Concrete floor - screeding - final brain check

[Tim S]

Hi again,

Quick recap - got to make up a floor with about 25-35mm screed. The current surface is solid, but it's a bit of a "farmer's concrete" special, meaning it was made with small round pebbles and in a few patches the pebbles are not well attached. ie they're attached but it doesn't take much to loosen them.

I suspect if we go digging the less well attached pebbles out we'll get to earth fairly quickly. No vapour barrier either. I should add, that the floor's been fine under quarry tiles so I don't see any need to go replacing it - I just want to make sure the screed gets a good bond.

How does this sound:

1) Soak the floor in a stabiliser.

2) Prime the floor with SBR/cement slurry

3) Screed with 4:1 sand/cement + SBR

4) Paint damp proof membrane on top.

5) Finish (ceramic tile or engineered wood depending on location) ?

=====

What would be good for 1): Liquid SBR? I think PVA will bite the dust due to the fact there's no vapour barrier.

Also, any good recommendations of a paint on DPC membrane?

Many thanks

Tim

[The Natural Philosopher]

It may go slightly soft, but it wont dissolve back to liquid. You probably could combine the two functions of stabiliser and DP by using something a bit more epoxy resin-ish than PVA.

There are coatings for exterior walls designed to reduce driving rain penetration.. I forget the name. I'd go that way.

[Tim S]

Hi,

The Natural Philosopher coughed up some electrons that declared:

Ah. OK - I'll do some research for epoxy products. I've seen some mention of aqueous epoxy solutions - I'll have a dig on google.

I saw those in Wickes yesterday. I could grab a tin and try it on a patch and see how it goes.

Many thanks indeed :)

Cheers

Tim

Right, off to do more leccy box sinking now...

[Bruce]

Some links here:

[stuart noble]

If there's no damp, you shouldn't need SBR at all, but it would be an extra precaution, at least for the stabiliser. SBR doesn't mix well with sand cement mortars. It has a tendency to go its own way and leech out if you leave it standing for any length of time. Pva mixes in better and the mortar is easier to use because of that. You could just use sand/cement for the screed and paint SBR on when it's dry. It's penetrates like crazy, which is better than creating a film on the surface

[meow2222]

I realise you dont want to be bothered, but now is your opportunity to dig it up and put insulation down that will save you n times the cost of doing the job. Where n is an unknown number that someone else might have more idea about.

NT

[Phil L]

The floor sounds like it's been botched up more than once in the past and now you appear to want to do the same again, while you might save a few quid, you are still left with a floor with no insulation and no damp proofing, and believe me, trying to rectify this once the units are fitted is nigh on impossible, not to mention astronomical in price.....doing it now while the house is empty and in such a state is both economical and sensible.

I would dig it out to a depth of at least 125mm, put down a vapour barrier and 75mm of polystyrene, then 75mm of concrete....the concrete will essentially be free because it will replace the cost of the sand/cement, the polystyrene (jablite) is about £15 per 8X4 sheet.

The only other factors you need to think about are getting rid of the old concrete and whether you want any ducting anywhere for cables, pipes etc.

[jim]

ISTR that SBR mixes can be made (as long as enough SBR used) to be waterproof in themselves - that's one of it's main uses isn't it? I used it to construct two hearths that were previously damp being laid on earth - worked well enough for me...

cheers jim

[jim]

jim

[Tim S]

Phil L coughed up some electrons that declared:

I don't think it has been botched - the quarry tiles were the originals (1950's). I think it's just a rough old slab.

Having said that, there's never been issues with cracking or damp.

If I did, I would follow your excellent advice (indeed, I'm doing it in the shower room, partly because some of the floor had to come up for drains, so it wasn't much effort to take the rest of 3m2 up).

Unfortunately, I have to draw the line there - it would be good to have an insulated floor with UFH everywhere, but the house loses more heat in other places (roof mainly) so I'd rather put the time and money towards that and my budget's already tight.

But thanks for the suggestion (NT too).

Cheers

Tim

[Tim S]

stuart noble coughed up some electrons that declared:

Oh. I thought SBR was the stuff for sand and cement. Never mind.

