DPC and depth of oversite

[c0wb0y]

Hey folks,

So, I’m building a 7m x 4m double skin (brick and block) workshop at the back of garden - no building regs/planning required.

1). I’m planning the oversite ‘sandwich’ but can’t see the need to screed. Why can’t the top 100mm concrete layer become the finished floor which I plan on tampering and levelling against the perimeter blockwork? It’ll be a rough finish but fine for a workshop surely and I can lay rubber interlocking tiles afterwards. What am I missing?

2). As its a light duty workshop, my proposed sandwich was as follows:

50mm hardcore type1
25mm sand blinding
Plastic membrane
70mm celotex (or polystyrene perhaps?)
Polythene
100mm concrete

Will the above be ok?

3). Do I need 25mm celotex around the perimeter as i’m concerned about the concrete cracking? Is the perimeter insulation intended to insulate or act as an expansion gap?

4). Why must the oversite insulation be PIR - why can’t it be the cheaper polystyrene / jablite? Is that just down to U values? This will be a back-of -garden workshop afterall

5). Does the cavity between walls need to be 100mm? Can it be 50mm?

 

[noseall]

1). I’m planning the oversite ‘sandwich’ but can’t see the need to screed. Why can’t the top 100mm concrete layer become the finished floor which I plan on tampering and levelling against the perimeter blockwork? It’ll be a rough finish but fine for a workshop surely and I can lay rubber interlocking tiles afterwards. What am I missing?

No need for screed. We use a screed when building an extension, so that the floors can be accurately married in, after the knock-through has been completed.

2). As its a light duty workshop, my proposed sandwich was as follows:

50mm hardcore type1
25mm sand blinding
Plastic membrane
70mm celotex (or polystyrene perhaps?)
Polythene
100mm concrete

Will the above be ok?

100mm hardcore, especially if you are using large granular units.

3). Do I need 25mm celotex around the perimeter as i’m concerned about the concrete cracking? Is the perimeter insulation intended to insulate or act as an expansion gap?

It's there to insulate, but it does inadvertently act as an ex-con strip.

4). Why must the oversite insulation be PIR - why can’t it be the cheaper polystyrene / jablite? Is that just down to U values? This will be a back-of -garden workshop afterall

5). Does the cavity between walls need to be 100mm? Can it be 50mm?

It's your wallet and your heat loss. Is this a building you wish to heat and use often?

 

[c0wb0y]

@noseall - cheers pal. Sound advice from someone who knows what they’re talking about. Its a workshop so yes, i’ll be working a few hours a week in it but I feel 100mm Jablite will be fine. Cant justify the cost of PIR in the floor especially as the rubber interlocking floor tiles will surely be boosting the U values too. Will stick to 100mm cavity in walls though