new extension - wooden joist floor has sunk

[rajk]

Hi - I have got a floating wooden joist floor.

The joist are 6inches by 2inches. Covered with 18mm ply. Then I have got polyplumb overlay panels for under floor heating. Then ceramic tiles for finishing.

Its been just over a year and now the floor has dropped by about 1/4inch all round. Gap formed near skirting boards, splash back and work top.

Is this normal? Will it drop more?

Major problem is my tiles are cracking and I have got underflooring heating directly underneath the tiles.

 

[maltaron]

I would suspect that the heated floor has dried out the joists and caused them to shrink. That much shrinkage is possible if the joists were still damp fron outside storage when installed. also 18mm ply is too thin for floor tiles, should be at least 22mm.

 

[noseall]

My guess is that the skirting has shrunk also. Always does.

We went to great lengths to ensure that our suspended timber floor was adequately supported before tiling.

This involved using additional supports to the sleeper walls and a whole series of noggins where the plywood board ends would fall.

We were able to use 18mm WBP but only because of all the additional support and stiffening. It has not budged one iota but the skirts have shrunk a bit.

 

[rajk]

skirting board are MDF - checked and have not shrunk.

There are no sleeper walls for extra supports - to add the sleeper wall means removing the floor.....sounds expensive.

Fortunately, its shrunk approx 5mm everywhere - will the wooden joist shrink/dry out anymore?

The joist were treated and looked wet when installed.

 

[maltaron]

It is unlikely that further shrinkage will occur. You mention no sleeper walls but have not told us the span. For your info 6 x 2 joists will span 8ft without support but would normally have herringbone or solid strutting half way.

 

[IanAnderson]

This is a common problem with heated floors. Not normally a 'support' problem, simply everything shrinks, just that with carpet you don't notice! Been in loads of older properties and seen black edges to the carpet because the skirts/etc have shrunk and that is without underfloor heating.

I have just done my own electric underfloor over wood with a tile finish and here in Norway we have to put 50mm squared mesh down with 20mm of self leveling stuff (special/expensive!) over the wires. Then tiles etc. Shrunk like a bu**er.

Admitedly I used softwood skirts, but now I too have 2-4mm gap underneath.

Oh well, it will make keeping the tiles paint free easy next time I paint the skirting boards