Example foundation and slab detailing for timber frame

[Ben100]

Hi guys,

 

I'm looking for some advice and example foundation and slab detailing for timber frame construction. This is what I have in mind so far.

 

Foundation:

• 300mm of concrete (my soil type is hard ballast)
• Single 140mm block up to DPC
• Start of TF construction with 145mm sill plate

Slab:

• Hardcore
• Sand
• 60mm floor grade insulation
• 150mm RC slab
• 65mm screed

 

One more important detail is that the new construction is set 135mm below ground level, so the internal first floor level is below the external ground level. What options do I have here with regards to the foundation? I can see two options.

 

1. Dig a trench around the building around 375mm so the exterior ground does not come in contact with the house, and maybe add in a french drain.
2. Have a higher DPM to external ground level and add some insulation to cope with the external ground  being in contact with the foundation above first floor level.

 

Any thoughts and comments very much appreciated as always 

 

Cheers!

[lizzie]

When you say first floor level is that what I think of as ground floor?

 

Why can’t you just lower the ground all around rather than a having some below ground and digging a trench. We lowered our ground so nothing below ground level but we still have a french drain and trench all around.

[ProDave]

60mm is way too little floor insulation, I doubt that even meets building regs let alone coming anywhere close to "good"

 

What is the TF being clad with? no provision for example for a block or brick outer wall in that foundation.

 

And yes why 150mm below ground?  Are planning that strict on a maximum height?  I would at least lower a good path width of ground all around the house to avoid that.

[PeterW]

Ditch the screed, transfer 50mm of the RC slab thickness into insulation and you’re possibly at building regs level insulation depending on your P/A for the slab area. 

 

That brings the  floor up by 65mm, and makes it a relatively simple task to excavate a 1000mm wide path around the house at 100mm below DPC. Bed this on sand and cement, with 600mm slabs in the centre of 200mm bands of deeper gravel as a French drain. 

 

As @ProDave says though - 145mm TF build, what is the outer skin going to be and what are you doing with the blockwork..? 

[Onoff]

Have a read of these two foundation beginnings:

 

@PeterStarck

 

@JSHarris

http://www.mayfly.eu/2013/08/part-ten-out-of-the-ground/

[Ben100]

19 hours ago, lizzie said:

When you say first floor level is that what I think of as ground floor?

 

Why can’t you just lower the ground all around rather than a having some below ground and digging a trench. We lowered our ground so nothing below ground level but we still have a french drain and trench all around.

 

Yes, sorry, I mean ground floor.

 

I am considering the idea of lowering the ground all around the new extension, but this would mean having a trench all around the property or lowering the entire garden. Both ideas are not ideal...

[Ben100]

18 hours ago, ProDave said:

60mm is way too little floor insulation, I doubt that even meets building regs let alone coming anywhere close to "good"

 

What is the TF being clad with? no provision for example for a block or brick outer wall in that foundation.

 

And yes why 150mm below ground?  Are planning that strict on a maximum height?  I would at least lower a good path width of ground all around the house to avoid that.

 

Ok, thanks, I'll check out the 60mm insulation with BR. This is the exact floor build-up specified.

 

Floor type 2. new ground bearing concrete floor:

• New floor to achieve U-value not less than 0.22w/m2.k
• Floor finish TBC
• 65mm reinforced S/C screed
• separating membrane
• 60mm Celotex GA4000
• 150mm min ground bearing RC slab
• Well compressed hardcore.

 

We plan to clad the TF with vertical timber cladding to as close to the ground as possible. At the moment we're planning a single block foundation.

 

We're actually going to be 375mm below ground level. The reason for this is a mixture of design and planning restrictions.

 

thanks!

[Onoff]

29 minutes ago, Ben100 said:

I'll check out the 60mm insulation with BR

 

Is it a typo maybe when they meant 160mm?

[Jeremy Harris]

58 minutes ago, Ben100 said:

 

Ok, thanks, I'll check out the 60mm insulation with BR. This is the exact floor build-up specified.

 

Floor type 2. new ground bearing concrete floor:

• New floor to achieve U-value not less than 0.22w/m2.k
• Floor finish TBC
• 65mm reinforced S/C screed
• separating membrane
• 60mm Celotex GA4000
• 150mm min ground bearing RC slab
• Well compressed hardcore.

 

We plan to clad the TF with vertical timber cladding to as close to the ground as possible. At the moment we're planning a single block foundation.

 

We're actually going to be 375mm below ground level. The reason for this is a mixture of design and planning restrictions.

 

thanks!

 

 

The line I've highlighted seems odd.  Ideally you want the floor U value to be a lot less than that 0.22 W/m².K figure.  The lower the U value the better, so I'd expect that line to read "New floor to achieve U-value not MORE than 0.22 W/m².K".  A good target would be to aim for a floor U value of around 0.1 to 0.12 W/m².K if you can.  That will just about halve the floor heat loss (assuming that there's no UFH) and mean warmer feet.  If you're looking to install UFH then I'd very definitely look to improve the insulation, as UFH will increase the heat loss to the ground even more.

