Tiling Screed'd Floor - PVA first, Y/N?

themacster

No, don't use PVA - it will serve no useful purpose. Is this a new screed or an old one? If its just been laid you should let it dry out before you tile it.

Arfa__

Interesting debate there! So some say yah, some say no way. I've also read guides that reckon you must put the tiles down on the PVA whilst its still wet and tacky.

The floor I have, was concrete, followed by an inch or two of screed (to cover pipes/electrics). I don't know much more about it than that. I've already picked up a huge tub of Unibond all purpose tile adhesive/grout.

Think I might just skip it.

keystone

Arfa__ wrote: »

Interesting debate there! So some say yah, some say no way. I've also read guides that reckon you must put the tiles down on the PVA whilst its still wet and tacky.

No do NOT tile onto PVA there will be no bond with the substrate. Tiles will lift and grout will crack.

The floor I have, was concrete, followed by an inch or two of screed (to cover pipes/electrics).

Cement screed or latex SLC?

I've already picked up a huge tub of Unibond all purpose tile adhesive/grout.

Combined addy/grout is useless carp. Take it back for a refund. You need a cement based adhesive as has already been suggested.

Think I might just skip it.

The Unibond or the job?

Cheers

Arfa__

Pretty sure its a cement based screed. The tub actually suggests putting PVA down first on porous surfaces (5:1 dilution).

OK, still have receipt, and also got a 15% off voucher for B&Q (must use today though). Will this stuff do the job? http://www.diy.com/diy/jsp/bq/nav.jsp?action=detail&fh_secondid=10137349 I'll prob go to B&Q tonight, as I want to crack on with job this weekend, is there a better choice they stock?

I only picked up the combined adhesive/grout was it was premixed and just seemed the most convenient. That said, a fast set one, would be handy too, the stuff I got says to leave 24 hours before walking on it. With fast set, I can start in centre of room and do all the edges in same day. I've not used powder adhesive before, always premixed, I presume its no worse than polyfiller, plaster, powder grout etc for mixing.

Alan_M_2

Something like BAL SBR (if it's still available) is designed for this job. (It's actually a modified PVA from memory - but the modification makes it suitable for this use)

As already suggested it's to stop draw (the act of pulling out the moisture from the adhesive when spread onto the dry screed).

It's also partially to stop a chemical reaction occurring between the adhesive and any gypsum that may be in the screed (Google "ettringite reaction" for greater scientific explanations)

themacster

SBR = Styrene Butadiene Rubber, it isn't a derivative of PVA. SBR is atill available. You're supposed to mix it with sand if using as a bonding agent.

Alan_M_2

Yup, you're right I'm thinking of something else.

Anyhow here's the product sheet for Bal SBR - including all usages and instructions for said usage.:-

http://194.223.92.131/pdf/Technical%20Data%20Sheets/B%20&%20C%20Technical%20Data%20Sheets/BAL_Bond_SBR.pdf

Plus a guide explaining "What's the difference between priming and not priming?"

http://www.bal-adhesives.co.uk/mediacentre/article14