DPM and Underfloor heating

[Komri]

Hi,

I'm refitting the kitchen in our 1930s house. I've taken up the old ceramic floor tiles and found some 15mm thick red (quarry) small tiles undernear, which I've taken up also. These red tiles go under the internal and external walls I think (atleat under the plaster). Underneath is a concrete floor and a small section of wooden floorboards.

I planned to put down self leveling compound, then the adhesive for the insullation boards, followed by the warmup underfloor heating, another layer of leveling compound, then adhesive and tiles.

I'm unsure if the red tiles were forming a damp proof membrane, there wasn't a black bitchaman layer, but a thin yellow rubbery layer ontop of the red tiles. I've been reading and have learnt that ideally I should measure the humidity in the floor, but I don't have the meter. Where can I hire this, or find someone to measure it for me in Birmingham (paying for their time obviously)?

I had considdered putting down an epoxy DPM, like stopgap 75, to be on the safe side, but I can't find anywhere to buy it, the primer 131 and f Ball's leveling compound. Any suggestions?

I really need to get on with the kitchen installation, but don't want to assume the floor will be ok without a DPM. There were no problems before, but no underfloor heating which I gather draws moisture up and could introduce a problem.

Any help/advise would be much apprechiated.

Simon

 

[Ian]

Just a quick question. How big is the area of floor boards, and how does it meet with the concrete? A picture or two of the area to be prepped would be handy. Welcome along to the forum.

 

[Komri]

Wow, that was a fast reply, thanks.

The floorboard area is 1.1m by 1.2m, the floor area of the room is 10m2. The wooden floor is higher than the concrete by about 15mm, I intend to level this off roughly by using 6mm insulation over the wood and 20mm over the concrete, then the self levelling compound to get it exact. The plywood is still to go down over the floorboards.

 

[Ian]

I see now, its never a good thing when two different substrates meet like that. Is there any way you can pull the boards up and replace with 22/25mm ply? Or find out what is underneath and fill with concrete to match in with the rest of the floor

 

[Komri]

Below the floor boards is a half meter gap to the rubble of the foundations, so filling with concrete isn't an option, but would be fine to ply over with 22/25mm ply.

 

[Ian]

In ordinary circumstances an expansion joint would be put in where two different substrates meet like this, but that would look very odd in this situation. The thing is if a grout joint falls on the join or a tile is over the join it is highly likely a crack will appear. One option is to remember where the join is when you come to tile and use a silicone joint instead of grout in this area, mapei do a range of silicones that match their grout colours so it wouldn't be noticeable. In responce to your original post, stopgap 75 is a brilliant product and f ball have a new levelling compound called stopgap 700 superflex which is fantastic and also suitable for UFH. Just make sure you use the 131 primer neat over the DPM before you put the UFH and levelling compound down.

 

[Komri]

Thanks for the advice Brian. The original floor had tiles over this wood/concrete junction which had been down for 20 years and hadn't cracked, hopefully I can get the same result.

Where can I buy the F Ball products from? I've search online and my usual suppliers and I can't find it anywhere, the local sellers which are on F Ball's website are either closed for Christmas, or have stopped selling it. Do you know of a supplier who will sell to the public?

 

[Ian]

Hopefully you will be fine, I just thought I'd make you aware of the possible problems! Just make sure the adhesives and grouts you use are flexible. I get the ball products from my local Jewsons, do you have one near you? What tiles are you laying and what size?

 

[Komri]

Yes I do have a Jewson's nearby, I'd checked their website and it didn't come up, but hopefully they will have it instore, I'll try tomorrow.

The tiles are 40cm square porceline, from Topps, using Topps flexiable adhesive and BAL microban grout. Will this be OK? From your earlier post, am I correct that need to get the tile joint over the substrate joint to avoid cracking?

 

[Ian]

When I get the ball products from Jewsons it is special order as they don't keep it in stock but, its only a couple of working days to get it. I have nothing bad to say about the topps adhesive, I use BAL mainly, only because I'm used to it and the 25 year guarantee gives me added confidence to guarantee my own work. If you can get a grout joint to fall over said area thats great, but don't set the room out around this if it will compromise how the rest of the floor will look ie, small cuts elsewhere.

 

[tfs]

f75 is not suited to under floor heating to the best of my knowledge. Have read this on the literature in the past so I would give them a call first just to be sure.

 

[Ian]

f75 is not suited to under floor heating to the best of my knowledge. Have read this on the literature in the past so I would give them a call first just to be sure.

I've just read the data for f75 and f76 and neither can be used for UFH. So thats another thing I've learnt! Good spot tfs. There is no info that I can find on whether insulation boards can be put down then the mat, so a call to find out will be necessary.

 

[Ajax123]

F75 I think is a paint on epoxy DPM. When you are using paint on DPM's they should not be used as a means to trap moisture in the heated substrate. That is not as far as I can see what you are doing. You are trapping the moisture in the ultimate substrate and then uncoupling everything else with insulation boards so the use of F75 as a DPM is perfectly acceptable from a technical stand point. If you are worried about the DPM manufacturer removing any warranty try Tremco Universal One Coat which IS suitable for use with Underfloor heating.

 

[tfs]

Yes, it is a two coat (roller applied) epoxy dpm.