sorry. I meant as opposed to having a block / brick outer skin. 

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9 minutes ago, the_r_sole said:

if you have the external ground level that high, why not do a solid floor construction rather than a suspended one?

 

SE requirement based on ground conditions. I initially showed a preference for a solid floor but was told not possible.  

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3 minutes ago, Mr Punter said:

Have you looked at Foamglas Perinsul or similar around the edges to help with cold bridges? I think they do 140mm wide. Line it up with the bottom of the floor beams and you will still have enough room for a proper 140mm block for the sole plate to sit on (unless the sole plate can sit on the Foamglas).

 

Incorporate some UFH pipework.

 

Thanks will take a look. 

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38 minutes ago, ProDave said:

Not related to your question but I would be very uncomfortable with a sub floor level lower than the finished ground level.  I am not even sure building regs (at least here in Scotland) allow that.  In our case the water table can rise almost up to ground level and unless perfectly tanked that would fill with water, and may then sit there for a very long time before it drains away.

 

Interesting observation, thanks. The water table is quite high so a serious concern. 

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48 minutes ago, AnonymousBosch said:

In haste, I'm almost sure @TerryE has solved this (or similar) problem. He details it in his blog.  

 

Thanks, I'll take a look. 

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6 minutes ago, Andrew said:

 

SE requirement based on ground conditions. I initially showed a preference for a solid floor but was told not possible.  

 

But you can have a "suspended" slab with no void (i.e. the bearing is taken onto the foundation walls rather than the ground)

as soon as you introduce a void, you introduce a need for ventilation - below ground level voids seem to add potential for disasters

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If you have a high or variable water table you will need to incorporate some drainage arrangement under the floor.  I suggest that the concrete blinding is laid to falls. @nod did this 

 

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1 minute ago, the_r_sole said:

 

But you can have a "suspended" slab with no void (i.e. the bearing is taken onto the foundation walls rather than the ground)

as soon as you introduce a void, you introduce a need for ventilation - below ground level voids seem to add potential for disasters

 

I understand that if the soil is clay and subject to heave then the void must remain.

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2 hours ago, Andrew said:

 

Yes, I do, sorry. I meant as opposed to having a block / brick outer skin. 

Don't be sorry I'm learning new construction methods every week on bh, it would have just added to the list. 😁

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2 hours ago, Mr Punter said:

I understand that if the soil is clay and subject to heave then the void must remain.

 

Is it not possible to use a sacrificial EPS layer such as CellCore in place of the void? e.g. https://cordek.com/products/cellcore-hxs 

 

Its what I plan to use under my reinforced concrete raft and the raft's EPS insulation. My raft will be attached to piles because of clay and for tree root protection.

 

(Note: a SE has not yet designed my raft, that will be done later by the timber-frame supplier, which in my case will do both frame and foundations).

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Posted (edited)

Andrew - Cross section  1-1 appears to show a half block (?) resting on the insulation under the inner leaf? Not sure of the point of that. I recommend speaking to the frame company to see what they normally recommend. eg Ask if the full width of the frame must be supported or is a single width 140mm block enough.

1-1.jpg.b801d18cc308b287d5b573a2477084bf.jpg

 

 

Edited by Temp

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I bored two holes on Sunday  Next  to the foundations Each 600 deep 

Both immediately filled up with water 

As pointed out above It was on the engineers advice that I angled six cube of concrete and installed a drain with a flood valve 

I thought it was overkill at the time 

But in light of the recent weather Its looking like money well spent 

and Somthing you can’t do retrospectively 

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Posted (edited)
4 hours ago, Andrew said:

As I see it, one challenge moving to a 140mm block is that on the elevations without the brick outer skin (e.g. Wall Section 3-3 on the attachment) the blocks will be exposed at ground level. There is the option to build a small decorative plinth or perhaps even using brick slips. Is there a standard detail for this arrangement? 

 

Regarding 3-3

 

Not very keen on the detailing. Shouldn't there be some sort of drip bead where I've shown to stop water running in under the cladding? Perhaps that's not shown because this is just the foundation detail drawing? Has the frame company provided any drawings?

 

Again there appears to be a half block on the inside (see my earlier post).

 

3-3.thumb.jpg.e36d9602fcf2d8b8c072e31475a18d6e.jpg

 

 

 

Edited by Temp

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6 hours ago, Temp said:

Andrew - Cross section  1-1 appears to show a half block (?) resting on the insulation under the inner leaf? Not sure of the point of that. I recommend speaking to the frame company to see what they normally recommend. eg Ask if the full width of the frame must be supported or is a single width 140mm block enough.

1-1.jpg.b801d18cc308b287d5b573a2477084bf.jpg

 

 

 

I believe this is a mistake on the part of the SE. The insulation would stop at the blocks. The tetris system has closure blocks that fill the gaps between the beams and on top of this would be a block to get up to sole plate level. As it's shown in the drawing doesn't really make any sense. I've attached the standard tetris details for info - the SE has had these from the beginning. 

 

Standard frame company detail is to just have a single course of 140mm wide blocks and sit the sole plate / frame on top of that. They are used to the frame having a brick outer skin as this is how most of their projects are constructed. 

Concrete Options - TETRiS.pdf

Construction_Details_-_TETRiS.pdf

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6 hours ago, Temp said:

 

Regarding 3-3

 

Not very keen on the detailing. Shouldn't there be some sort of drip bead where I've shown to stop water running in under the cladding? Perhaps that's not shown because this is just the foundation detail drawing? Has the frame company provided any drawings?

 

These are just foundation details, but thanks for the observation.

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Thanks for all the helpful replies. Are there any thoughts on the thermal bridge aspect if the original details are used. The biggest concern is that the 140mm frame is sitting on 200mm wide blocks / bricks and hence there will be a section of the base which does not have insulation sitting on top??

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7 hours ago, Temp said:

Andrew - Cross section  1-1 appears to show a half block (?) resting on the insulation under the inner leaf? Not sure of the point of that. I recommend speaking to the frame company to see what they normally recommend. eg Ask if the full width of the frame must be supported or is a single width 140mm block enough.

1-1.jpg.b801d18cc308b287d5b573a2477084bf.jpg

 

 

When Scotland made the general move from 100mm frames to 140mm frames, I saw several like that (though without the half block) where the foundations had been done for a 100mm frame and then a 140mm frame arrived and it overhung the blockwork on the inside.

 

Now they use 140mm blocks for the inner leaf.

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i'd be removing the course of bricks under the kit replacing it with a course of 140mm marmox under the blockwork to break the cold from below. i'd also be staggering, where possible the courses to block on flat.

 

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Andrew, why the floor void? This really complicates this type of floor.  Quite a few of us use MBC raft designs. Their SE demands a geo survey and the approach that he adopts in perhaps 90% of cases is to build the raft on a sub-base made up of 50 mm layers of course crushed gravel. The depth varies account to soil conditions, but we had 450mm for ours on Gault clay, IIRC.  We used MOT 1 for the top 50mm to blind it and this was then topped by a 50mm sharp and leveling layer and the slab poured into structural grade EPS which both acted as the former for the slab and the insulation. No void at all.

 

We had a land drain around the perimeter of the sub-base emptying into a sump that we could pump out before the proper drainage was in place.

 

Having a void below ground level is a bad idea IMO. It's not a good idea to have a lake below your living area.

 

And either way foamGlass should easily support your frame. Get your SE to do calcs. It solved a bridging issue that we had and our SE signed it off.

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