Raising garage floor by 170

[Fozzy]

I’m converting my garage, and making it into a part of a kitchen diner. 

 

The front of the garage will stay the same as storage, so a small brick wall will be built to box in the new floor, and then a stud wall will be built on top of this. This is all to plans and signed off by BC.

 

 The garage floor is 170mm below the final finished floor height.

 

The size of the area is 4.8 x 3.2m 

 

What method would you be using to raise this?

 

From the research I’ve done, I’m leaning into the option of putting down 100mm insulation board with DPM above and below, then 70mm of semidry screed on top. It would be approx 1 cubic m of screed that I would need. I’ve been quoted £475 just for the supply of this ready mixer. I’ve called a few more companies but they’re unwilling to quote for such a small amount. 

 

Mixing the screed myself seems like a lot of effort and the potential to do it wrong is much higher.

 

I’ve also had it suggested to use jackoboard with a20mm screed.

 

What method would you use?

[Nickfromwales]

Self level the slab before you start to get it nice and flat/level, and have zero screed?  

 

Check how out of level it is and fill to get it dead flat. Lets assume 10mm of SLC goes down.

 

Just put down PIR and then top it with 2 layers of 18mm P5 deck boards, staggered joints and glued and screwed, and then put your finished floor over that. You'll be surprised at just how solid a floor this gives. The SLC prep makes the sub-floor flat and level, so don't skip that bit!

 

Levels are achieved by mixing 2 layers of insulation at different thicknesses to get the height you need; 170-10mm = 160mm to fill, so allow for 1x18 +1x22mm=40mm deck, then 2x layers of 60mm sheets of PIR. That's 170mm total build up. 1x 120mm PIR sheets sounds faster and simpler, but won't conform and settle down as well as 2 layers.

 

DPM/DPC's as required.

Edited August 7 by Nickfromwales
40mm not 36mm.

[Fozzy]

I didn’t even consider that. Sounds like a great option. Thanks very much!

 

If I was to use that method, where the top board butts up to the old slab floor around the edge, how would you infill that gap? Slc?

[Nickfromwales]

9 hours ago, Fozzy said:

I didn’t even consider that. Sounds like a great option. Thanks very much!

 

If I was to use that method, where the top board butts up to the old slab floor around the edge, how would you infill that gap? Slc?

Illbruck FM 330 expanding foam. Defo nothing that would bridge damp such as wood / mortar and the like.

 

Would need some DPM there obvs, but that foam goes off way tougher than the usual foams from builder merchants etc. You'd foam and fit the boards as you went along, so the boards are perimeter sealed with the 330 foam, both in front of and behind the DPM.

 

FYI the 330 foam is closed cell so won't bridge damp like most regular off-the-shelf foams, plus it kills off the cold junction between the existing floor and the new, and is much much hardier all round.

 

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