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Timber Frame - Dwarf Wall, or not?

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dunny1234

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Hi, I'm about to construct a timber frame building 10m X 6m. It's going to go on a concrete slab of the same size. Obviously, once the cladding goes on, the building will be a tad larger than the slab. A few people have recomended laying a single course of engineered bricks on the slab, and then place the sole plate on top of this?

Is this neccessary? If so, what do i do with the DPM from the slab?

Would you then need to leave ample DPM from the concrete slab to lap over the layer of bricks, and then lay a DPC on top, and then place the sole plate down? (please see Diagram A - DPM over dwarf wall)

Or, does it go under the single course of bricks, and then a strip of damp proof course on top the bricks, and then the timber frame on top of that?

Really quite stuck, and amazingly there's little info on this specefic subject. Any advice would be great. Thanks
 

twothumbs

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Your sketch detail has not come up.

As a general approach I would suggest no brick if you can, as you will find it a nuisance and easily bumped loose, and it is another line to level up to. I good fixing into the slab is best for you when you can pack up off if necessay, with your dpc under the sole plate. Dpc's are difficult to pull into tidy lines and are not like they are shown in catalogues and drawings. Lets see the sketch and we can see what is happening with the exposed face of the slab and where the ground level finishes, which is what is the important factor, how you are putting a small shutter in for the slab edge and so on.

Come back with the sketch and we will look at it.
 

GPB

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I would say you just need to ensure the sole plate is at least 4 inches (6 would be better) above ground level. So if the concrete slab is out of the ground, put dpc between that and the sole plate with the cladding hanging down past the dpc. If the slab is close to ground level then you will need to run a course, or 2, of bricks around as you say.

Cheers
Graham
 

bosshogg

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You haven't said whether you intend to finish the exterior in brick/block, timber lining or what, which would be really helpful.

I presume the advice to put down a course of perimeter bricks, dpc and sole plate on top of that would be to raise the building above the storm line effectively. If your intention is for a timber lining, this is good advise, as hard rain and dare I say it snow (in Cornwall?) would quite easily soak up through the exposed bottom edge, no matter what you did!
You could however cast a raised plinth around the exterior of the concrete slab? this would require a shutter of some sort.
On the other hand if the external is to be clad in brick/block (& render) then building on a double/triple course of finishing bricks incorporating a dpc, is a good point also.

Are you intending using the concrete slab as the floor? I really think you need to provide some more information at this point, so we can give you the benefit of our experience...bosshogg :)
"Be a first rate version of yourself, not a second rate version of someone else." Judy Garland :) :)
 

mark

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I don't mean to be a pessimist but a 10x6M building would surely come under the auspices of Building Control - ask their advice before it's too late!

Good Luck

Mark
 

dunny1234

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andrews0001.jpg
Thanks very much for the info guys, much appreciated!

The slab should protrude about 3-4 inches up from ground level, so i dony think i'm going to lay any dwarf wall. The slab will only be large enough for the timber frame and sheathing, and once i clad the building in horizontal shiplap, any rain, or splash back will fall onto the pea shingle below.

I think i'm going to place the timber frame on the slab with DPC inbetween and then run the DPM up the outside of the timber frame for 300mm or so. Then have my external breather membrane come down over the top of the DPM.

I'll then attached the soleplate to the slab using Tapcon type anchors. These dont appear to have a large diameter, so shouldn't damage the DPC???? Does this sound okay, or can anyone foresee any problems??

Or, is it worth using a precast 'J-Bolt' style fixing to fix sole plate to slab, and avoid puncturing the DPC??

Thanks again

 

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twothumbs

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I think the fixing to the slab is best for you and if you crank-up against the sole plate and tighten up to flat the dpc it may be ok. This is done all the time and what you are planning is ‘normal’. This will reduce down the number of variables for you. A circle of sealant on the sole plate, if it is loose, before you set it down but it depends on your sequence of working. Less fiddling about you have the better you are. It can be that the dpc is only of value to resist the initial drawing of moisture out of the slab. Not so vital now with Tanilised timber but still worthwhile. A starter hole for the bolts would be good also a strong driver but don’t force it. Do it before your slab matures too hard.

Pea gravel around the edge gives less ‘bounce’ to rain than you would get with say paving. Your dpc pinned up the studs and MB carried over it is good. Carry the MB down to hang down to the edge of the first board which if fixed with an inch say of cover over the slab edge and studs.

It depends what the building will be used for, but the bottom board is always going to be a possible sacrificial one somewher in the long future. Give it a coat of your treatment on the back face and any ends before fixing. A dawrf wall on the face gets complicated with a need for foundation or extended edge to slab and so. Is it under the studs or set outside and so exposed on the top and then how do you protect it and so it goes on. Best not go there.

