Waterproof plywood under tiling?

[STEVEROG]

1. Most Important - floor structure (joists for me)

Make sure the floor deflection at mid span point is 1mm under a two fat bloke load. Pretty simple to do just hand a plumb line to just touch the floor then load it and see what happens. If, as with most standard timber floors it sags a bit then I suggest you sort this out by one or more of

(a) doubling up joists,
(b) Fitting bigger joists
(c) breaking the span by adding a mid-span suport

Note that fitting a sensible thickness of plywood (18mm or more) will also reduce the floor deflection if the plywood is fitted correctly (see later)

2. plywood

Go direct onto the joists with WBP (now called something line EN636-3s). Seal the edges before fitting. Thickness should be minimum of 18mm for 400 centres.

When fixing the plywood screw AND glue. This may sound a bit mad but it will dramatically improve the structural aspects of the floor. Screws want to be decent size, I would run with at least 70mm a nice fat size 6 head. Screws MUST repeat MUST be either stainless steel or ceramic coated to ensure they don't corrode and cause tile blow.

3. Tile substrate

Many like to run with a cement based board over the plywood. If you can afford this its a very sensible way to go. For a wet-room this is an absolute definate (plus full tanking).

If you don't put down a cement board you need to make sure the adhesive you use is compatible direct to plywood.

4. Tile adhesive -

Simple - Go for BAL or MAPEI, and you MUST ensure you use the latex additive to get maximum tolerance to movement.

5. Consider getting a pro tiler in

Oh BTW, while your floor is up you will definatley want to insulate between joists if you are on a ground floor.

Enjoy

 

[Richard C]

Make sure the floor deflection at mid span point is 1mm under a two fat bloke load.

When fixing the plywood screw AND glue. This may sound a bit mad but it will dramatically improve the structural aspects of the floor.

Never done that or know of anyone else that does it; will make little if any difference over screwing every 150-200mm IMO. Overboarding with backer boards is another matter.

Screws MUST repeat MUST be either stainless steel or ceramic coated to ensure they don't corrode and cause tile blow.

Again a little OTT & not necessary unless its wet room floor but it won’t do any harm.

Many like to run with a cement based board over the plywood. If you can afford this its a very sensible way to go. For a wet-room this is an absolute definite (plus full tanking).

If you use the right thickness of WBP based on the joist size/pitch/span & the use the area is being subjected to, additional backer over boarding isn’t necessary. It will also raise the threshold which (for most) is one of the main reasons for replacing rather than over boarding existing.

4. Tile adhesive -
Simple - Go for BAL or MAPEI, and you MUST ensure you use the latex additive to get maximum tolerance to movement.

I use BAL exclusively but Mapei (not the one sold in B&Q), Webber, Granfix + some others. You don’t need expensive latex based addy if you prep the floor right & deflection is within addy tolerance; but if you’ve a deep wallet, it won’t do any harm.

5. Consider getting a pro tiler in

I wouldn’t disagree with that.

 

[macadonald]

Once again, thanks for all the detailed responses guys.

I will most probably take your advice on gettign a pro to do the tiling itself, but want to do all the prep myslef.

Can I throw a spanner in the works and ask how all your advice is affected if I want to add under floor heating. I believe the heating matts are set in a screed based substance. I assume you get a flexible equivelent like the adhesive or can you simply place the matt and apply the adhesive/tiles on this?

 

[Richard C]

Under floor heating really need an insulating tile backer board underneath to direct the heat upwards through the tiles rather than it going into the floor structure; more efficient & considerably reduces reduce warm up time. Although not essential, a self leveling compound should be applied over the element & tile over that. It protects it during tiling. If you just use addy to bed the mat, the tiles can be uneven if you don’t know what you’re doing & if you ever needed to repair the mat, you will never get the tiles up without further damaging it.

Be very careful when selecting your tiler; there are some cowboys around just now & not everyone who says they are a tiler actually is or, in fact, can even tile!

 

[macadonald]

Excuse my ignorance Richard, but what is the point in sealing WBP when it is already water and boil proof?

About to place my order and still unsure whether Travis Perkins 18mm or 25mm is best. Like I previously said, they are slim tiles (2-3mm) thick, 400mm joist spacing and I will be applying a loose wire underfloor heating. All going to add some thickness which I don't really want too much of.

What about 18mm with noggins in the high traffic area. Only a small bathroom.

 

[mointainwalker]

Re UFH

You don't have to use the hyper-expensive pretty useless insulation that is normally proposed for fitting beneath the mat. You can insulate beneath the floor using much thicker, much cheaper normal insulation materials which will also minimise height- loss in bathroom.

Maybe you mentioned you are doing this anyway, but I didn't want to re-read thread.

 

[Richard C]

Excuse my ignorance Richard, but what is the point in sealing WBP when it is already water and boil proof?

Let’s be clear, you only seal the underside & edges not the tile face. The adhesive used to bond the WBP plies together is waterproof so it won’t dissolve & fall apart when wet. Ply is wood & wood will still absorb moisture if left untreated & moisture can get into the back & edges causing it to expand or curl which can cause tile failure. Even seasonal ambient moisture variations can be enough & you will find that sealing the underside & edges is a warranty condition of most adhesive manufacturers

About to place my order and still unsure whether Travis Perkins 18mm or 25mm is best.

I would think you can get cheaper than TP that unless you’ve already haggled; do they not do 22mm?

Like I previously said, they are slim tiles (2-3mm) thick, 400mm joist spacing and I will be applying a loose wire underfloor heating. All going to add some thickness which I don't really want too much of.

