Foundations for timber frame build

[SeanDean]

Hello all,

 

I'm after some help understanding if the following drawing is correct for a timber frame building to sit on. 

The building is a two-story extension and is to be clad with timber feathered edge, not a brick cavity wall.

 

Any help would be greatly appreciated, Cheers. 

[Russell griffiths]

Not really that’s probably as shite as it could get, do a bit of a search on here, the same thing was covered a few weeks ago. 

 

Have a read up on stuff previously covered and come back and we will sort it out, but it will help once you have a bit more knowledge of terminology and stuff.  

[SeanDean]

Hello Russell,

 

Thank you for getting back to me. I was searching for a while and could not find what I was after then I realised I can search specific parts of the forum so found some threads that were super helpful. 

 

Based on what I have seen and read I have updated my shite plan with this one, am I going in the right direction?

 

[Russell griffiths]

thats Getting better, gone from 10 out of 10 shite to 4 out of 10 ??

 

the way to picture it is imagine you have a big pointy stick, this stick is the cold, look at any areas that that stick can penetrate into your structure, then you have another pointy stick, this ones heat loss, look at where this can escape from the building. 

 

You can do loads more, but you need to be realistic, I think you said it was an extension. 

 

Tell us a bit more on the design. 

For example if you have a freezing cold stone cottage on 5 walls and this extension is just forming 1 wall it is pointless building this to passive spec if the rest of the house is air leaky and cold. 

 

As as opposed to an extension that will wrap around 75% of the house so is worth upgrading specs as it will improve the overall house. 

[Gus Potter]

19 minutes ago, SeanDean said:

Hello Russell,

 

Thank you for getting back to me. I was searching for a while and could not find what I was after then I realised I can search specific parts of the forum so found some threads that were super helpful. 

 

Based on what I have seen and read I have updated my shite plan with this one, am I going in the right direction?

 

 

Well done you for developing this up.. design is a journey! Often you draw something (professionals often) and realise it can be improved once you have slept on it.

 

One thing to take account of is that you need to tie the timber frame down. When you don't have a heavy outer skin of masonry to provide ballast this gets a little more tricky. Some say.. there is no way that the wind suction on the roof will lift the building off the founds.. and yes you don't see too many failures like this... but if you live in a windy spot with certain pitches of roof then you can get uplift when you don't have a masonry outer skin.

 

Often if you have an extension with big doors on the front and small panels each side (no goal post) which may want to tip over, like a column, they need to be anchored to something heavy.

 

Sean.. If you can post some of your elevation drawings then you may get some more pointers, I'll give you some if I can. Have a look for tie down (holding down) strap details etc on BH.

 

You can embed tie down straps into the founds but they can get in the way, folk can trip over them. It''s also hard to get them to line up with the stud positions, but here you can introduce dummy studs unless you are using SIPS... this means that you don't need to be so accurate when pouring the concrete. You can also fix the holding down straps to the top of the founds and there is a way of doing this in your case which will work as you have plenty edge distance for the fixings (strap to concrete found) and so on.

 

Draw in your tie down straps on the inside of the frame (5.0mm thick) with a bent leg facing in fixed to the founds if you can, to see how that might work. Put a big washer ( 50 x 50x 6.0mm thick) that extends to the inside of the bend of the strap. Depending on what tensile loads you have in the strap you may be ok.. if not you may need a bespoke strap... not a massive cost issue. Much will depend on what tension you have in the strap, hence my point about the elevation drawings.

 

You can also try and use the slab to provide the ballast.. but as you have only light anticrack (A142) reinforcement in the slab it becomes an issue in terms of technical design and justification.

 

Happy researching. Well done again for sticking with it.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[SeanDean]

22 hours ago, Russell griffiths said:

thats Getting better, gone from 10 out of 10 shite to 4 out of 10 ??

 

the way to picture it is imagine you have a big pointy stick, this stick is the cold, look at any areas that that stick can penetrate into your structure, then you have another pointy stick, this ones heat loss, look at where this can escape from the building. 

 

You can do loads more, but you need to be realistic, I think you said it was an extension. 

 

Tell us a bit more on the design. 

For example if you have a freezing cold stone cottage on 5 walls and this extension is just forming 1 wall it is pointless building this to passive spec if the rest of the house is air leaky and cold. 

 

As as opposed to an extension that will wrap around 75% of the house so is worth upgrading specs as it will improve the overall house. 

 

Hi Russel,

 

I'm happy to hear it's going in the right direction, cheers.

 

The build is a two story extension that is 11m long x 4.6m wide, the front half of the ground floor is a garage area and the rear half is a workshop. The extension is 4 wall with half of one wall making use of the existing house (highlighted in yellow on the plan. To the front of the build is a tiled pitched roof and towards the rear is a flat green roof.

 

There is a step down halfway making the garage half 300mm below the original DPC. It has 3m bi-folds to the front of the garage and 4m bi-folds at the side of the workshop, both have SHS columns bolted into the foundations with UB's so basically two goal post. 

 

Here is a basic plan of the build and render of the rear.