Good luck
 

dunny1234

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Thanks Twothumbs. So i bolt the sole plate onto the slab, and that should be fine with the DPC?

I was going to make the timber frame with untreated timber, because i'll be plasterboarding the inside and didnt wants cracks due to shrinkage. Is it worth me using treated timber for just the sole plate?

Thanks again, really helpful!!!!
 

heimlaga

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The sill should be at least 12 inches above ground. Preferably 16 inches.
That is an old rule in the Nordic countries. People tend to forget it theese days and that usually ends with trouble.
There should be a strip of bitumen based roofing felt under the sill. On top of that comes a thin strip of insulation to stop draught and then the sill. The sill is bolted down.

Leave the concrete bare to the outside so any moisture disappears outwards. If you use painted wooden siding there should be a 7/8" air space between the breathing membrane and the siding. The air must be able to cirkulate in the vertical direction in that space.

Up here we prefere using a porous wood fibre board wit a slightly water repelling surface. That is way better than those breathing membranes. It breathes more and does not flap in the wind and stiffens the building in the erection stage.

I am a construction engineer but I do not know your climatical conditions over there.
 

bosshogg

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twothumbs":298xzi0e said:
I think the fixing to the slab is best for you and if you crank-up against the sole plate and tighten up to flat the dpc it may be ok. This is done all the time and what you are planning is ‘normal’. This will reduce down the number of variables for you. A circle of sealant on the sole plate, if it is loose, before you set it down but it depends on your sequence of working. Less fiddling about you have the better you are. It can be that the dpc is only of value to resist the initial drawing of moisture out of the slab. Not so vital now with Tanilised timber but still worthwhile. A starter hole for the bolts would be good also a strong driver but don’t force it. Do it before your slab matures too hard.

Pea gravel around the edge gives less ‘bounce’ to rain than you would get with say paving. Your dpc pinned up the studs and MB carried over it is good. Carry the MB down to hang down to the edge of the first board which if fixed with an inch say of cover over the slab edge and studs.

It depends what the building will be used for, but the bottom board is always going to be a possible sacrificial one somewher in the long future. Give it a coat of your treatment on the back face and any ends before fixing. A dawrf wall on the face gets complicated with a need for foundation or extended edge to slab and so. Is it under the studs or set outside and so exposed on the top and then how do you protect it and so it goes on. Best not go there. Good luck
Actually what you're contemplating is far from normal practice, in fact it borders on bad practice.
In all my years as a Projects Manager in the construction industry, I've never come across anything like this.
First, putting the dpm up the outside of a raised conc slab, is wrong, exposing a dpm to the atmosphere, even 3-4", will allow uv breakdown, moisture ingress or anything else, even though it's tucked up behind a breather membrane!
Have a look at this-
slab.jpg
You are putting up a structure the size of a house adhering to good practises will pay you dividends, go down the opposite route...
I can help devise something that will work, if you want, otherwise as noted above @ 10x6m your structure will probably require a building control certificate, at minimum, and I'm fairly certain the sketch showing your proposal would not pass muster, sorry...bosshogg :)
"Be a first rate version of yourself, not a second rate version of someone else." Judy Garland (hammer)
 

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twothumbs

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Dear Bosshogg

You are quite correct. For whatever reason I did not see the detail put up otherwise my answer would have been framed rather differently. It is very wrong when you make suggestions without any real understanding of what the full situation is, as I have done, and not being aware of the basis or detail of the original question. Of course the dpm should not turn up the slab face. I was working on the basis of it turning up from under the stud and so had a possible way of doing things, to part answer what I saw the direction it was going in. Of course that was wrong.

What is important is learning the lessons on giving ideas to a question when there is so little information to go on. There is a so much more to know than is given to us on this one. The advice being sought would attract a substantial amount of fees if professional guidance was necessary and it is beginning looking that way.

With a starting point of a 10x6 m building, I list below a series of possible information necessary to questions to illustrate to others what may have to be taken into account before making considered and knowledgeable possibilities. I make no excuse for the length of the list by way of showing how complex matters can be; before an apparent simple question is resolved, hence why professional consulting is often expensive. It is also important to appreciate the audience may not always be in position to make informed interpretations and judgement of what is being offered....with all due respect to everyone.

