Discuss Suspended Timber Floor Tile Prep in the Tiling Forum area at TilersForums.com


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Hey guys,

I normally post over in diynot for general DIY but wrt tiling over timber I need some specialist advice...

I've recently replaced a very bouncy/squeaky subfloor constructed out of 4x2 (unknown grade) @ 16ish centers on a 2 course sleeper wall (sleeper at either end and mid span @ 1.8m clean span).

There wasn't a lot I could do with the sleepers other than make good where it had become loose and install replacement 4x2 (c24 grade) @ 12" centers either side of a fireplace and doubled up the middle 3 joists (where foot traffic will be) with m12 bolts and 50mm timber connectors (the OC of the middle 3 joists is 15"). Perimeter noggins fitted with 1 inch gap between internal walls.

I've laid 18mm ply staggering joints onto the sistered 4x2's so each board end is on a full 4x2 with fixings at 200mm along the joist and 150mm on board edges. Noggins fitted along all other board ends.

The floor is pretty stable now, it doesnt feel bouncy (we have a few of these upstairs the house is around 125 years old) but my laser is showing small amounts of deflection across the span of the room (maybe upto 2mm in places if I place the laser and impose my weight around the laser). However I think this is localised to the load is at its worst when the laser is between the joists. I need to do some further testing to confirm this.

Anyways, the wife has enquired about running the porcelain tiles we're having in the extension (1200mm x 300mm wood effect 10mm) through into this room as theres an opening rather than a door and I'd be remiss if I didnt at least take this opportunity to educate myself so hopefully you guys can help! FYI the tiles would be going across the joists if that makes an iota of difference.

Personally, I understand why its not preferable to tile on this sort of subfloor and I would expect most tilers to turn it down its just not worth the aggro for them however if I'm happy with the subfloor prep then I can make an informed decision on whether or not to go ahead or if I need to do some futher work.

I went ahead and put the joist sizes into JB's deflection calculator (for both 4x2 @ 14" span 1.8m and 4x4 @ 16 span 1.8m) which yielded min L/550 which seems adequate.

The dead load of the ply, adhesive and tiles would be approx 36kg/m2 which is 0.35kN/m2. Checking the joist span tables using the above values looks like its sufficient (max clean span approx 2m). As to how much this will cause natural sag over time I just don't know.

There will only be a sofa in the finished room (TV and units wall mounted). Moderate foot traffic through the center of the room where I put additional supports.

So a couple of questions:

- Am I measuring the deflection correctly? Localised deflection along the joist and and between joists doesnt seem to be causing deflection at the other end of the room. The laser showing 2mm on the opposite wall is just the result of trigonometry. I would be better to find out at what point the load stops impacting the laser. Likewise I should put it on board joints to see if there are any weak spots that might force boards up into tiles.

- What can I do to reduce deflection further? The least intrusive is to remove ply and sister a few more 4x2's (or even 6x2's with notches around sleepers). The most instrusive would be additional sleeper walls at mid span. Deflection between the joists can only be remedied by more subfloor (another 8mm ply perhaps going the other way)

- In an ideal world there would be zero deflection, but most timber floors will move. Is there some minimal tolerance I should be looking to achieve?

- How much flex is in porcelain tiles. These tiles I have for the extension do have some "give" in them due to their plank shape. Putting them on a flat surface gives about 1mm in the center which when put under the the load of me standing on it flattens out happily.

Sorry for the long post! I guess more information is better than less in these cases :)


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Were they the only two images?

At the risk at getting in to an overly complicated discussion, I’ll attempt to give you some extra information so that you can make an informed decision.
Using the laser to detect deflection is a good indicator.
However, don’t get bogged down in believing any movement is all bad.
It’s more about the residual vibration than the amount of actual movement.
If, as you say, you can detect 2mm of deviation when you apply your weight in its vicinity, the question is, when you remove your weight, does it return to its original position and stop ‘dead’ or does it vibrate to a standstill?
If it vibrates there’s more work to be done for sure, if it comes to a virtual standstill immediately, you’re very close to a conclusion.
Obviously any indication of movement by way of laser is only local to its position.
It’s unable to measure the level of deflection at the other end of the room.
I would place the laser in many locations and imposing your weight in its local vicinity and watch for deflection in the laser line on the nearest wall.

You can increase the stability of the floor in several ways.
Probably the most cost effective route would be to use a 6mm cement board and overlay the ply.
They should be laid perpendicular to the ply and stuck with tile adhesive and then mechanically fixed.
You can checkout the installation method on their website.
You could lay an extra 12mm layer of ply, firstly glued to the original layer and then screwed every 200mm in to the ply below and NOT the joist beneath. Of course it would be staggered and perpendicular to the original.

You could remove all the ply and use an inert structural floor board like GIFA from Knauff or a 22mm board from ‘No more ply’
They’re both structural quality boards and are both inert, therefore uneffected by heat, or moisture, which are actually your main enemies along with deflection.
Or there is Blanke Permat, which is a mesh like mat which will increase the rigidity of the substrate.

And if you need to go further, then combine one of these techniques along with an upgrade in adhesive.
You can go all the way up to a two part (bottle & bag) S2 class adhesive. Which is highly deformable and designed for areas of vibration.

But as with most of these methods, you should talk with the Tec deapartments of specific materials and get them to give you some kind of method statement in writing to give you some peace of mind that what you are doing is correct, in their eyes at least. And if you want products to supply any kind of guarantee, then getting a method statement from them is important.

One thing tho, if you end up with ply as you’re final layer of substrate, it will have to covered with another inert product before it can be tiled.

Typically 6mm cement board, an xps insulation board, or an antifracture matting. (Which only deals with lateral movement)

Alternatively, you could either, get yourself 20yrs of experience as a tiler and know instantly when walking on it if it’s suitable, or
Get a tiler round to either consult for a fee or get him to price actually Tiling your floor and take guidance from him about extra prep.

Did I say something about it not getting too complicated!


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Professional Tiler
I'm in an old house too. My floors have lots of brick pillars under the joists which chock up and remove the deflection very well.
But then I have wood and carpet. No tiles!


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Thanks 3_Fall for that comprehensive information!

I did some more testing last night (my wife did the jumping :)) and there are two suspect areas which I can lift the ply easily and improve structurally. I would call is pure deflection rather than vibration. Point loading around the laser in various places showed minimal movement otherwise. The areas where I had doubled up the joists already were rock solid.

I think with these improvements the overall subfloor will be very solid. I dont really want to rip up the existing subfloor, but you're right I need a tile underlayment. However the current subfloor height now matches the screed height in the adjacent room. Both of these areas would be tiled, and the screed has water UFH. I would be looking to maybe use Ditra matting or similar to help movement between the substrates expansion/contraction from the UFH.

Unfortunately the old floor was pretty uneven so at least now the current subfloor is level between the openings either end of the room, and and maybe 2-3 mm lower that the wooden floor in the hallway so with Ditra we're going to end up maybe 15mm above which is OK I suppose.

Anyone know someone in the Warwick area? A second opinion would always be welcomed.


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I’d pop some steel angle in down to the floor. Concrete pads under the steel if it needs it. Then bolt into the joists. You won’t need all that many and there will be no deflection at all.

I’d be over boarding the ply too. Any joining floors that are screened I’d build the level up with a Foam Tile backer board.


Professional Tiler
Is 4 x 2 suitable? I would think it’s a bit skinny 6 x 2 I would think min. The 2 x 4 needs support underneath
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