This is the 1st part of my 2 part guide to tiling over wooden floors – see Tiling on Wooden Floors (Part 2 – Tiling) once you have finished this article. Tiled floors require proper preparation to ensure their longevity.
Most houses in England are terraced or semi detached houses and most bathrooms are upstairs.
This means that in 9/10 cases, bathrooms have subfloors constructed from floorboards or chipboard screwed or nailed onto joists.
Both require specialist attention in order to prepare them for tiling.
Proper surface preparation is essential on wooden floors as they have the tendency to move, bend, bow, stretch and shrink. In contrast, tiles (even when fixed with flexible adhesives & grouts) are relatively solid.
Any stresses between the movement of the wood and the tiles can cause failure of the tiles to grip to the substrate, resulting in loose tiles or cracks forming in the grout or across the tiles.
Sub floor preparation
You can do this by doing a combination of the following:
1. Ensuring the joists are strong enough to take the additional weight of the new tiled floor.
2. Ensuring that all the floorboards are securely screwed down to the joists, ready for over-boarding.
(Floorboards & chipboard should not be tiled onto directly)
Alternatively, you may wish to replace all of the floorboards at this time with plywood for reasons outlined below:
3. Over-boarding the floor with a product to stabilise the area ‘as a whole’ and provide a good base for tiling.
4. Adding a decoupling layer that acts as a buffer, absorbing lateral movement in the floor so that it is not transferred to the tiles.
In some instances (where movement is detected) it is sensible to strengthen the floor, as tiles will break or dislodge if the floor surface bends (deflects) under the increased load of the tiles.
There are a couple of ways to measure floor deflection, which will determine whether or not you floor needs strengthening prior to floor tiling.
One way is to measure the joist span and divide this by a pre-determined figure (360 for ceramic tiles, 720 for stone tiles) to give a total maximum allowed deflection. The actual deflection of the floor under an estimated load is then measured and compared against this figure. Type ‘measuring floor deflection’ into Google if you want to know more.
A more practical (less technical) method is to stand in the middle of the room and jump up and down. If you feel the floor move under your jumping weight, chances are that there is a deflection problem that will need to be remedied prior to tiling. Better still, look at someone else doing this and you will see the movement.
You will now have to either strengthen the sub floor with the techniques I am about to explain or consider an alternative flooring choice. This may involve lifting the floorboards to get at the joists underneath.
Please see my article on bathroom flooring options for alternatives to tiling that would be better suited to a floor that deflects significantly under load.
Here, rotten joists have been removed and replaced with new equivalent timber using 6x100mm Turbogold woodscrews on one of my bathroom installations in Leeds.
This will ensure a more solid floor.
Ensure joists are fully secured
Ensure that the joists are tightly packed in the wall junctions (they should not be able to be moved in any direction – up or down, left or right, in or out) by hammering in slate and using rapid-set cement to fill in the gaps. Slate is used because it is one of the few substances that cannot be compressed.
Also, if there is a wall supporting the joists (as in the picture above) check that there is no gap between the underside of the joist and the top of the wall. If there is, you are missing out on a chance to strengthen the sub floor for very little effort.
If these gaps are small, pack them with broken slate, or if bricks are missing build up the brickwork until it touches the underside of the joists.
The goal is to stop the joists sagging under the heavy load of the new tiled floor.
If the joists are not very deep, like those above which are less than 6” deep, then this becomes crucial.
These pictures shows how noggins have been added between the joists to add strength.
They have also been added at strategic points (beneath where 2 sheets of plywood flooring will meet) to ensure that the edges of the plywood will not flex relative to each other.
The picture above shows how replacing the entire floor (floorboards > plywood) allowed for the easier running of the shower waste which ran under a large part of the bathroom in this particular installation.
Use a cross cut saw to cut noggins rather than a hand saw – this way you can ensure accurate, right angled cuts that can be wedged in-between the joists tightly (as you can shave off tiny amounts which is impossible by hand.)Hammer noggins in place with a wooden mallet and secure with large woodscrews e.g. 5 x 75mm turbogolds.