 

 

 

 

[SeanDean]

21 hours ago, Gus Potter said:

 

Well done you for developing this up.. design is a journey! Often you draw something (professionals often) and realise it can be improved once you have slept on it.

 

One thing to take account of is that you need to tie the timber frame down. When you don't have a heavy outer skin of masonry to provide ballast this gets a little more tricky. Some say.. there is no way that the wind suction on the roof will lift the building off the founds.. and yes you don't see too many failures like this... but if you live in a windy spot with certain pitches of roof then you can get uplift when you don't have a masonry outer skin.

 

Often if you have an extension with big doors on the front and small panels each side (no goal post) which may want to tip over, like a column, they need to be anchored to something heavy.

 

Sean.. If you can post some of your elevation drawings then you may get some more pointers, I'll give you some if I can. Have a look for tie down (holding down) strap details etc on BH.

 

You can embed tie down straps into the founds but they can get in the way, folk can trip over them. It''s also hard to get them to line up with the stud positions, but here you can introduce dummy studs unless you are using SIPS... this means that you don't need to be so accurate when pouring the concrete. You can also fix the holding down straps to the top of the founds and there is a way of doing this in your case which will work as you have plenty edge distance for the fixings (strap to concrete found) and so on.

 

Draw in your tie down straps on the inside of the frame (5.0mm thick) with a bent leg facing in fixed to the founds if you can, to see how that might work. Put a big washer ( 50 x 50x 6.0mm thick) that extends to the inside of the bend of the strap. Depending on what tensile loads you have in the strap you may be ok.. if not you may need a bespoke strap... not a massive cost issue. Much will depend on what tension you have in the strap, hence my point about the elevation drawings.

 

You can also try and use the slab to provide the ballast.. but as you have only light anticrack (A142) reinforcement in the slab it becomes an issue in terms of technical design and justification.

 

Happy researching. Well done again for sticking with it.

 

 

Hello Gus,

 

Cheers for your input its greatly appreciated.

 

If I'm honest I just assumed the soleplate could be bolted down to the brick/blockwork but now saying that I can see how that's not a great idea as it will puncture the DPC.

 

I will start researching ways to strap it down like you have pointed out and I have now provided some drawings and a description of the build in the post above.

 

 

[SeanDean]

23 hours ago, Russell griffiths said:

thats Getting better, gone from 10 out of 10 shite to 4 out of 10 ??

 

the way to picture it is imagine you have a big pointy stick, this stick is the cold, look at any areas that that stick can penetrate into your structure, then you have another pointy stick, this ones heat loss, look at where this can escape from the building. 

 

You can do loads more, but you need to be realistic, I think you said it was an extension. 

 

Tell us a bit more on the design. 

For example if you have a freezing cold stone cottage on 5 walls and this extension is just forming 1 wall it is pointless building this to passive spec if the rest of the house is air leaky and cold. 

 

As as opposed to an extension that will wrap around 75% of the house so is worth upgrading specs as it will improve the overall house. 

 

Am I correct in thinking that where the soleplate is I should extend my up-stand PIR insulation up?

Should i be using Marmox THERMOBLOCK under the soleplate?

Edited February 13, 2021 by SeanDean

[SimonD]

I've just been looking at this with great interest, especially the green roof and what looks like a cantilevered balcony/entrance door on the 1st floor. It looks pretty cool and I'd love a workshop that looks like this with a vehicle lift. The cantilever together with green roof isn't entirely a trivial thing to design from a timber frame structural perspective and will feed into your foundation and ground floor wall design (e.g. where you may need double or triple studs and maximum cantilever distance and load for the selected joist dimensions). I guess you've had SE input for this?

 

Otherwise, I just have some minor points of note, that may or may not feed into your end design.

 

Stud wise, you're either looking at carcassing timber which is 47 x 150mm, with a nominal finished size of 45 x 145 (which some framers do use) or the more appropriate 50 x 150mm CLS which has a finised size of 38 x 140, which is the 'proper' framing timber.

 

Also have a look at your need for the Intello Plus vapour check as I think this is actually designed for fibrous insulation materials rather than PIR. It's expensive and another vapour check and airtightness layer may serve you better.

 

The current sole plate arrangement does look like a cold bridge area as currently designed. I personally prefer the timber frame designs that have a layer of insulation outside the frame as this reduces cold bridging across the whole frame and performs better at junctions and can easily be taken down to cover the sole plate. For example you could use 60mm pir between studs and 60mm outside, or some other arrangement.

 

Hope some of that is informative but I'm now being hassled by kids and have the check out! ?

 

 

 

 

[SeanDean]

On 13/02/2021 at 20:31, SimonD said:

I've just been looking at this with great interest, especially the green roof and what looks like a cantilevered balcony/entrance door on the 1st floor. It looks pretty cool and I'd love a workshop that looks like this with a vehicle lift. The cantilever together with green roof isn't entirely a trivial thing to design from a timber frame structural perspective and will feed into your foundation and ground floor wall design (e.g. where you may need double or triple studs and maximum cantilever distance and load for the selected joist dimensions). I guess you've had SE input for this?

 

Otherwise, I just have some minor points of note, that may or may not feed into your end design.

