Slate floor and underfloor heating problem

[ikcdab]

We had a new extension built 5 years ago. The floor is a concrete slab with underfloor heating embedded in the slab. Over this we have large slate tiles, approx 500 X 500 X 10mm.

From the very beginning some of the slates felt a bit loose but I did nothing about it.

Just before Christmas, we had a water leak in the kitchen and a large number of the slates have come completely loose.

I have lifted them up and the adhesive just fell away. At least they will be easier to refit.

I was surprised to see voids under the slates as they had been laid on "dabs" rather than a continuous bed of adhesive.

I don't think the tiler did them right in the first place.

I have also been surprised to find wet under the tiles when I lifted them. The water leak dried out three weeks ago so I don't think it's that.

I wonder if it's some kind of sweating under the tiles. Maybe the voids get hot from the ufh and then condensation from the colder tile above? Has anyone seen this?

By the way, we have a tiler coming to refix the tiles and he has said he needs to use a special adhesive. I don't think that was used first time round.

Any comments???

[chris p bacon]

2 hours ago, ikcdab said:

I was surprised to see voids under the slates as they had been laid on "dabs" rather than a continuous bed of adhesive.

I don't think the tiler did them right in the first place.

I have also been surprised to find wet under the tiles when I lifted them. The water leak dried out three weeks ago so I don't think it's that.

 

The 'dabs' would depend on what the underside of the slate is like, if it's one good side (uppermost) and a rough side for fixing that might be why the original tiler used that method.

As for damp, How long was the leak for ? when concrete slabs get wet they take weeks to dry out. If you've only just taken the tiles up then there wasn't any airflow to disipate the moisture. The loose tiles could have been due to the tiles being laid when the slb was still drying the first time.

[ikcdab]

I think your right. The slabs are single sided. I don't know how long the leak was there. It was under the sink and I came down one morning and when I stood on a tile it "squelched". A pinhole leak had developed in a copper fitting and there was a very fine jet of water. I can't imagine it was there for more than 24 hours,maybe 48 but surely no longer or I would have seen it under the sink. Maybe it is still drying out. I have lifted the tiles so it will have had a few days to dry out before the tiler comes on Saturday.

[chris p bacon]

1 hour ago, ikcdab said:

A pinhole leak had developed in a copper fitting and there was a very fine jet of water

 

It's quite likely it started as a weep and turned into a pinhole, it may have been leaking for a long time before you noticed. I'd make sure the slab is well and truly dry before re-tiling, it could take several weeks. 

[didcot]

If you have under floor heating are you able to turn it on with the tiles removed. That might help dry it out.

We have a couple of large slate slabs that form the hearth on which our log burner sits. They have started to become loose and we dont stand on them. The backs are machine gouged and I applied pva to the backs and in the mortar mix to help bonding. I think slate is so impervious its difficult to sometimes get it to key.

[Grovenor]

I would have thought that leaving an air gap between slab ant tiles would reduce the efficiency of underfloor heating.

[Giles]

Nearly three years ago we had a limestone floor put in in our extension with underfloor heating, and it was laid on Detra Mat, the orange stuff, which de-couples the stone floor from any movement of the slab below, thus minimising any cracking etc.. if yours doesn't have something similar, it may explain why the tiles loosened off quickly?

 

Our build-up was reinforced concrete,  insulation, heating pipes, screed, Detra,  adhesive grout (full bed), slabs.

 

 

Edited January 8, 2021 by Giles

[tomparryharry]

If you take the underfloor heating much above 25 degrees, the concrete underneath won't always like it. We have done repairs in the past, where overheating has produced hairline cracks. We used to recommend a degree a week to dry/cure the floor. Copper plumbing must always be shielded to avoid corrosion where it goes through the concrete, or be prepared for a leak later on.

 

Edit: Depending on your square area, you might consider electric underfloor heating.  Our house here is a suspended floor, but I also have an extension to build.

Edited January 12, 2021 by tomparryharry
Information.

[peterfgf]

You'll probably find that water has leaked under the entire area of the slabs - it is incredibly searching stuff - and I would lift all the tiles up and start again.  As the previous poster said, you need to guard against corrosion of copper in concrete.  Presumably the underfloor heating is plastic pipe (partly because of corrosion but also because it is easier to lay). Slate is impervious to water and was used for damp proof courses in the past.  Detra decoupling is excellent stuff and is essential when laying tiles on suspended wooden floors.  The procedure described by Giles sounds like the way to go - to paraphrase "take a shortcut, do it twice".  Sorry, not to be more helpful.

Peterfg

[ikcdab]

Thanks for your advice. Indeed, we had to lift the tiles in the whole kitchen and they came up very easily. The slab has dried and now to be relaid.

Thanks

[KeithMacdonald]

We're just having a new underfloor heating laid, with plastic pipe (not copper) under the concrete (less corrosion) and a continuous layer of flexible bonding between the concrete slab and the ceramic tiles on top. Can upload pics if that helps anyone?

[ikcdab]

7 hours ago, KeithMacdonald said:

We're just having a new underfloor heating laid, with plastic pipe (not copper) under the concrete (less corrosion) and a continuous layer of flexible bonding between the concrete slab and the ceramic tiles on top. Can upload pics if that helps anyone?

All I would say is that the underfloor heating is the best thing we ever did. Just love it. It runs through our kitchen, dining room and conservatory. We just leave it on all the time and the space is always at a comfortable temperature and the floor cosy too.