If the majority of the joists are rotten or not deep (strong) enough (6×2” is standard but this varies depending on the span) then it might be easier to start again from scratch.
In this bathroom in Leeds, we essentially started from scratch using new joists and joist hangers. This allowed us to fit a perfectly level, structurally sound floor with no deflection, ensuring that the bathroom installation would last for many years to come.
Please see my Top 5 Hidden Bathroom Installation Costs for things like this that can crop up (and throw your budget out of the window) if you haven’t expected them.
Sometimes you may just need to add an extra couple of joists in-between the existing joists if the distance between the joists is too large to provide adequate support.
Here, a previously heavily notched joist has ben re-enforced by securing a similar sized joist alongside it. This can be done by bolting the 2 joists together with 10mm bolts.
If your floorboards are in good condition, then you may be able to overboard these with a suitable board (see below.)
If this is the case, ensure there are no boards missing (or rotten) and that they are all securely screwed down to the joists below, taking care to avoid screwing into any cables or pipework underneath.
Plane off any high points where the boards have twisted.
Replacing the existing floor with plywood
If you’ve had to take up a large proportion of the existing floor to….
- work on the joists (as above)
- run electrical cables and pipework
- replace damaged, missing or rotten floorboards
…. then it might be worth biting the bullet and replacing the whole floor.
This approach allows you to replace the existing floor with a new one with excellent rigidity that will provide you with a surface onto which you can directly apply tiles or apply a thin tile backer board on top for a truly professional approach.
This method, although more work, will allow you to inspect the joists and carry out remedial work if required, and it will also make running the pipework a lot easier if a bit more treacherous:
Don’t fall through the joists and never rest your weight anywhere other than the joists – you will fall through the plasterboard that is screwed to the underside of the joists (this is downstairs’s ceiling).
After you have removed the existing floorboards or chipboard, you can then lay a new plywood floor directly onto the top of the joists (after all 1st fix work has been done – see bathroom installation process for information relating to the order of bathroom installation work.)
There is no reason to replace tongue & groove chipboard with plywood from a structural point of view, but you cannot tile directly onto chipboard. It will have to be over-boarded with plywood or tile backer board (see 3. below) prior to tiling.
Material choices for replacing your floor
I would recommend using 22mm or 25mm plywood rather than 18mm, particularly if the gaps between your joists are larger than 300mm and there are few noggins. This will help limit defection or sagging of the plywood between joists.
This may have an impact at the door threshold where it meets the existing floorboards. When the backer board and tiles are laid on top, you may have a small step up into the bathroom which will need to be covered with an appropriate threshold.
a) Marine ply
|PRO||Manufactured to the highest standards|
|Uses the best waterproof adhesives|
|Provides the best resistance to moisture|
|CON||Expensive (approx £50) -18mm+ thick 8”x4” sheet|
|Max thickness 18mm|
b) External ply sometimes referred to as WBP ply = Weather & boil resistant
|PRO||Designed for external use|
|Uses moisture resistant adhesives|
|Cheaper than marine ply – approx £35 for 8”x4” sheet|
|More readily available than marine ply|
|Available in 22mm & 25mm thicknesses|
|CON||Quality not as good with regards to moisture resistance compared to marine ply|
If replacing the floor I would recommend using 22mm+ external ply, over-boarded with a 6mm tile backer board, particularly if you are having underfloor heating or would prefer a belt ‘n’ braces approach.You save money off the marine ply but still have the waterproof & heat reflective qualities of the tile backer board above, which is the best compromise in my opinion.If installing stone tiles I would also recommend the use of a decoupling mat – see overboarding option c) below.
Please see my article on screwing down structural plywood to joists for installation tips and tricks.
Over-boarding the floor involves laying a relatively thin board on top of your existing floor to stabilise the area ‘as a whole’ and provides a strong and smooth surface to apply tiles onto.
