Garden office on existing concrete slab

[Al Parker]

Hi all,

I recently purchased a house that has a fairly newly built 4.5m x 3m insulated concrete slab at the bottom of the garden. I’m now looking to build a garden office on this, i’ll match the office to the size and I was given the specs via an invoice as part of the purchase. The slab finishes 100mm above ground level on the right hand side and then due to a slope finishes 250mm above on the left. Is it a terrible idea to build the timber frame, separated by DPC, straight onto the slab? The vast majority will be above the suggested 150mm

Edited January 25, 2024 by Al Parker

[oliwoodings]

What do you mean by 'suggested 150mm'?

 

Nothing wrong with putting it straight on top, but depending on the detailing of the slab you could get cold bridging at the edges if the sides of the concrete are exposed. Do you have details of the build up of the slab?

[Al Parker]

 I thought it was suggested that timber frames should start 150mm above ground level and to use a course engineering block to build it up if it didn’t. 

 

the document I have states “hardcore, sand binding, dpm, 100mm insulation, dpm, mesh, concrete”

[Moonshine]

How thick us the slab if it's 250mm above ground level? Can you see the slab edges?

 

It is recommended that the timber sole plates are 150mm above ground level.

 

When i built mine which the slab was level with ground I used two course of brick to bring it up 150mm.

 

I would probably do the the same for yours personally 

[Moonshine]

This was how I did it, but my slab wasn't insulated from below

 

[oliwoodings]

Personally I'd be worried about introducing further cold bridging with a brick course given you aren't planning on any additional floor insulation, unless you insulate the inside of the brick course somehow.

 

If it were me I'd put oversized dpc under the sole plates, extending out either side and once the walls are up I'd lap the dpc up underneath the breather membrane so the sole plate isn't exposed at all. That's what I did on mine, worked well