Building up (ground) floor

[Piers Finlayson]

Am after some advice form the group. I have a large room (approx 11m x

5.5m) which currently has a floor over three slightly different levels

- each is a couple of courses bricks lower (or higher depending your pov) from the next one. The reason for this is that previously this was three rooms, and the building is built on a slope. The current floors appear to be a few inches of concrete on top of DPM, with no insulation.

I want to build up the floor to the same level - so it's all at one level. I'm not interested in digging the higher levels down because that would take the current top level underground, and as we live in a very wet area I'd need to sort out tanking, and ceiling height isn't an issue. (Generally I'm of the opinion that building above ground is better than below, if below can be avoided.)

Our plan, once we have the floor at the same level is to install wet UFH, so will install insulation, UFH and then screed on top.

So, to my question, what are sensible ways to build up the floor?

I've discarded filling it with concrete, cos of the drying time (months AFAICT).

I've also discarded building a raised floor out of timber, because a) it'd have to be really strong to cope with the UFH and screed b) I'm worried about damp under the floor roting the timber.

Friendly local builder has suggested polystyrene, on the basis that a) its cheap b) it provides insulation c) it doesn't compress.

b) I'm not fussed about, cos I'm going to put decent amounts of insulation in all over anyway for the UFH. c) I'm a bit worried about, as I've seen some reports on the web of floors "flexing" in a joint between concrete and polysytrene.

SWMBO suggested stacking a load of concrete blocks - enough to make it all the same height. Seems vaguely plauslble but haven't run the costings yet or figured out how easy it'll be to get to the right height (i.e. without cutting blocks, etc).

So, any advice or suggestions?

Thanks, Piers

[Piers Finlayson]

Thanks for your advice, but I can't quite figure out what you're suggesting to build up the floor, currently on three different levels before fitting the underfloor heating, and associated bits.

Are you suggesting I fill it with concrete (I think drying time of months, and that's before even fitting the UFH and adding screed on top), or insulation (so parts of the floor will be much more insulated than others, and will cost me 1 arm + 1 leg)?

Piers

[Steve Walker]

When we built our conservatory, we prepared the base finishing at ground level, added 4" of insulation, with underfloor heating clipped down, 1" of polystyrene upstand around the edge as an expansion joint and then 3" of screed on top, bringing it neatly level with the existing house floor. The weight of the screed pushes the insulation down hard and once set, nothing moves.

In our case, ours had plenty of time to then dry naturally, but the underfloor heating instructions recommended 2-3 weeks of drying and then putting the heating on, on its lowest level.

Polystyrene will certainly support a great weight. I live close to junction 8 of the M60 and when they widened it, adding double slip-roads running parallel to the motorway from there to junction 6, they built them on polystyrene.

SteveW

[Piers Finlayson]

This is what worries me. If I fill the different levels of floor with different amounts of insulation and then screed on top fo the UFH, each of the three different heights of insulation is going to compress different amounts and then I'm going to end up with a non level floor.

[Steve Walker]

No problem, you're not adding a fixed depth of screed, you're putting it in to a level (and compression would only be a millimetre or two anyway)

- they all end up at the level you want and they are not going to change afterwards.

SteveW

[Piers Finlayson]

Ah OK, ta.