This approach is used after the subfloor (floorboards, chipboard or newly laid plywood) is solidly secured to the joists with no deflection.
Over-boarding does not add much structural strength to the floor (i.e. it will not remedy floor deflection.) Therefore, you may still have to follow steps 1. and 2. above prior to installation.
Do I need to overboard?
In short, yes.
If you have floorboards or chipboard you will need to carry out one of the over-boarding options listed below.
If you have replaced the existing floor with 22mm+ plywood, you may still wish to overboard it with a tile backer board for its waterproof and / or thermal qualities. Also, tiles adhere better to backer-boards such as no more ply, which is what I use.
a) Overboard with plywood
British Standards recommend a minimum of 15mm thick plywood to be used for over-boarding purposes (any less will not provide the rigidity required). This involves screwing a sheet of plywood over the top of the existing floorboards to provide extra rigidity and a flat surface onto which to tile.
If tiling with stone tiles or on a floor with slightly more movement, you may wish to overboard with an even thicker plywood (18mm) and use decoupling matting on top – see part c) below.
Tips of the trades for overboarding with plywood
Please see the article Screwing down structural plywood to joists and add:
- Screw down every 150mm in every direction with 4 x 40mm Turbogold woodscrews directly into the joists where possible rather than just into the T&G floorboards / chipboard below, taking care to avoid concealed pipes and cables. You will need a lot of screws!
- Ensure plywood sheets do not join directly over a floor joint below. They should be offset or staggered to limit potential flexing relative to each other.
- Should be external WBP or marine ply.
- If water makes its way through to the plywood it can swell, causing your tiles to lift or grout to crack
- Changes in humidity in bathrooms could cause the plywood to swell and contact (even if there is no ‘water’ in direct contact with it, leading to similar problems.
- Fungal growth caused by rotting plywood can react with tile adhesives, causing your tiles to lift.
- There are also reports of the preservatives present in marine ply (to keep it waterproof) leeching out into the grout, giving it an orange stain, though I can’t say that I’ve seen this personally.
- 12mm ply (when combined with the thickness of the floor tile and adhesive) can result in a sizeable step into the bathroom, which can represent a trip hazard.
- You need hundreds of screws to make sure its secured correctly (every 150mm)
- Plywood is a natural product and can vary in quality.
Some of these problems can be overcome by opting for option b) below when over-boarding.
The theory to over-boarding with tile backer boards is to provide a rigid, stable, waterproof base for tiling onto. This can normally be achieved with a thinner board than the plywood equivalent, meaning there is less of a step into the bathroom.
It is essentially glued and screwed over the top of the existing floorboards, plywood or chipboard subfloor.
Depending on the manufacturer, this may be in the form of a tubed construction adhesive or a thin-set tile adhesive which is better suited to level out uneven floor coverings.
Very uneven floors may need self-levelling compound to be applied in addition (see below.)
There are many tile backer board options available but I choose to use no-more ply because it is:
- Thin – As the board is only 6mm thick, you can prevent a large step up into the bathroom and the threshold problems associated with using 15mm ply
- Sold in my local tile merchant of choice at a good price
- Relatively cheap per board (120 x 60cm = <£10)
- Waterproof – can be submerged in water and not loose its structural integrity – although it needs to be primed with SBR primer 1st (after laying)
- Heat resistant – this is good for bathroom underfloor heating as it helps to reduce heat up times, ensuring your floor gets up to temperature quicker as heat is not lost ‘down’ into the floor, but reflected back ‘up’ into the tiles. Again, the SBR primer helps with this
- Easy to handle – Its small size (120 x 60cm) allows you to fit it in your car and not dint your hall walls when you carry it upstairs
- Soundproof, in that it reduces noise transmission to the rooms below
- Gets a great bond with rapid-set adhesive so tiles are firmly stuck, especially if primed with SBR primer 1st
- Quicker to lay than plywood as less screws are needed (8 per board) than when overboarding with plywood. This is because cement based tile backer boards are more structurally sound and do not expand / contract with changes in humidity and therefore do not need lots of screws to hold them still
- The glue that no more ply is glued down with absorbs any slight movement in the subfloor beneith, and also waterproof any joints, making no more ply suitable for wet rooms
- It integrates with the kaskade wet room system which I think is the the best wet room system
- They are simpler to cut that plywood (in that no power tools needed) as they can be cut with a special knife. Top tip – Cut shapes with a grit edged jigsaw blade and drill holes for pipes with a diamond coated drill bit
For detailed fitting instructions, please see
Other options along the same lines are HardieBacker® 250 Cement Boards, and with these boards, the manufacturer recommends reinforcing the joints with tile adhesive and scrim tape (rather than a tubed adhesive as with no more ply) to join all of the floor together as a whole for added strength.