That sounds like a plan. Think I need to go and read the SBR data sheets...

Cheers

Tim

[Tim S]

jim coughed up some electrons that declared:

Thanks for that - I'll read it when I have my tea.

Cheers

Tim

[Tim S]

Tim S coughed up some electrons that declared:

And it's extremely useful - thanks - I've saved that to my info collection.

All the details of the sealing, priming and screed are covered nicely.

BTW - What's Category 1 BS 13139:2002 sand? Does that mean "Builders sand" as sold by B&Q or something a bit more specific?

Cheers

Tim

[Phil L]

I'd use grit sand - sharp sand with very small stones in it, sets very hard....mixed about 3 or 4 to 1 cement, grit sand is available in bulk bags and is about the same price as other sand.

Builder's sand is no good for screed, it's too fine and will dust up.

I'd take that "BS 13139:2002" with a pinch of salt, I C&P it into google only gives four results and they all pertain to the flowcrete site.

[Tim S]

Phil L coughed up some electrons that declared:

Cool - thanks Phil, I know the stuff...

Cheers

Tim

[Tim S]

[e-mail] coughed up some electrons that declared:

Well, an interesting discussion came about at work today, between me and the boss.

He's an engineer and as engineers do, he'd been doing a casual study of flooring insulation, and I'm sure he won't mind me repeating it here.

He has a kitchen, tiled onto concrete with no insulation. Using an accurate temperature meter, he'd noticed a 4 celcius difference between air just above the floor (colder obviously) and air at about 1m above the floor.

Did the same at work and the difference was 0.5C.

We work in a converted apple shed, so teh subfloor is farmer's concrete onto the ground.

Over that is 25mm Jablite and chipboard panels.

================

Observation - although not upto building regs for newbuilds, even 25mm Jablite (I assume it would be Jabfloor 70) makes a massive difference.

So I priced it up for my house, 2 ways:

Method 1:

25mm Jabfloor 70 overlaid with 18mm water resistant T+G flooring chipboard as recommended by Jabfloor's data sheet = (for 95m2 ish, whole ground floor) £731 inc VAT.

Same with 25mm TB3000 Celotex and 18mm chipboard = £888

Both methods need additional materials such as DPC barrier, glue etc.

=================

The Celotex method looks like it would achieve a U-value of about 0.3 which doesn't seem bad for a loss in room height of 43mm. Including final flooring, door frame headroom would be reduced to 1.91m which seems acceptable.

UFH isn't an option, but I could believe the cost of materials might actually be recovered through less heating. I'd have to do some calculations to prove it though.

The only thing that bothers me is having chip under my nice tiles and what would happen with water ingress (it's going to be impossible to seal the top layer 100% and hard for the substrate to dry out).

What do folk think?

Cheers

Tim

[Tim S]

Tim S coughed up some electrons that declared:

to be another option. Nothing rottable there AFAICS and everything's glued to everything to no voids for water to collect.

Twice the price though (around 2k). But half the work to lay. Anyone know anything about these "insulation backer boards"? They're new to me...

Cheers

Tim

[Rod]

Living in a house with uninsulated solid concrete floors (on the ground floor!), I would very much appreciate anything that would allow insulation. The 6mm might just about be usable.

My own highly sensitive thermal detectors[1] show a difference between warm and FF when stepping down from bottom stair to floor. So I strongly concur that doing something is highly desirable.

Although I only looked quite quickly - I couldn't see the area of the tile-backer boards! Is that per square metre or a 2.4x1.2 or what?

[1] - Slightly less noticeable with fluffy slippers on them.

[abuse]

On Wed, 31 Dec 2008 00:46:32 +0000, a certain chimpanzee, Tim S randomly hit the keyboard and produced:

How far away from the external wall did you take the measurements? Heat is only lost via the edges of a floor, so buildings with large floor areas can have very little heat losses from the centre of the floor. Most large buildings don't need to be insulated to comply with requirements for heat loss, or if they do, only need to insulate the slab close to the external walls.

[Tim S]

Hugo Nebula coughed up some electrons that declared:

OK - so if I understand this, once the slab and some of the ground under the house has warmed up, net heat loss there is minimal?

Cheers

Tim