 

Stud wise, you're either looking at carcassing timber which is 47 x 150mm, with a nominal finished size of 45 x 145 (which some framers do use) or the more appropriate 50 x 150mm CLS which has a finised size of 38 x 140, which is the 'proper' framing timber.

 

Also have a look at your need for the Intello Plus vapour check as I think this is actually designed for fibrous insulation materials rather than PIR. It's expensive and another vapour check and airtightness layer may serve you better.

 

The current sole plate arrangement does look like a cold bridge area as currently designed. I personally prefer the timber frame designs that have a layer of insulation outside the frame as this reduces cold bridging across the whole frame and performs better at junctions and can easily be taken down to cover the sole plate. For example you could use 60mm pir between studs and 60mm outside, or some other arrangement.

 

Hope some of that is informative but I'm now being hassled by kids and have the check out! ?

 

 

 

 

Cheers for your input.

 

Yes SE has gone over everything and done all the calcs for the steel, the balcony is actually on SHS columns but the render is old and does not feature them. 

I did a lot of research on the VCL but maybe I missed that it's not designed for PIR so thank you for pointing it out, I will contact the manufacturer. I will also search the BH for another solution.

 

I have an updated drawing of my improved foundations based on the help and direction you guys have all been giving me, so thank you all. 

 

[SeanDean]

So here is the drawing update, what do you all think?

 

I have not incorporated any straps yet as I'm still looking into solutions for this and keep coming across chemfix and self-sealing DPC

[Cpd]

Only speeded through this thread so may already be mentioned - but is it ok to have the battens supporting the feather edge board attached straight onto the OSB / membrane or should there be a counter batten in there to allow more airflow ? If water was to get in there it would possibly sit on top of the batten with nowhere to go. 

[Russell griffiths]

8 hours ago, Cpd said:

Only speeded through this thread so may already be mentioned - but is it ok to have the battens supporting the feather edge board attached straight onto the OSB / membrane or should there be a counter batten in there to allow more airflow ? If water was to get in there it would possibly sit on top of the batten with nowhere to go. 

Vertical battens, no counter batten required

horizontal batten, counter batten required. 

 

The battens ideally should go through the osb and into the studs for extra support.  

[ProDave]

My only input is the TF walls don't look very thick = not enough insulation?

[SeanDean]

On 17/02/2021 at 09:11, ProDave said:

My only input is the TF walls don't look very thick = not enough insulation?

 

My original wall PIR was just 120mm and was giving me a U-value of 0.24W/m²K when calculated, the updated design now has 140mm PIR along with an additional 25mm PIR internally between the studs and battens so I'm guessing the U-value should be improved. 

 

It's my understanding that the min wall U-value on a new build cant be any higher than 0.3 W/m²K so we are good regarding building regs.

I think my bigger issue is the thermal bridging my SHS columns are going to create because they sit directly on the trench foundation and are in the single leaf wall construction. I currently have no clue how to combat these issues and that's the next thing I need to research.

Edited February 18, 2021 by SeanDean

[SimonD]

4 hours ago, SeanDean said:

 

My original wall PIR was just 120mm and was giving me a U-value of 0.24W/m²K when calculated, the updated design now has 140mm PIR along with an additional 25mm PIR internally between the studs and battens so I'm guessing the U-value should be improved. 

 

It's my understanding that the min wall U-value on a new build cant be any higher than 0.3 W/m²K so we are good regarding building regs.

I think my bigger issue is the thermal bridging my SHS columns are going to create because they sit directly on the trench foundation and are in the single leaf wall construction. I currently have no clue how to combat these issues and that's the next thing I need to research.

Install an extra layer of insulation outside the timber frame. Frankly ,this is a better way to build timber frame walls from a thermal performance perspective. If you have an extra layer of insulation across the studs internally, you'll get more cold bridging than if the same layer is external. TRADA has some free materials available demonstrating this and using an external layer to the timber frame also helps your SAP calculations as it will reduce Y value for cold bridging of the frame.

 

Why we still build timber frames with internal insulation to the frame I have no idea.

[SeanDean]

On 16/02/2021 at 21:20, SeanDean said:

So here is the drawing update, what do you all think?

 

I have not incorporated any straps yet as I'm still looking into solutions for this and keep coming across chemfix and self-sealing DPC

@Russell griffiths

 

Would be great to get your feedback on this updated design please.

[SeanDean]

Hello All,

 

Does anyone have any ideas/solution for the Marmox Thermoblock course at the point my foundations step down 300mm?

I have had a search but I can't see this mentioned anywhere, I have contacted Marmox but have not heard back from them as of yet. 

 

Any help would be greatly appreciated. 

[SeanDean]

So I received a reply from Marmox and they have suggested the use of  50mm Marmox Multiboard up-stand like this. 

 

 

 

[Russell griffiths]

What’s the 4 studs for, do they carry a point load, if not I see no reason for them. 

[SeanDean]

I thought it best because the Marmox Multiboard is not a load-bearing product.

 

Do you see it having just two studs like this?

 

[Russell griffiths]

Why not, if the load above is the same as all the other studs why double them up. 

Obviously check with your structural engineer designer.