TBH I think electric UFH in a suspended timber floor bathroom is a bit of a novelty & pretty much a waste of time if it’s on the 1st floor. Personally I would ditch the undertile board, heater mat & SLC rather than compromise on the rigidity of the tile base. It’s not going to get that cold in there & the warm up time (even with an undertile board) sort of self defeats the object a bit, if you forget turn it on 20-30 minutes before showering, you’ll be done & dusted by the time it gets up to temperature.

What about 18mm with noggins in the high traffic area. Only a small bathroom.

Noggins are required for unsupported edges but won’t do an awful lot to increase the overall floor stiffness; just as important as the pitch is the joist size & span, you still haven’t said what they are!

 

[Richard C]

Re UFH
You don't have to use the hyper-expensive pretty useless insulation that is normally proposed for fitting beneath the mat. You can insulate beneath the floor using much thicker, much cheaper normal insulation materials which will also minimise height- loss in bathroom.

Under floor insulation is essential in a ground floor slab; it may even be of some merit in suspended floors although the negative side is you won’t get much heat migration from the floor below. You’re rather missing the point about using under tile heat reflectors though; even with substantial slab (or suspended floor) insulation, a lot of the heat will be absorbed by possibly 50+mm of concrete screed & up to a 25mm of wood in the case of a suspended floor. An under tile heat reflector just below the hater mat will reduce heat absorption considerably reflecting much of the heat back into the tiles, reducing response & warm up time.

As I’ve said above, I think electric UFH is pretty much a waste of time in a bathroom or kitchen; in fact it can turn your kitchen into a sauna. I do have it in my conservatory though with 3” of slab insulation & under tile heat reflectors directly below the mat. Response & warm up time is very good & it heats the cons very well though it can be somewhat expensive to run.

 

[mointainwalker]

Richard

My comment was made for the above post where there is no screed in question. There is obviously no problem in laying aluminium foil on the floor for extra efficiency, but if insulating under the joists, I am not sure how great the benefits are. In this case however, insulating under the joists will be far more efficient - and much cheaper - than these poxy little sheets that they push with electric UFH.

Moving to your general points: why would anyone want a screed on a suspended wooden floor if they are then putting heating wires on that and tiling on top unless they wanted the screed as thermal mass ?

An under tile heat reflector just below the hater mat will reduce heat absorption considerably reflecting much of the heat back into the tiles,

Do you have any links for the above ? As said above I don't think the reflective element is of significance but don't know.

 

[Richard C]

Moving to your general points: why would anyone want a screed on a suspended wooden floor

Didnlt mean that, does this make more sense; --- will be absorbed by possibly 50+mm of concrete screed OR up to a 25mm of wood in the case of a suspended floor.

Do you have any links for the above ? As said above I don't think the reflective element is of significance but don't know.

The idea of the under tile mat is to get reflective insulation directly below the heating element to avoid heat loss into either the screed floor or timber floor as in suspended floor.

 

[mointainwalker]

The idea of the under tile mat is to get reflective insulation directly below the heating element to avoid heat loss into either the screed

Yes, that is clear, but do you have details/ links for the

An under tile heat reflector just below the hater mat will reduce heat absorption considerably reflecting much of the heat back into the tiles

as i don't believe the reflecting part is very significant whereas you do. However I can't remember where I read about this and wonder if you have any confirmation.

 

[Richard C]

Marmox, Hardi, Aquapanel Thermal, Wedi are some of the most well known.

This site;
http://www.floorheating.com/fastwarm-technical-information.php
shows a useful U value comparison table of some of them but there are many others around.

 

[mointainwalker]

Ah, ok , we have been talking at cross-purposes (slightly). Since in all your posts you have talked about reflective insulation, I believed you were talking about the foil-type as this is reflective.

The others that you mention are not reflective but insulate because they are very poor thermal-conductors i.e. traditional insualtion.

The point remains however, that in the thicknesses overwhemingly used for electric UFH, these are pathetically useless.

If we look at the figures you quote, the thicknesses that will be most popular with electric UFH will be 6 mm and 10 mm ( previous threads will bear this out ) which have respective U values of 4.8 and 2.9 !!!

In the light of this , I absolutely cannot agree with your comments about the considerable effect of these boards. A very small effect , yes.

An under tile heat reflector just below the hater mat will reduce heat absorption considerably reflecting much of the heat back into the tiles

Under floor heating really need an insulating tile backer board underneath to direct the heat upwards through the tiles rather than it going into the floor structure; more efficient & considerably reduces reduce warm up time

Agreed though, in a bathroom, it may not be important, but in other spaces, the costs will be very high with such poor insulation.

 

[Richard C]

Again you’re missing the point. In the case of a ground floor structural slab which may be insulated with, say, 100mm of Celotex with a further 50mm screed over the top; a backer board over this will provide an insulation barrier immediately beneath a heater mat which will minimise heat loss into this screed; I agree a 6mm board on its own would be pretty useless in this case but the main floor insulation is in the slab below that. You cannot lay tiles directly over insulation material as this would be more or less a floating floor which is problematic. You have to cover it with something to fix the floor & provide a suitable tile base, on a ground floor this is usually a screed. The backer boards are used to minimise heat loss into the screed which must, in turn, accelerate warm up time for the UFH. Likewise, with a suspended timber floor, a backer board over the timber & directly beneath the mat must be better than nothing at all; it’s a question of personal choice weather or not you consider it to be worth the additional work & expense. As I said, I think secondary UFH is pretty much unnecessary & a waste of time in such situations anyway but, again, that’s a matter of personal choice.

Perhaps you should contact Marmox technical dept to see if they can provide performance figures both with & without their products in order to substantiate their claims of improved warm up time which seem logical to me. I’ve no wish to keep arguing with you about it or the choice of wording in my posts; I’ve had enough of that for one week & the OP seems happy.