Any advantage? Over-board in 12mm or new subfloor of 25mm

[finishthehouse1day]

Tiling bathroom - large format (450sq) porcelain tiles. Existing floorboards old, butchered over the years, many in short lengths and with tongues missing.

I've read the sticky and I think our options are
1. Replace the floorboards and then over board with 12mm WBP ply
2. Remove the existing floorboards and re lay a 25mm wbp ply floor.

If we did 1. would this give extra strength/rigidity? I assume you'd ensure that the joints in the ply were at right angles to the floorboards?

Ta!

 

[Richard C]

Assuming the original boards are 18mm (3/4”), are proper timber boards rather than crap chipboard & providing they are suitably rigid you could overboard what you have but the size/pitch/span of the floor joists is also a factor here. Boards must be securely fixed (use additional screws), with no unsupported edges but the odd gouge & missing tongues won’t matter too much. Over boarding should be 12mm WBP minimum fixed through into the joists every 150mm – 200mm max & not just into the top of the original boards (check for pipes & cables); the biggest draw back with over boarding is it raises the door threshold significantly. Avoid any board joints in the centre of the room or by the door & support long joints down the centre line of a joist. Acrylic prime the back & edges of WBP (not the tile face) before you lay the board. In some cases, a quality 6mm waterproof tile backer board will be sufficient but it depends on the floor construction.

Option 1 is rather belt & braces & I see little point unless the joists are flimsy; but then your better off beefing those up & going with new floor 18-25mm WBP (see below). Sometimes you can overboard with a quality 6mm waterproof backer board but this is only usually required in a wet room; again it depends on the room use & floor construction.

If you decide to replace the floor completely (usually my preferred option) go for option 2 & replace with 18 – 25mm WBP; 18mm is usually sufficient in light load/use areas such as bath/shower rooms but actual thickness you require will depend on the size/pitch/span of the floor joists. Again, acrylic prime the back & edges of WBP (not the tile face) before you lay the board.

A flat floor is critical with large format tiles so it’s important the floor/joists aren’t bowed. Use a large format trowel & flexy powder adhesive/grout. It’s important to use only quality trade tilling materials of the correct type for your tiles & tile base; cheapo own brand & DIY stuff is mostly crap. You’ve read the Tiling Sticky but I would also advise you read through the Tiling Forum Archive posts before doing any work or buying materials, it could prevent you making disastrous & potentially expensive mistakes.

Which type porc tiles do you have; some require sealing before you lay but some come pre-sealed; check with your supplier. Post back if you need more.

 

[finishthehouse1day]

Thanks for a really helpful reply.

My only concern with laying new WBP boards directly onto joists is about sheet size. I assume for maximum strength we need to use the largest size boards with the minimum number of joints? Getting large sheet ply into the room is going to be an issue.

What would you think would be the smallest boards to do the job? The floor area is 3m x 2.3m. Joists are 6" x 3" at 450 centres, on the 3m span. The floor has been additionally braced from below (garage) as there used to be a cast iron bath (no longer). Floor is pretty rigid now but we need to lift it to correct long standing "sag" to centre of the room before tiling. (Already levelled/relaid bedroom floors but not tiled them ).

Thanks!

 

[Richard C]

Even when over boarding, the smallest boards you can do the job with are the largest one’s you can get up the stairs & into the room but with a little thought, some help & even brute force, you'd be surprised how large you can actually fit them into a clear room . Every joint you have increases the risk & on a replacement WBP floor, any cross joist board joints must be supported with noggins as must the door threshold if the joist run is against you.

6” joist depth over 3m is getting a little marginal as far as floor rigidity goes & I like to see 400mm pitch rather than 450mm; 3” wide joists is also a little unusual, this is presumable an attempt to compensate for the joist depth/pitch. It may explain the long standing “sag” you have, how are you proposing to correct it? You say there is additional bracing from below, can you explain what sort?

If you want to post a little sketch showing position of the door, bath & joist run etc, I can have a look & advise in more detail.

 

[Symptoms]

1day - garage below!... then also consider putting in some suitable insulation