Foundation/ base for garden room.

[Jsmith7]

I’m looking to build a timber frame cladded garden room in the region of 7m x 3m or 6m x 4m. I don’t want a wooden base.  What’s the best way to do a concrete foundation / base?

ideally I need it not to be damp inside and it’s to try and stop vermin getting in. 
Thanks 

[JohnMo]

Hardcore, sand, DPM, concrete. You could add insulation between the DPM and concrete.

 

Or heavy 3x2 slabs on a cement mix.

 

Lits of different ways really.

 

This what I did

 

 

[eandg]

I'm planning for something similar next year but will be looking to get the found done in the next month or so while I've machines/groundworkers in.

 

Ideally looking for something well insulated - plan is for 89mm CLS studded timber frame with mineral wool fill, then 50mm PIR the other side so decent enough u-value. Wood burner inside. Was thinking we'd want maybe 150mm PIR/200mm EPS floor insulation but not sure what build up should be and I'll need to decide for getting levels done. 

[Temp]

What I did was lay a flat concrete slab (excavate, compacted hardcore shuttering, sand blind, DPM, concrete), then two courses of engineering bricks, a DPC and timber frame (Mostly 2x4 and 4x4).

 

Then on the outside WBP to prevent racking. A vapour permeable membrane, then vertical battens and horizontal cladding. I used oak boards but T&G also works.

 

I extended the battens and cladding down over the bricks stopping about 25-30mm above the concrete.  Gravel around the outside of the slab to reduce splashing. Ideally water running down the cladding would drip onto the gravel not the top of the slab but the engineering bricks keep out any water that does.

 

The roof is pitched and tiled but flat roof with fall also possible. The front of my building has a stable like roof overhang to shelter the door. 

 

The wall plate of the timber frame was bolted down to the slab. I did this by pushing "bolts" made from threaded rod and nuts into the wet concrete slab. These were tall enough to go through the two courses of bricks and the wall plate, big washers and nuts. Caution: I tripped over and nearly impaled my head on one of these "bolts". After that I put 50mm plastic pipe off cuts over them. Scary moment.

 

I also put some electrical duct/pipe in the ground before pouring the slab so that later I could run SWA cable up in through it. I used a so called "garage CU" which has 1 power and 1 lighting breakers. Electrician installed Earth rod etc.

 

The inside was insulated between the frame and another layer of WBP painted white. I wanted it strong due to tiled roof. 

 

Edited December 12, 2023 by Temp

[oliwoodings]

For my 6x5m garden room I did a 100mm reinforced slab over dpm and 100mm of mot1. I then put oversized dpc under the sole plates of the walls, which I could lap up the outside underneath the breather membrane once I'd built the walls. Sole plates themselves were anchored with M10 concrete bolts. For floor insulation, I put down more dpm, taped to the inside edges of the dpc sticking out from the sole plates, and then put 100mm of pir on top, taping the seams with foil tape. Then 22mm t&g (glued) osb subfloor floating on top of that.

Edited December 12, 2023 by oliwoodings

[Jsmith7]

So if my concrete base is going to be level with the lawn I need to dig out around 300mm, 100mm type 1. 100mm insulation and 100mm concrete?

thanks 

[Russell griffiths]

11 hours ago, Jsmith7 said:

So if my concrete base is going to be level with the lawn I need to dig out around 300mm, 100mm type 1. 100mm insulation and 100mm concrete?

thanks 

Your concrete base needs to finish 150mm higher then ground level. 

If not the bottom of your external cladding will get wet from rain bounce.