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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Building Questions >> Concrete/Screed flooring project https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1521114606 Message started by andyl3004 on Mar 15^th, 2018, 11:50am

Title: Concrete/Screed flooring project
Post by andyl3004 on Mar 15^th, 2018, 11:50am

Hi

I have a new brick porch around the base of an old UPVC one. The concrete base with quarry tiles is still in place. The new bricks went ontop of this and the DPM is two courses up with some polythene folded back down to the old floor. I need to in fill this 2 x 0.5m space with about 110mm to bring it to the right level for new tiling and not to bridge the new DPC.

After laying down a full piece of polythene and taping it to the old pieces, and taping a foam perimeter, can I just fill with polystyrene leaving minimum 50mm to screed on top of? Or am I best to fill with concrete rather than screed? I'd rather not do 50mm of both as will not be able to insulate at all and it will be more work.

Thanks

Andy

Title: Re: Concrete/Screed flooring project
Post by thescruff on Mar 15^th, 2018, 1:17pm

Not a pro for this they will be along later. I would think you'd need at least a 75 mm screed on insulation, maybe less with reinforcing mesh

Title: Re: Concrete/Screed flooring project
Post by woodsmith on Mar 16^th, 2018, 11:15am

The Scruff is right you will need at least 75mm of screed on top of your insulation. You can get foiled sided insulation board in various thicknesses and I would pick one that gives you space to lay 75mm of screed on top.

Alternatively you could fit 90mm of insulation and lay 22mm flooring grade chipboard on top, which you could then tile on.

If you are planning to extend the tiles from another room into the porch I would fit a flexible joint between them as you are just asking for a crack to appear if you tile over any joint between two rooms.

Title: Re: Concrete/Screed flooring project
Post by CWatters on Mar 20^th, 2018, 7:39pm

woodsmith wrote on Mar 16^th, 2018, 11:15am:

If you are planning to extend the tiles from another room into the porch I would fit a flexible joint between them as you are just asking for a crack to appear if you tile over any joint between two rooms.

I have a crack in a stone floor to prove it.

Title: Re: Concrete/Screed flooring project
Post by andyl3004 on Mar 21^st, 2018, 6:47pm

Thanks all. Two follow up questions. I thought foil backed insulation reacted with some Portland cements so should be avoided?

Second, this space after insulating requires about 1 cubic metre of screed. At a 4:1 mix what number of bags of cement would this require? I have a tonne of old plastering sand which was for tender to which I will add some smaller bags of sharp sand for extra grit, just want to have enough cement before starting.

Thanks again

Title: Re: Concrete/Screed flooring project
Post by thescruff on Mar 21^st, 2018, 10:44pm

2m x 0.5m x 11cm works out at approximately 0.37yd³. (0.116 cubic metres)
or 10ft³ without insulation, so you are going to have a lot left over unless you have other work

Title: Re: Concrete/Screed flooring project
Post by andyl3004 on Mar 22^nd, 2018, 7:47am

Thanks Scruff. Just rechecked the app based calc I did and I inputted the wrong info so thanks for checking that.

Title: Re: Concrete/Screed flooring project
Post by woodsmith on Mar 22^nd, 2018, 9:45am

andyl3004 wrote on Mar 21^st, 2018, 6:47pm:

Thanks all. Two follow up questions. I thought foil backed insulation reacted with some Portland cements so should be avoided?

You would need a damp proof membrane between the concrete and the insulation so there would be no contact, but I've just looked on the Celotex website and they are not recommending using it under a slab so you are right it's better to find a different insulation material, but you may have problems finding a suitable one thin enough.

For .1 cubic metres you will only need a couple of bags of cement but I wouldn't use the plastering sand it's likely to crack. Best to use all sharp sand.

Title: Re: Concrete/Screed flooring project
Post by CWatters on Mar 22^nd, 2018, 12:25pm

+1

and if you were planning on putting the DPM under the insulation you need another plastic sheet above to prevent water in the screed floating the insulation.

Title: Re: Concrete/Screed flooring project
Post by andyl3004 on Mar 24^th, 2018, 1:08pm

Too late. Screed laid on top of insulation which sat on Dpm. Let's see ...