Marmox or Wedi boards are a more expensive option, but they have better thermal qualities, although I would question their structural strength vs no more ply as they easily bend & dent.
Also, some of these other boards are secured to the floor with a polymer modified (flexible) cement based tile adhesive, which does not absorb as much movement in the subfloor as the no more ply tubed adhesive (in my opinion.)
Whichever tile backer board you use, follow the manufacturers fitting instructions. Ask the retailer you buy the boards from for this, or look on the manufacturers website.
Please see my article on Which Tile Backer-Board Is Best?
Cement based tile backer boards contain respirable crystalline silica, which is carcinogenic.Therefore, use the score and snap method of cutting boards wherever possible, and use a specialist circular saw blade (Hardieblade) combined with dust extraction if necessary.Limit dust inhalation by cutting outside and always wearing a good quality dust mask.
Underfloor heating can then be laid on top of the no more ply prior to tiling.
c) Overboard with 12mm ply / no more ply + decoupling matting
Belt, braces and the whole 9 yards.
Schluter ditra matting can be installed after the sub floor has been stabilised and overboarded as per the instructions above.
The matting, which is rolled out and glued to the floor with flexible tile adhesive, acts as an uncoupling device in that it prevents small, gradual (lateral) movement in expanding/contracting timber from being transmitted through to the tiles, thus preventing cracking and lifting tiles etc. It also creates a waterproof floor (provided it is installed correctly (joints & wall junctions sealed) and can be tiled onto immediately following its installation.
However, Schluter ditra matting cannot compensate for deflection of the floor under load (vertical movement) so it is not a cure all that negates the need to do the stabilising work discussed earlier in this article prior to its installation.
It also has no thermal qualities so this is another reason why overboarding is also sometimes necessary.
Other makes are available but Schluter is the brand leader and I have heard problems with some imitation brands, specifically with the matt itself separating.
Always follow manufacturers recommendations with regards to adhesive used etc.
Sometimes floors will be uneven, even after overboarding, making tiling very difficult – You cannot expect to tile a floor perfectly level if the floor you are tiling onto is not level.
In these instances floors may have to be levelled prior to tiling by using a self-levelling compound.
This is essentially a bagged powder that is (usually) mixed with water until a thick creamy consistency is reached, which is then poured onto the overboarded plywood / tile backer board and then levelled off with a float. It is then left to dry for at least 24 hours before tiling (depending on the thickness.)
Flexible options are available and should be used over wooden floors. Some manufacturers advise the addition of a latex additive rather than water when mixing to increase flexibility in these situations – it just depends on the manufacturer so read the back of the pack.
Self levelling compound can be applied up to approximately 5mm thick in one coat, but with very uneven floors (that vary by more than 5mm) sharp sand can sometimes be added to the mix to bulk it out – check with the manufacturers instructions.
Please see applying self-levelling compound for details on how to apply self-levelling compound.
See tiling on wooden floors (part 2 – tiling) for step by step instructions of how to lay out, tile, grout & finish. If you have enjoyed this article please leave a comment below or ‘like’ it on Facebook.
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