1 What is the proposed use of the building. Space for army tanks or just bicycles, etc
2 Is the building habitable. Hay barn or dining hall.
3 What are the ground conditions re. A choice of strip or raft foundations, floor edge, etc.
4 Is there drainage in the building- how does it become external under the founds/edge.
5 Is there a screed or loose floor finish
6 Is insulation required in the floor, also cold bridging problems.
7 What form of services are there – re. pipes again.
8 Form of roof spanning onto external wall studs and so stud sizes and holding down for strength.
9 Are there internal walls to break the roof spans and loads.
10 Is a dpm required under the slab – not ness. the same as a poly. membrane.
11 The expected loading on the slab- and so extent of calculated steel/fabric reinf.
12 Is the Bldg. Regs. (in Scot.) of 150mm distance between ground and dpc the min. required.
13 What form of rainwater downpipes to be provided- trenches adjacent wall base/founds.
I4 Is a structural fire rating necessary.
15 Is there an internal finish. P/b has been mentioned.
16 Is there a finish to the slab edge – a render, board, and so.
17 Is the slab to be waterproof.
18 Should the external boarding be spaced on runners for ventilation.
19 Wall fixings to slab to withstand the combined roof loads, wind loads, uplift, and dead loads.
20 Is there a need to sheath the studs after the above is resolved- re. for struct. racking.
21 Does it have an agricultural use.
22 Is there exemption from planning and building regulations.
23 Height of studs.
24 walls made up in panels or sticks re. form sole plate and holding down.
25 floor loading re. thickness and reinf.
26 and so on, and so on.

I do hope that the loose linkage across these questions is seen and understood and how one question poses yet another, and another, until a round view is achieved of elements of importance.

For me, “once bitten, twice shy” now applies. Sense will prevail from now on; I have been overzealous to assist. Hope this clarifies matters and perhaps helps others appreciate the initial complexities. One last point is; you ask for comments and you get several opposing responses. They may not be wrong, just different as everyone sees a different potential. It would be easy to pass criticism both your detail Bosshog and the good information provided by Mike G. but that doesn’t necessarily make them wrong. That is the nature of the question and answer sessions.

Thank goodness I didn’t suggest meeting up with you at the local tool shop for a bit of rioting, burning and looting. We could be sharing the same prison cell by now.

Over and out and returning to base.
 

bosshogg

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twothumbs":172l7hm7 said:
Dear Bosshogg


You are quite correct. For whatever reason I did not see the detail put up otherwise my answer would have been framed rather differently. It is very wrong when you make suggestions without any real understanding of what the full situation is, as I have done, and not being aware of the basis or detail of the original question. Of course the dpm should not turn up the slab face. I was working on the basis of it turning up from under the stud and so had a possible way of doing things, to part answer what I saw the direction it was going in. Of course that was wrong.

What is important is learning the lessons on giving ideas to a question when there is so little information to go on. There is a so much more to know than is given to us on this one. The advice being sought would attract a substantial amount of fees if professional guidance was necessary and it is beginning looking that way.

With a starting point of a 10x6 m building, I list below a series of possible information necessary to questions to illustrate to others what may have to be taken into account before making considered and knowledgeable possibilities. I make no excuse for the length of the list by way of showing how complex matters can be; before an apparent simple question is resolved, hence why professional consulting is often expensive. It is also important to appreciate the audience may not always be in position to make informed interpretations and judgement of what is being offered....with all due respect to everyone.

1 What is the proposed use of the building. Space for army tanks or just bicycles, etc
2 Is the building habitable. Hay barn or dining hall.
3 What are the ground conditions re. A choice of strip or raft foundations, floor edge, etc.
4 Is there drainage in the building- how does it become external under the founds/edge.
5 Is there a screed or loose floor finish
6 Is insulation required in the floor, also cold bridging problems.
7 What form of services are there – re. pipes again.
8 Form of roof spanning onto external wall studs and so stud sizes and holding down for strength.
9 Are there internal walls to break the roof spans and loads.
10 Is a dpm required under the slab – not ness. the same as a poly. membrane.
11 The expected loading on the slab- and so extent of calculated steel/fabric reinf.
12 Is the Bldg. Regs. (in Scot.) of 150mm distance between ground and dpc the min. required.
13 What form of rainwater downpipes to be provided- trenches adjacent wall base/founds.
I4 Is a structural fire rating necessary.
15 Is there an internal finish. P/b has been mentioned.
16 Is there a finish to the slab edge – a render, board, and so.
17 Is the slab to be waterproof.
18 Should the external boarding be spaced on runners for ventilation.
19 Wall fixings to slab to withstand the combined roof loads, wind loads, uplift, and dead loads.
20 Is there a need to sheath the studs after the above is resolved- re. for struct. racking.
21 Does it have an agricultural use.
22 Is there exemption from planning and building regulations.
23 Height of studs.
24 walls made up in panels or sticks re. form sole plate and holding down.
25 floor loading re. thickness and reinf.
26 and so on, and so on.

I do hope that the loose linkage across these questions is seen and understood and how one question poses yet another, and another, until a round view is achieved of elements of importance.

