tricky floor project

[seantownsend]

I am tiling my bathroom floor and have decided to remove the old 20mm floorboards and replace with 25mm WBP ply, the only problem i have is a 3m steel rsj running down the centre of the bathroom. The joists have been run in the I section of the rsj and the floor boards are about 2mm higher than the top surface of the rsj.

Can i run the ply either side of the rsj and infill with a thinner piece or would i be better off chiselling out a section of ply (18mm) to allow a perfect fit over the rsj. I only ask this as i don't know whether there will be any lateral movement either side of the rsj that could break any tiles that span that particular area. Any advice would be welcome.

 

[dextrous]

Your description seems to contradict itself. Do you mean that the joists are currently 2mm below the top of the rsj which is forcing a gap between the floorboards and the joists in the proximity of the steels, which would support your proposed solutions?

If so, then routing out a couple of mm of ply will work, and you can run some adhesive along the rsj to refill any lost thickness in the ply and guarantee full support. The other solution would be to place thin timber fillets on top of the joists to enable the ply to bridge over the steels, but this would make your floor even higher. Not sure I'd go for your other solution of butting up the plyy to the sides of the steel and infilling, since one of the purposes of the ply is to use it's integral lateral strength, plus you may have a problem with unsupported edges of the ply between the joists unless noggins have been fitted.

Conclusion - routing out a groove sounds the best bet.

 

[seantownsend]

Thanks for the reply, the top of the floorboards vary from being flush with the top of the rsj to being 2mm above it, if i remove the 20mm floorboards and use 25mm ply i will have to route/plane between 20 and 22mm out of the 25mm ply, not an easy job when working with a 4x8 sheet and having to move it about a small bathroom without breaking it.

Another option i have read about it using aqua panel/cement board over the existing floorboard, does anyone have any experience of using this, would it be a viable option?

I want to be certain that i have the best possible base for the tiles, i don't want to be re-tiling in a few years time.

 

[dextrous]

So the RSJ is actually exposed anyway? Bit odd. However, nothing to stop you reinforcing the floor with 12mm ply screwed to the existing floor, ideally through to the joists as well as intermediate screws.

Haven't heard about aquapanel being used on the floor, so can't comment.

 

[jefoss]

i think i know what you mean about the joist, i was doing a floor recently that had this, simplist option was as mr dextrous says overboard. you don't want to be trying to route it out, it will be a bit of a nightmare for a beginner.

 

[seantownsend]

I wanted to avoid overboarding the existing floor with ply (if at all possible) as it will raise the floor level too much, that's why i was considering the aqua panel/cement board as it is only 6mm plus 2mm for adhesive.

 

[jefoss]

you can use aquapanel on floors but ive never done that, its not what it was made for.

why dont you bite the bullet then and rip out the floor, router the bit out needed and jd, plenty of work but no step

 

[dextrous]

mr dextrous

This is either an insult or a demonstration of deep respect. Can't work out which

 

[dextrous]

Op, if you gor for the routing route, you can always screw 2 or 3 timbers across the position of the routed groove at right angles to it and on the "top" side of the board to strengthen it while you move it about. Then drop it on place and remove these battens

 

[seantownsend]

Yes i could, that would certainly make the process of moving it about a lot easier. I think this is the option i will take, just need to buy myself a router now.

Any suggestions on what adhesive to use for the tiles, obviously something flexible, but is there a specific type for using on ply?

 

[Richard C]

Any suggestions on what adhesive to use for the tiles, obviously something flexible, but is there a specific type for using on ply?

I use this;
http://www.bal-adhesives.co.uk/products/rapidset-flexible

& this for the grout;
http://www.bal-adhesives.co.uk/products/superflex-wide-joint

It isn’t cheap & you need to shop around & haggle for best price. Jefoss has his own preferences which I'm sure he’ll chip in with; I'm biased with BAL but I've never had a single failure. Keep away from the cheap DIY shed products.

Prime the back & edges of the ply with this before you lay it;
http://www.bal-adhesives.co.uk/products/bond-sbr

 

[gcol]

The whole point in replacing the floor with 25mm ply is to make it rigid. If you're talking about having a strip routed out so that it leaves the ply at a thickness of 3-5mm then I fear you're going to have problems with the ply flexing over this area. If I were doing this job, I'd be overboarding with 12mm ply and living with the extra floor height. At least this way you'll have piece of mind.

 

[jefoss]

The whole point in replacing the floor with 25mm ply is to make it rigid. If you're talking about having a strip routed out so that it leaves the ply at a thickness of 3-5mm then I fear you're going to have problems with the ply flexing over this area. If I were doing this job, I'd be overboarding with 12mm ply and living with the extra floor height. At least this way you'll have piece of mind.

but that 3-5 mm is going to be placed on a metal joist....

Jefoss has his own preferences which I'm sure he’ll chip in with; I'm biased with BAL but I've never had a single failure. Keep away from the cheap DIY shed products.

ive never not said i wont use bal, however i do use weber, very similar in useage and quality, however i do prefer weber grout but the range of colours is rubbish.

 

[gcol]

The whole point in replacing the floor with 25mm ply is to make it rigid. If you're talking about having a strip routed out so that it leaves the ply at a thickness of 3-5mm then I fear you're going to have problems with the ply flexing over this area. If I were doing this job, I'd be overboarding with 12mm ply and living with the extra floor height. At least this way you'll have piece of mind.

but that 3-5 mm is going to be placed on a metal joist....

You miss my point. Any up/down movement on one side of the RSJ will try to be transfered to the other side. Unfortunately, if the ply is only 3-5mm it will flex - it's too thin not to.

 

[dextrous]

The whole point in replacing the floor with 25mm ply is to make it rigid. If you're talking about having a strip routed out so that it leaves the ply at a thickness of 3-5mm then I fear you're going to have problems with the ply flexing over this area. If I were doing this job, I'd be overboarding with 12mm ply and living with the extra floor height. At least this way you'll have piece of mind.

but that 3-5 mm is going to be placed on a metal joist....

You miss my point. Any up/down movement on one side of the RSJ will try to be transfered to the other side. Unfortunately, if the ply is only 3-5mm it will flex - it's too thin not to.

That's why I recommended squirting a load of adhesive on top of the rsj prior to laying the ply. When it sets, will it not be rigid?