1700x800 shower tray laying as low as possible

[john2k]

Currently I have a bathtub that was installed by developers when house was a new-build about 14 years ago. I’m planning to remove the tub. The entire bathroom floor is currently what looks like tongue and groove chipboard and the floor is tiled upto the bathtub.

I’ve purchased a 1700x800x25mm Mira Flight Safe stone resin cast tray to get the wet room look. I want to install the tray as low as possible so that by the time the floor is tiled then both levels are very similar height. When I remove the bathtub I will cut away the chipboard as I’ve been reading that it’s not a good base for laying the tray on. Once I remove the chip board one option I have is to lay ply to replace the chip board but this will bring the tray up again so it will get a higher level. Instead I’ve seen people screw timber battens along the side of the joists sitting lower with the thickness of the ply so that then panels of ply can be slot in which then essentially makes the floor level the same as the surface level of the joist. I’ve the seen people tank over it before laying a bed of sand and cement.

Anyone have any tips on this method? What thickness ply to use? Battens are often quite thin so when the ply sits on it, how do you screw it to the batten without the batten splitting?

A bit like what Roger Bisby has done in the start of the following video:

 

[noseall]

Replace the whole floor with ply, then bed your new tray onto that. We added another thinner layer of ply before fitting the Karndean type flooring, which served to minimise the shower upstand.

 

[robinbanks]

What thickness ply to use?

25mm. Double it up with glue and screw if you're really paranoid

Battens are often quite thin so when the ply sits on it, how do you screw it to the batten without the batten splitting?

Use decent ones, not full of knots. Predrill them and then glue n screw. If using countersunk screws don't go mental sinking the head into the batten. Don't screw near the end of the batten. You'll already have ply and you'll have offcuts of it. Use them as battens. If you can't cut the ply straight enough, use the factory edge as the top surface of your ply batten

Be really wary about water getting to the edge of the chipboard - if it does it'll expand and ruin the job. I'd impregnate it with some suitable sealer and go over it with something waterproof, then do a really good job of ensuring the tiles were sealed to the tray with flexible sealant, not grout

 

[john2k]

25mm. Double it up with glue and screw if you're really paranoid


Use decent ones, not full of knots. Predrill them and then glue n screw. If using countersunk screws don't go mental sinking the head into the batten. Don't screw near the end of the batten. You'll already have ply and you'll have offcuts of it. Use them as battens. If you can't cut the ply straight enough, use the factory edge as the top surface of your ply batten

Be really wary about water getting to the edge of the chipboard - if it does it'll expand and ruin the job. I'd impregnate it with some suitable sealer and go over it with something waterproof, then do a really good job of ensuring the tiles were sealed to the tray with flexible sealant, not grout

What sort of thickness size battens do the one in this picture look like. So, pre-drill the battens and countersink screws through them going straight into the joists. Any particular type of screws? and how much screw length should pretude after it comes out of the batten? in other words how much of the screws needs to be on the joist?

 

[cdbe]

18mm ply, for the "battens" use 38x63mm CLS, 6x 100mm woodscrews at 150mm spacing. You will really need an impact driver or a decent short cordless to properly drive the screws home, the holes won't need countersinking - the heads will go into CLS no problem.

 

[john2k]

18mm ply, for the "battens" use 38x63mm CLS, 6x 100mm woodscrews at 150mm spacing. You will really need an impact driver or a decent short cordless to properly drive the screws home, the holes won't need countersinking - the heads will go into CLS no problem.

Thank you

 

[robinbanks]

38x63mm CLS, 6x 100mm woodscrews at 150mm spacing

I think it's slightly overkill but I did want to point out that 100mm screws may well end up sticking out the other side of the joist

Measure the screw length required

Also be aware that 6x screws will probably need a PZ3 bit to drive them

I'd glue n screw a narrower batten with a shorter screw

Try to use screws that are only partly threaded (smooth shaft on the part under the head). Using fully threaded screws means they might not pull the batten tight against the joist

A good rule of thumb for screwing things together is "2/3 the screw in the substrate, 1/3 in the thing being fixed" - e.g use 75mm screws to fix a 25mm batten

If I was screw shopping for this task I'd be looking at using Screwfix's Turbo Coach (but they need a socket bit because they're a bolt head)

 

[cdbe]

100mm screws may well end up sticking out the other side of the joist

Fair enough, screws to suit overall width, but I stand by the 6mm - you want those battens really solid and the extra width of the CLS gives the fixing screws for the ply plenty to grip. As for overkill - I can't see the point in 25mm plywood for a span that will be even less than average.

 

[john2k]

If I was screw shopping for this task I'd be looking at using Screwfix's Turbo Coach (but they need a socket bit because they're a bolt head)

Socket bit idea is probably better as I prefer driving using socket because it doesn't slip like screw heads do. So something like this: https://www.screwfix.com/p/turbocoach-hex-flange-coach-screws-m6-x-70mm-100-pack/22717

This will be equivalant to the 6mm that was recommended by @cdbe and I just need to measure the thickness of the joist and add the thickness of the CLS batten to work out closest length that won't come out the other side

 

[robinbanks]

you want those battens really solid

You do..

and the extra width of the CLS

..though increasing width progressively defeats the solidity goal as the more you project the batten into the void, the more of a lever, riving on the fixings, it becomes

In the limited space available, I'd use narrower batten material to make life easier. For me that would be 18x50 or 25x50 rips of the same ply, glued n screwed, that I was installing as the infill..

..but if I'd tasked you to do the job and you installed CLS I wouldn't make you take it out

 

[robinbanks]

I just need to measure the thickness of the joist and add the thickness of the CLS batten to work out closest length that won't come out the other side

Dont forget to consider the length of your cordless, plus bit, plus a screw sticking out the end of it; if it totals to longer than the width of the void minus two thicknesses of CLS the job becomes a bit more awkward

A quick clamp or two might save you some hassle and are a very useful purchase too

 

[john2k]

Dont forget to consider the length of your cordless, plus bit, plus a screw sticking out the end of it; if it totals to longer than the width of the void minus two thicknesses of CLS the job becomes a bit more awkward

Good point. I have one of those 90 degree angle attachments for my driver. So that may come in handy. I also have a ACDelco Cordless Ratchet ARW1209P that i use for car work and works a treat. Can tighten upto 61nm. So that might help in tightening the hex head socket driven screws