For me, “once bitten, twice shy” now applies. Sense will prevail from now on; I have been overzealous to assist. Hope this clarifies matters and perhaps helps others appreciate the initial complexities. One last point is; you ask for comments and you get several opposing responses. They may not be wrong, just different as everyone sees a different potential. It would be easy to pass criticism both your detail Bosshog and the good information provided by Mike G. but that doesn’t necessarily make them wrong. That is the nature of the question and answer sessions.

Thank goodness I didn’t suggest meeting up with you at the local tool shop for a bit of rioting, burning and looting. We could be sharing the same prison cell by now.

Over and out and returning to base.
twothumbs --- and al other interested parties

Sharing information is, hopefully, what this forum is about. I have to say, sometimes, when posting a reply, I do so tongue in cheek, so to speak, but when I see things are going a little too far in the wrong direction, by my assumption, I say what I say regardless. I'm afraid this was just one of these cases, but that's good as clarity tends to ensue and good advice often follows, as is the case with your reply post. I feel many would benefit from listening to responsive replies, as sadly is sometimes not the case.
The important thing here is that the original question receives a few good responses, enough that the originator can choose a wise course of action, and I fell in this instance that is being achieved...only dunny can answer this one.
twothums, you have my respect...bosshogg :)
I never teach my pupils. I only attempt to provide the conditions in which they can learn.
Albert Einstein (hammer)
 

dunny1234

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Thanks for all the info from everyone, but i'm even more unsure than when i started.

1 What is the proposed use of the building. - The building will be for a toilet/shower block on my campsite.
2 Is the building habitable. No, probably summer use only, but not habitable.
3 What are the ground conditions - Good solid ground conditions, 150mm of compacted gravel, layer of sand, DPM & 150mm Concrete slab
4 Is there drainage in the building- 50mm water drainage from showers/basin in slab, and waste/soil drainage 110mm vertical pipes accessing toilet areas.
5 Is there a screed or loose floor finish - No, tiling straight onto concrete slab once ready
6 Is insulation required in the floor, also cold bridging problems. - No insulation in floor
7 What form of services are there – 50mm water drainage from showers/basin in slab, and waste/soil drainage 110mm vertical pipes accessing toilet areas leading to outside septic tank area
8 Form of roof spanning onto external wall studs and so stud sizes and holding down for strength. Timber frame panels - 100mm x 50m at 600mm centres, and roof truses at 600mm centres
9 Are there internal walls to break the roof spans and loads. - Central partition lengthways down the centre and another partition across the width (please see diagram attached)
10 Is a dpm required under the slab – 1200 grade DPM under slab
11 The expected loading on the slab- Other than building, light traffic, people going to the toilet, and having showers
12 Is the Bldg. Regs. (in Scot.) of 150mm distance between ground and dpc the min. required. - Need to ask BCO, initial conversation suggested no, as it's not a habital dwelling.
13 What form of rainwater downpipes to be provided- trenches adjacent wall base/founds. - hunters rain water good, and french drain all around.
I4 Is a structural fire rating necessary. - My structual calcs dont mention it
15 Is there an internal finish. P/b has been mentioned. - PLasterboard interior
16 Is there a finish to the slab edge – a render, board, and so. - ?
17 Is the slab to be waterproof.- ?
18 Should the external boarding be spaced on runners for ventilation. - all cladding will be on treated 25mm x 50mm battens
19 Wall fixings to slab to withstand the combined roof loads, wind loads, uplift, and dead loads. - i intended to bolt it down as you first suggested?
20 Is there a need to sheath the studs after the above is resolved- re. for struct. racking. - I'll be steathing the studs with OSB 3 9mm, then a breather membrane
21 Does it have an agricultural use. - NO
22 Is there exemption from planning and building regulations. NO
23 Height of studs. 2.4m
24 walls made up in panels or sticks re. form sole plate and holding down. - Timber panels
25 floor loading re. thickness and reinf.
26 and so on, and so on.

Twothumbs, your original suggestion stated that if i was to use a single or double course of bricks on the slab, it could be a nuisense and bump loose. With this in mind, i want to avoid this?

The frames will be made of untreated timer, but should i use treated wood for the sole plate?

Or, could i bolt a layer of treated timber on the concrete base, and than attach my untreated timber frame panels to this?

And with regard to the DPC and DPM position? I take it putting the DPM up the outside of the building, and breather membrane over it, may not be the best solution? What if i placed the DPM on top of the concrete slab, but under the timber frame and attached it to the inside of the building would be a better solution?? (please see Timber frame DPM inside building diagram)

Any help on this subject would be really grateful. I must admit, i'm slightly more confussed than when i started.

Thanks
 

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