Building off a concrete slab

[theflyingpig]

I've a double garage sized concrete slab to build off. As I far prefer working with wood my plan is..
Build
1. Drill into slab at every 600mm and chem-fix long bolt around edge of slab.

2. Lay 100mm concrete blocks on their flat around edge of slap, working as round the protruding bolts..

3. Lay dpm over blocks, then fix down a 6*2 as a sole plate using bolts..

4. Build 6*2 framing off sole plate

Any comments on this plan?

Thanks!

 

[^woody^]

What is going to pull up the walls vertically that will require chemical anchoring to resist?

How thick is the slab?

How are you dealing with water splash at the bottom of the frame?

 

[theflyingpig]

What is going to pull up the walls vertically that will require chemical anchoring to resist?

How thick is the slab?

How are you dealing with water splash at the bottom of the frame?

I was only thinking of chemical anchoring as a, way or securing the threaded bar.... Was thinking to fix them every 600mm

Slab is 150 /200mm thick, reinforced with fibres.

Externally I was thinking to wrap framing in usb, dropped down cover my course of concrete blocks, wrap in membrane, then drop cladding lower to cover. Or maybe use something like a plastic eaves starter to cover bottom of usb....

Havnt started this yet! So all ideas welcome....

 

[23vc]

Trouble is, if slab extends beyond walls, it’s going to be tricky to waterproof the bottom part, anything made of osb or batten or timber cladding dropped down to <100mm is going to be at risk of water splash.

I did similar, but with a course of blue brick, no need for any anchors to the ground, and the bricks were on the edge of the slab. Unless you’re building a 215mm ish thick timber frame, it sounds like you’ll have a cold bridge issue if using blocks laid flat.

 

[Ivor Windybottom]

I'd say build in wood or in masonry. What you're suggesting is an awkward hybrid, where there's just enough masonry to mean you can't bolt your timber into the foundation, and not enough for it to hold itself down by gravity.

I don't know how timber framed buildings normally attach to their foundation. But if I was going to build one I'd find out how, and do it that way rather than using blocks, bolts and sticky tape!

I don't know how timber frames work as I've never built one. I don't see what advantage they have, as masonry is only a bit more expensive but doesn't need treatment, doesn't rot and (usually) doesn't blow away.

 

[theflyingpig]

Trouble is, if slab extends beyond walls, it’s going to be tricky to waterproof the bottom part, anything made of osb or batten or timber cladding dropped down to <100mm is going to be at risk of water splash.

I did similar, but with a course of blue brick, no need for any anchors to the ground, and the bricks were on the edge of the slab. Unless you’re building a 215mm ish thick timber frame, it sounds like you’ll have a cold bridge issue if using blocks laid flat.

Good point....

So you built with timber off the top off the bricks? I could set my bricks/blocks to the edge of the slab, and we rounded the edge of slab off to help splash....

 

[theflyingpig]

I'd say build in wood or in masonry. What you're suggesting is an awkward hybrid, where there's just enough masonry to mean you can't bolt your timber into the foundation, and not enough for it to hold itself down by gravity.

I don't know how timber framed buildings normally attach to their foundation. But if I was going to build one I'd find out how, and do it that way rather than using blocks, bolts and sticky tape!

I don't know how timber frames work as I've never built one. I don't see what advantage they have, as masonry is only a bit more expensive but doesn't need treatment, doesn't rot and (usually) doesn't blow away.

Material cost might not be much different, but for me it's the labour cost - timber I can do, masonary I'm rubbish at....

 

[Ivor Windybottom]

I'd worry about using any steel below DPC, as it will inevitably turn into a brown puffed-up lump at some point in the future.

 

[^woody^]

Slab is 150 /200mm thick, reinforced with fibres.

That may be enough to precent edge shear cracking, but depends on the actual loading and ground below. Fibres are not reinforcement and won't help one bit.

There are lots of threads about the building on slabs and the damp detailing required. It's very important to get right, as its almost impossible to cure afterwards if you don't.

The recommended way is to have at least 150 of engineering quality bricks above any external ground level to prevent splashing to the cladding/frame, suitable protection and drip detail at the cavity to project the frame at floor level, and you need to prevent ground water entering the slab.

 

[23vc]

Single course of blue engineers, (treated) frame on top, frame protected by breather membrane, and non-timber cladding has worked for me.
Dpm on top of internal slab and lapped up walls. 75mm celotex on top, floating floor on top.

2 courses of brick would take you to the 150mm above ground mark, but you’d have to double up on insulation or more concrete to bring finished floor height up.

These things tend to all be built fairly similarly with decent size roof overhangs which helps a lot with the ground splashing thing

The main reasons people do it this way (I’m talking garden room/offices) rather than masonry, aside from it’s probably more attractive to DIYers, is you can get a 100mm+battens+cladding thick wall, with 100mm celotex in it, which you’d struggle to achieve with masonry, and get away with building off a half decent slab rather than needing trench/raft

 

[Ivor Windybottom]

Does the building bolt to the slab or are you relying on gravity?

In fact are timber buildings normally bolted down?

Roofs definitely are, they have whacking great straps stretching a few courses down the bricks. But perhaps the weight of a wall is considered enough to defeat the wind? I don't think our 1930s or 1950s houses had anything other than gravity holding the roof on, neither has taken off yet at least.

But things would be different with a flat roof, which weighs much less and can get sucked upwards as the wind blows across it.

 

[23vc]

Fixed mine with a few concrete screws I think.
Hasn’t blown away yet, but I suppose you’re right, it’d blow away before the house did.
Still a fair old weight with all that osb, timber, bifold doors, plasterboard…

 

[theflyingpig]

Single course of blue engineers, (treated) frame on top, frame protected by breather membrane, and non-timber cladding has worked for me.
Dpm on top of internal slab and lapped up walls. 75mm celotex on top, floating floor on top.

2 courses of brick would take you to the 150mm above ground mark, but you’d have to double up on insulation or more concrete to bring finished floor height up.

These things tend to all be built fairly similarly with decent size roof overhangs which helps a lot with the ground splashing thing

The main reasons people do it this way (I’m talking garden room/offices) rather than masonry, aside from it’s probably more attractive to DIYers, is you can get a 100mm+battens+cladding thick wall, with 100mm celotex in it, which you’d struggle to achieve with masonry, and get away with building off a half decent slab rather than needing trench/raft

This sounds good. Is your framing just 4*2? Not fixed down to slab?? And, what sort of non timber cladding have you used?....

Thanks!

 

[23vc]

This sounds good. Is your framing just 4*2? Not fixed down to slab?? And, what sort of non timber cladding have you used?....

Thanks!

Was fixed down, but only by a few concrete screw/bolts here and there.
I used corroline corrugated sheets. Not for everyone but I love how it looks and surprisingly seems an amazing acoustic barrier.

I did a thread on it here with photos at each stage https://www.diynot.com/diy/threads/another-man-cave-garden-room.594447/

 

[noseall]

And, what sort of non timber cladding have you used?....

Thanks!

I used Cedral cladding on mine....

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• noseall
• 4 Dec 2023

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• noseall
• 4 Dec 2023

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• noseall
• 4 Dec 2023