Slab for timber garden room

[Dan8000]

Hi all!

It would be great to get advice and opinions for a garden room I am looking to build.

I’m a complete novice when it comes to blocks, concrete, slabs, mortar etc and have carried out a bit of research but I think to get the best and right answers, this forum may be it!

I’m looking to build a timber garden room/man cave approximately 6m x 3m.

I’ll be building on a 6” concrete slab (on 4” of MOT Type 1 and reinforced with rebar) and the slab will be bigger than the footprint of the room.

It appears that it’s best to have an air gap between floor and concrete slab so I have been working out the best way to create this. I think there are two options but here is where I’d be grateful for any expert guidance:

1) Place 100mm concrete blocks on top of the slab to provide the air gap and start the timber floor on top of these. If this can be done, is a perimeter of 1 block high enough to carry the weight? And is it a must that the blocks are secured with mortar or can an adhesive or even brackets be used to secure these to the slab?

Or

2) Place 215mm hollow blocks within the slab area so that they become part of the concrete slab pour and sit above the top of the pad. Fill the hollows to include rebar which will extend above block in preparation for the timber base which will have holes drilled for the rebar to go through. This (I think) would make the structure secure without movement?

I hope this makes sense trying to get the thoughts in my head into words lol.

Any help is much appreciated and I look forward to any replies. I can draw a picture of any of the two options above if needed!

Thanks all!

Dan

 

[^woody^]

Why can't the floor joists just go on the slab and vent each end?

 

[Dan8000]

Hi @^woody^

Thanks for the reply.

The only thing I was thinking regarding putting the joists straight onto the concrete (with a DPC) is that the joists would be too close to the ground and may be less resistant to weathering etc? Or am I looking at this too deeply lol!

Never thought of venting each end but how would I do it?

 

[^woody^]

Use tanalised timber, DPC underneath only (not wrapped around), slab surface flat so no ponding and it will be OK if vented.

Vent only really needs to be a continuous minimum 25mm slot. This can normally be hidden by the cladding, and mesh screened to keep insects and rodents out.

 

[Notch7]

slab will be bigger than the footprint of the room

it is best really if the slab is the size of the framework and the cladding runs down past the concrete

if slab is bigger than structure then rain will run across the slab eventually rotting joists

 

[^woody^]

Angle the slab perimeter.

 

[Dan8000]

Use tanalised timber, DPC underneath only (not wrapped around), slab surface flat so no ponding and it will be OK if vented.

Vent only really needs to be a continuous minimum 25mm slot. This can normally be hidden by the cladding, and mesh screened to keep insects and rodents out.

Thanks Woody. First bit makes sense…The novice is question is - how do I create a continuous 25mm slot if it’s DPC then wood on top of this?

Sorry if this sounds simple!

 

[Dan8000]

it is best really if the slab is the size of the framework and the cladding runs down past the concrete

if slab is bigger than structure then rain will run across the slab eventually rotting joists

Thanks Notch! Was thinking that the slab needs to be slightly bigger as the rebar needs to be 6 inches from the edge from what I’ve seen? I may be wrong?

With double batten for cladding, this should take it past the slab at the bottom so no run off on the slab?

 

[catlad]

Once you pour your slab you can stick some metal in it before it goes off to connect your new floor joists to. I built a summer house recently and I used some left over block pavers to keep it off the ground, with all the rain we have had lately I'm glad I did, Although it was never going to be any other way as timber and the ground don't like each other for long.

 

[Notch7]

Thanks Notch! Was thinking that the slab needs to be slightly bigger as the rebar needs to be 6 inches from the edge from what I’ve seen? I may be wrong?

With double batten for cladding, this should take it past the slab at the bottom so no run off on the slab?

reinforcing needs minimum of 50mm cover.

I would aim to be making the slab around 15 to 20mm smaller than the measurement of the cladding taken from internal face to internal face - that allows you a fair bit of margin for the slab out of squareness or wavy edges

Don’t forget to use plastic chairs for the reinforcing - what are you using A142

 

[Dan8000]

reinforcing needs minimum of 50mm cover.

I would aim to be making the slab around 15 to 20mm smaller than the measurement of the cladding taken from internal face to internal face - that allows you a fair bit of margin for the slab out of squareness or wavy edges

Don’t forget to use plastic chairs for the reinforcing - what are you using A142

I was looking to use 65/75mm plastic mesh chairs to provide the required cover on a 6 inch slab? With A252?

This would be without padstones and the castles directly on the membrane.

 

[noseall]

Hi all!

It would be great to get advice and opinions for a garden room I am looking to build.

I’m a complete novice when it comes to blocks, concrete, slabs, mortar etc and have carried out a bit of research but I think to get the best and right answers, this forum may be it!

I’m looking to build a timber garden room/man cave approximately 6m x 3m.

I’ll be building on a 6” concrete slab (on 4” of MOT Type 1 and reinforced with rebar) and the slab will be bigger than the footprint of the room.

It appears that it’s best to have an air gap between floor and concrete slab so I have been working out the best way to create this. I think there are two options but here is where I’d be grateful for any expert guidance:

1) Place 100mm concrete blocks on top of the slab to provide the air gap and start the timber floor on top of these. If this can be done, is a perimeter of 1 block high enough to carry the weight? And is it a must that the blocks are secured with mortar or can an adhesive or even brackets be used to secure these to the slab?

Or

2) Place 215mm hollow blocks within the slab area so that they become part of the concrete slab pour and sit above the top of the pad. Fill the hollows to include rebar which will extend above block in preparation for the timber base which will have holes drilled for the rebar to go through. This (I think) would make the structure secure without movement?

I hope this makes sense trying to get the thoughts in my head into words lol.

Any help is much appreciated and I look forward to any replies. I can draw a picture of any of the two options above if needed!

Thanks all!

Dan

When I built my shed (on concrete slabs), I laid a DPM on the slabs fist. This DPM extended past the edges of the slab by 600mm on all sides. I then laid out the timber frame base for the shed in 75mm x 50mm timber (laid flat), to the perimeter of the shed only. I infilled the 'frame' with 50mm Celotex insulation. I laid the ply floor onto the frame and insulation. I then erected the sides of the shed in 75mm x 50mm timber. I then wrapped the additional 600mm of DPM up the sides of the shed before cladding.

shed.jpg

• noseall
• 4 Dec 2022