Maximum screed depth / ideas for building up ground floor height

[CivilianJenny]

We are in the process of converting our attached garage, roughly 5.5 x 5.5m, into living accommodation (two floors).

The ground floor level in the garage needs to be raised by 420mm to bring it in line with the house. The existing garage floor is a 200mm reinforced concrete slab.

We intended to put a DPM on the existing slab, then 100mm PIR insulation, then vapour barrier, then 320mm of concrete.

Building control officer came today and said we should use a reinforced screed instead because we already have a concrete slab in place. I said it would have to be pretty thick screed and he thought it wouldn't be a problem if it was reinforced.

I've considered a block and beam floor but we've already been told we can build the new inside leaf for the cavity wall straight off the existing slab. I don't really want to concentrate ground floor loading around the perimeter and prefer the idea of distributing the additional load by keeping a solid floor construction. Plus I'd possibly need to build dwarf walls for the long span and I'd have to ventilate the void.

So options I've considered are:

• Buying more insulation and going with 200mm rigid insulation + 220mm reinforced screed.
• Sticking with 100mm insulation and 320mm concrete on top.
• Going with some kind of suspended floor like block and beam.

What do you think is the best way to make up the 420mm height needed?

 

[noseall]

Id go with Insulation concrete and screed. Standard stuff.

 

[^woody^]

Just span some joists across.

320mm concrete? Wtf?

 

[Notch7]

concentrate ground floor loading around the perimeter and prefer the idea of distributing the additional load by keeping a solid floor construction.

What do you have against a suspended timber floor?

Isnt that the obvious solution.

or are you having a screed underfloor heating?

 

[CivilianJenny]

I used to be a tiler so that's what I've got against suspended timber floors. Long span (5.1m clear) means joists would need to be pretty bulky, they are really expensive (at least comparable in cost to the other options) and we need to tile it for a wheelchair user so would just prefer it to be solid if possible.

I think I'm going to go for 200mm rigid insulation just to reduce the load, but still not sure about making up the other 220mm. Access is another problem, our driveway slopes uphill towards the garage so we might have to pay for a pump for pre-mixed concrete. Apparently you can't pump screed because it's too dry?

Would it be totally mad to get sand and cement delivered and mix 5.5 cubic metres of screed on site in two mixers? I've worked out it would be about 60 mixes in total, so 30 each. It sounds too much but I'm thinking maybe with a couple of Red Bulls it would be just about possible in a day. A 220mm screed would take 7 months to dry though. Might have to stick with something like 150mm concrete and 70mm unbonded(?) screed and ignore the Building Inspector.

 

[Notch7]

Long span (5.1m clear) means joists would need to be pretty bulky,

Sleeper wall in the middle?

If solid floor, why not do say 350mm jabfloor70 and 70mm screed?

It cuts out the material and labour cost for the concrete and you get better u value.

Its light so sorts out your access issue.

 

[^woody^]

I used to be a tiler so that's what I've got against suspended timber floors. Long span (5.1m clear) means joists would need to be pretty bulky, they are really expensive (at least comparable in cost to the other options) and we need to tile it for a wheelchair user so would just prefer it to be solid if possible.

I was never a tiler, but what you do is use standard joists, fit noggins and strut the floor and it's solid.

 

[CivilianJenny]

I think the answer is probably more insulation. I suppose a sleeper wall and lots of noggins would also make a suspended timber floor possible, but I'm just not sure about whether that can be built off the existing slab and so it may require calcs. We already had to cut a short channel into the slab to add the drainage and it was soul-destroying so I wouldn't want to have to do it all the way across for a foundation.

Thank you all for your suggestions.