Floor prep in bathroom - with UFH

[MarkP80]

Hi all,

OK, I've read lots of different threads and am only succeeding in getting myself more confused than ever!

I'm about to start on a bathroom renovation project. I am using a builder friend of mine to do the first fix work (I consider him to be extrememly competent) and want to be sure I get things right for the tiling. I may get the tiling done professionally if I can be confident I can find someone to do a good job for me, but that's a little down the road yet.

The house was built in 1980 and has a chipboard floor. In the bathroom this is in a poor state due to having been butchered by a plumber when the central heating was replaced a couple of years ago. So, this will all be removed. The joists will all be checked at this stage to make sure they are all level and true, so we'll be starting with a clean slate.

The shower will be a conventional tray, not a wetroom type. Mrs MarkP wants, I think, limestone tiles on the floor, and underfloor heating. (Maybe not limestone, but you know the type I mean - big and look like real stone, sorry I'm not confident about the specific type)

So, my first thought was to have a complete new floor laid directly onto the joists of 25mm ply to form the basis for the new floor. However, from there I'm starting to get a little lost.

Is this the correct starting point?
What would come next? - the UFH (a cable-type mat?)
Does it need an insulating layer, or is the ply satisfactory?
Does it need anything to decouple the tiles from the wooden floor, for thermal expansion?
IT's not a wetroom, so should only be subject to splashing/wet feet - does it need tanking, or any other waterproofing treatment?

A few questions, but I want to get this correct from the outset. I don't want to get a 25mm ply floor laid, then have a tiler come round and say it needs to be something else!

Many thanks,
MarkP

 

[grumpygrouter]

25mm ply straight onto the joists is a great start. You would be advised to add noggins first at 300mm centres for extra support. If it was my floor, I would then lay 6mm thermal board on the top then your UFH. Skim over this with SLC and then you are ready for tiling. if you are going for a soft stone like limestone, then a decoupling membrane is a good idea in my opinion.

 

[wetdec]

Hi Mark

Overboarding directly onto joists with 22 or 25mm ply would be a good start screwing at 300mm centres.

Over the ply you should install an uncoupling membrane such as Dura-wp to aid in the prevention of latteral cracking resulting from expansion etc. Uncoupling or decoupling membranes do not prevent against vertical movement. The Dura-wp is just .6mm thick so will keep your height down, uncouple and act as a tanking membrane in one.

Under floor heating on a wood floor in a small area say up to 6m2 will not benefit greatly from insulation as ply of that thickness does not disperse heat greatly. In saying this if you wish to use aa insulation board feel free, the majour issue for you with insulation will be the build up of height as your talking of around 10mm. Putting insulation between the joists eg rockwool or celotex is another way of insulating if your keen to do it. The ufh cable can can be fixed above the the dura-wp membrane without a problem using a flexible afhesive or self leveling compound.

Tiles fixed to finish off should see you about done.

hope this helps




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[MarkP80]

Thanks for the quick response guys, appreciated.

You would be advised to add noggins first at 300mm centres for extra support.

- That's just the type of advice I was looking for, to make sure I get this first step done right.

Over the ply you should install an uncoupling membrane such as Dura-wp to aid in the prevention of latteral cracking resulting from expansion etc. Uncoupling or decoupling membranes do not prevent against vertical movement. The Dura-wp is just .6mm thick so will keep your height down, uncouple and act as a tanking membrane in one

- I hadn't realised it was as thin as that. It would seem daft to cut a corner by not using something like this.

That's great, certainly gives me some confidence I'm getting off on the right footing. I really don't want this floor to start cracking/moving once it's dwon. I'll be back with some more questions when she chooses the tiles.....

Many thanks,
MarkP

 

[wetdec]

Dura-wp is a light membrane suitable for uncoupling due to the fact it has an upper and lower fleeced surface. For bathrooms, showers, wet rooms etc where a well constructed structally sound floor not overly large is being worked on eg newly boarded ply as you are using it will tick all the boxes.

In living spaces such as lounges kitchens where floors may be a little suspect or large areas then an uncoupling / decoupling membrane such as Ditra or Dura -CI would be recomended these come in at around 3 or 4mm thickness due to their construction.



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[MarkP80]

Just another quick question -

The shower tray I'm looking at is a low-level (ish) type, I think it's 55mm. The installation instructions advise that it should be fitted directly onto the joists, ie the new ply floor fitted around it rather than under.

Any specific recommendations as to waterproofing this area? Once the tiles are laid up to the tray this seems as if it will be the area subject to possibly the most drips. Off the top of my head I would expect the tray to floor gap to be filled with Silicon sealant, but is tghere a special treatment for this type of thing?

The tray incidentally is from Roman's Sculptures range, Roman | Lifetime Luxury - Sculptures Standard Trays

Thanks again,
MarkP

Roman’s trays are manufactured from Roman Stone, a self-reinforcing material which allows the floorboards of your bathroom can be cut away and the tray can be installed onto the beams. This can then be tiled up to, giving completely level access and a truly minimalist feel.

 

[MarkP80]

I've now got hold of the instructions for the tray, and in fact it says it must be laid on a ply base. However, it also says to bed it on a 15mm sand/cement mortar layer. Not quite tiling, but do you guys have any views on this? Mortar doesn't strike me as having any flexibility whatsoever, so would anyone recommend anything different to this? I ask because the tile cement manufacturers seem to have a specialist adhesive for just about everything.

Also, reading through a few other threads, I've seen people advised to coat the underside of the ply with varnish. Is this applicable in my case (my floor will be a single layer of 25mm ply, not an overboarding layer).

Thanks again,
MarkP

 

[Gazebo]

Hi Markp,

yes i think the underside and also the edges of your ply need to be sealed prior to fixing. with regards to the bedding down of the shower tray with a mortar screed i would also be interested to know if there is a tile adhesive alternative, some trays stipulate a sand cement screed to aid with the levelling process, some trays say you can use Silicon sealant but if the sub floor is not level then this makes it hard to install correctly, if anyone does know of an alternative to sand/cement please let us know.

(and please dont say gripfill................)

Cheers:8:
Gary