[Ben100]

On 27/07/2019 at 14:45, PeterW said:

Ditch the screed, transfer 50mm of the RC slab thickness into insulation and you’re possibly at building regs level insulation depending on your P/A for the slab area. 

 

That brings the  floor up by 65mm, and makes it a relatively simple task to excavate a 1000mm wide path around the house at 100mm below DPC. Bed this on sand and cement, with 600mm slabs in the centre of 200mm bands of deeper gravel as a French drain. 

 

As @ProDave says though - 145mm TF build, what is the outer skin going to be and what are you doing with the blockwork..? 

 

Thanks for the advice!

 

The cladding will be timber on TF.

[Ben100]

On 28/07/2019 at 10:28, JSHarris said:

 

 

The line I've highlighted seems odd.  Ideally you want the floor U value to be a lot less than that 0.22 W/m².K figure.  The lower the U value the better, so I'd expect that line to read "New floor to achieve U-value not MORE than 0.22 W/m².K".  A good target would be to aim for a floor U value of around 0.1 to 0.12 W/m².K if you can.  That will just about halve the floor heat loss (assuming that there's no UFH) and mean warmer feet.  If you're looking to install UFH then I'd very definitely look to improve the insulation, as UFH will increase the heat loss to the ground even more.

 

Yes, looks like a typo.

 

I'm planning UFH, so I'll definitely look at adding some more insulation in the floor.

[Ben100]

For the time being I don't want to drop the entire site down 375mm and I'm not very keen on having a 375 trench around the extension either. My plan is to run the two rows of blocks and DPM up >150mm past the external ground level with cavity insulation, and just have the external ground level by the door dropped with some stairs. I'll add some french drains too for ground water, but it is a dry site with plenty of natural drainage already. Also, I'll increase the insulation in the slab.

 

Something like this (not to scale).

 

[Russell griffiths]

19 hours ago, Ben100 said:

For the time being I don't want to drop the entire site down 375mm and I'm not very keen on having a 375 trench around the extension either. My plan is to run the two rows of blocks and DPM up >150mm past the external ground level with cavity insulation, and just have the external ground level by the door dropped with some stairs. I'll add some french drains too for ground water, but it is a dry site with plenty of natural drainage already. Also, I'll increase the insulation in the slab.

 

Something like this (not to scale).

 

That looks completely arse about face to me, so the outer leaf of brickwork is wet leading to the insulation being wet, nope, I would treat that like a mini basement, needs waterproofing on the outside, lapped down onto the footing. 

I personally would rather prevent water coming in than let it in partly and tank on the inside like you are proposing. 

[Jeremy Harris]

+1  ^^

 

Designing it as a mini-basement seems a good way to tackle this.  Shouldn't be that complex a design, as the low perimeter walls of the "basement" section aren't seeing much inward force from the surrounding soil.

[Mr Punter]

Where does the timber frame start?  Looks like a fair amount of brick and block so far, so you may do better sticking with that. Maybe you could do timber frame kit for upper floor(s) and roof if you are keen on it.

 

You may need to build solid below ground as this is retaining a reasonable amount.  The small below ground bit makes this very awkward as you have extra issues with moisture, retaining soil and insulation bridging.

[Onoff]

Your insulation isn't continuous between wall and floor. I think that will give potential cold bridging issues. Cleverer people will confirm.

[Ben100]

On 30/07/2019 at 08:09, Russell griffiths said:

That looks completely arse about face to me, so the outer leaf of brickwork is wet leading to the insulation being wet, nope, I would treat that like a mini basement, needs waterproofing on the outside, lapped down onto the footing. 

I personally would rather prevent water coming in than let it in partly and tank on the inside like you are proposing. 

 

The insulation vendor states they have no issue with the insulation getting wet, but I understand your thoughts on that. I was thinking the same.

 

If I was to run the DPM on the outside of the external brick facing wall can I just run it down to lap over the footings or would I need to run a continuous DMP through the footings and back under the slab? I've seen both examples when doing some research. My preference would be to run a DMP down from the external facing brickwork and lap the footings, then another DMP under the slab and into the internal blockwork.

[Ben100]

On 30/07/2019 at 11:52, Onoff said:

Your insulation isn't continuous between wall and floor. I think that will give potential cold bridging issues. Cleverer people will confirm.

 

Good point. A line of insulation between the slab and internal blackwork should resolve this.

[Russell griffiths]

Your external dpc isn’t just a sheet against the building this just wouldn’t work, is a fully stuck on system, or painted on. 

Then a dimple mat against that to allow any moisture to run down to a drain rather than gather against the wall. 

[Ben100]

1 hour ago, Russell griffiths said:

Your external dpc isn’t just a sheet against the building this just wouldn’t work, is a fully stuck on system, or painted on. 

Then a dimple mat against that to allow any moisture to run down to a drain rather than gather against the wall. 

 

aha, ok, so tanking for example?

[Russell griffiths]

34 minutes ago, Ben100 said:

 

aha, ok, so tanking for example?

Tanking is normally associated with internal, the same just a slightly different context. 

[Onoff]

2 hours ago, Ben100 said:

 

Good point. A line of insulation between the slab and internal blackwork should resolve this.

 

Don't think so. The "yellow" needs to be continuous with no break.