Cracked tiles laid on 18mm WBP over wet underfloor heating!

[amuiphoto]

Hi All,

My kitchen extension was completed around a year and a half ago and has developed a major problem with the 600x600x15mm basalt (volcanic natural stone) tiles. The tiles are laid on 18mm WBP ply using Mapei Keraquick adhesive and there is NO ditra matting.
There is wet underfloor heating throughout, and the problem which has arisen is that the tiles on the ply joints have cracked which means you can see the outline of all of the 2400x1200 ply boards.
The builder isnt interested in fixing it as no decoupling layer was specified by the architect. The architect blames the builder for not switching on the underfloor heating before tiling and allowing the boards to dry out. Hence I'm left with this rather expensive problem.
My question is, should i remove only the cracked tiles, attempt to remove the adhesive and replace the cracked tiles? Or remove the whole lot and re-tile the whole area? Bearing in mind I have a flush finish with the sliding glass doors and the outside decking so if I relaid the whole floor and used 15mm tiles plus Ditra and didn't remove all of the adhesive I'd finish up high with the sliding doors and the decking.
If I did replace only the broken tiles which are in view, ie the cheaper option are there any adhesives I can use which will allow an amount of flex, for example a latex based adhesive?
Any suggestions would be greatly appreciated!
Cheers
Angus.

 

[Karis]

What type of floor is under WBP, is it a concrete screed or suspended timber floor? How is the WBP fixed?

 

[amuiphoto]

Hi, it's suspended timber joists (200mm I think) with the UFH pipes laid in a concrete screed in between the joists. The ply was screwed down by the builders at regular intervals but I couldn't say exactly what the distance between screws is. The problem has occurred because the wet screed was not allowed to dry properly before the WBP and tiles were laid and when the UFH was switched on the ply boards expanded causing the cracks.

 

[Karis]

Your problem may be a little more involved than your builder not leaving the screed to dry out before boarding out & tiling. IMO your tile base & even the suspended floor construction itself may be under specked, do you know what size/pitch span your joists are? Wet UFH over a suspended timber floor should also use a minimum C16/24 timber specification.

I’m a little surprised at your Architect’s low boarding spec. tiling over a suspended floor with wet UFH can be problematic if it’s not adequately specked. Using WBP ply is far from ideal, timber sheet boards are not dimensionally stable when exposed to largish temperature/humidity changes & expansion/contraction when the heating is in use & during the changing seasons will cause movement & hence cracks. A decoupling membrane should at least have been specified & any decent tiler should have questioned the lack of one even if your builder didn’t have a clue. Keraquick is also an S1 spec adhesive &, given the wet UFH, I’d have gone with a C2 spec, Latex adhesive.

The BS & Tile Association standard for tiling suspended timber floors is 25mm WBP but you can often get away with 18mm in light load/use areas such as bathrooms (depending on size/pitch span the floor joists). I understand the ply is supported by the joist screed but I wouldn’t have chanced 18mm in a heavy load/use area such as a kitchen. Over boarding WBP with a tile backer board would help make it more stable but using a dry screed flooring system would have made far more sense & been a much better bet; http://productsearch.bdonline.co.uk/Knauf-Drywall-Brio-dry-floor-screed-file018205.pdf
It’s also more heat transparent than ply & your UFH would perform better.

Other factors to consider;
• How were the ply boards fixed? Joists must be cross boarded using screw fixings every 150-200mm max with NO unsupported edges & there must be no discernable flex in the floor.

• Were the boards acrylic sealed on the underside & edges before laying?

• Were the board joints reinforced taped before tiling?

• Is the heating system correctly regulated, the temperature should be between 35-50 degrees maximum. The system should have been fully commissioned & left running (low) for several days, turned off 2 days before tiling & not turned on again for several days & then only at low temperature initially.

• What is the sq/m tiled area? Linear tile runs over 6m may require expansion joints & certainly anything over 10m will. You should at least have an expansion joint around the edge of the room.

As for what you can do now, you can try refixing the tiles with a C2 spec latex adhesive but you’re likely to see the join & I think it will have limited success as a long term solution. Commiserations but removing the tiles & re-specking the floor may be the only permanent solution.

 

[amuiphoto]

Karis, thanks very much for your detailed reply!

I think the joists are 200mm thick and around 5.5m long supported on brick pillars at 400mm spacing between joists.

The architect did actually spec 25mm ply but due to the height of the sliding door installation (by a separate company) and the desire to achieve a flush finish with the doors we decided to use 18mm ply - expensive mistake.
He didn't spec a de-coupling layer though which I think may have helped.

I wish I'd known about the drywall screed system at the time as that appears to be a very stable subfloor if laid correctly. Just to clarify does it need to be laid on a subfloor or can you fix it straight to the joists? Presumably it does't require a decoupling layer before you tile on to it?

In answer to your other points, the total area is approx 40m2 and none of the things you mention were done although I think the boards were screwed down every 150-200mm but certainly no acrylic sealant, reinforced tape or expansion gaps were put in place.

It sounds like the only option for a permanent fix will be to remove the kitchen, 40m2 of tiles and ply and start again... gulp!

 

[Karis]

I think the joists are 200mm thick and around 5.5m long supported on brick pillars at 400mm spacing between joists.

Should be fine then, do you know if the timber was C16/24 specification?

The architect did actually spec 25mm ply but due to the height of the sliding door installation (by a separate company) and the desire to achieve a flush finish with the doors we decided to use 18mm ply - expensive mistake.

Oh dear, did you not consult the Architect before reducing the ply thickness some 25%? Ill advised but I don’t suppose you’ll do it again!

He didn't spec a de-coupling layer though which I think may have helped.

I think that was definitely a mistake.

I wish I'd known about the drywall screed system at the time as that appears to be a very stable subfloor if laid correctly.

Most people won’t know or even care about product specifications, after all, it’s what you no doubt paid the Architect a considerable fee to do!

Just to clarify does it need to be laid on a subfloor or can you fix it straight to the joists? Presumably it does't require a decoupling layer before you tile on to it?

It does require fully supporting but I think it’ll perfectly OK straight over the joists as you also have the joist screed providing intermediate support, assuming it’s flush with the top of the joists. Brio 23 is specified over wet heating system & an uncoupling membrane doesn’t seem to be required but check with Knauf & do ensure you follow the installation instructions to the letter, using the correct materials. Brio is a gypsum based product so acrylic priming will be required before tiling with powder cement based adhesive to avoid any reaction with the cement.

In answer to your other points, the total area is approx 40m2 and none of the things you mention were done although I think the boards were screwed down every 150-200mm but certainly no acrylic sealant, reinforced tape or expansion gaps were put in place.

It’s a largish area & you should consider if expansion joints are required, it depends on the size & shape of the room.

It sounds like the only option for a permanent fix will be to remove the kitchen, 40m2 of tiles and ply and start again... gulp!

In the long term, I think that’s what your probably looking at!