Replace and strengthen utility room floor

Due to already spongy floor patches throughout and a leaking dishwasher it looks like I need to replace approx 6sqm of t&g chipboard which feels like it's going through. The replacement will abutt 19mm chipboard but I plan to use ply and can only get 18mm ply, would you add shims only directly adjacent to the t&g or all over the ply? Also, is it worth using marine ply directly under the appliances and standard for the rest?

Additionally, the washing machine bounces round like its on a trampoline, floor shaking away. Is there a way to strengthen that area? Maybe doubling joists or putting cross pieces between them? The house has trench foundations so soil directly under, is it possible to build something up to provide a more stable base for that area or is that overkill?

Thanks

For starters true marine ply is horrendously expensive and can be difficult to come by, so you may want to consider just a reasonable quality WBP (water & boil proof) exterior plywood instead. TBH 6 square metres is about two full 8 x 4ft sheets (actually 5.76 square metres), so I'd either shim the lot with 1mm shims on top of the joists or just skim the floor with a levelling compound afterwards were I going with plywood. My own preference in terms of sub-flooring, however, would be P5 T&G chipboard (ideally Egger) which will stand up to periodic water ingress pretty well (based on having worked on a job where our P5 flooring was constantly getting soaked over the course of a winter and we didn't have to lift a single piece afterwards - the trick was to not let water stand on the floor for days on end). It's the right thickness and it's relatively cheap, too. If leaks are an issue there's something to be said for the type of welded vinyl flooring that you see in commercial builds like bars, restaurants, etc because water cannot easily penetrate them

In terms of the washing machine bouncing the floor I doubt that solid strutting will achieve what you want - you'd need to climb underneath the floor and strut completely from side to side twice over (at 1/3 of the distance across) if doing it properly, but if your span is less than 16ft I doubt that you'll get any advantage. If it really is that bad you'd probably be as well off putting something like a treated 5 x 2in C24 CLS across the floor beneath the affected area (tied into the existing joists) then building a couple of masonry piers beneath the CLS to support it, ensuring that you put a bit of Visqueen (1000 gauge poly sheet DPM) between the top of the masonry and the bottom of the CLS. That's what they did in Victorian buildings and it works well, but are you so certain that there's not another issue such as the joist ends being a bit rotted where they enter the masonry (and therefore loose in the sockets)?

thanks, i'm planning to lay a vinyl sheet on top, though the floor in the property it pretty poor anyway, a leak from underneath an appliance which was concealed by click laminate tiles was the final staw for this area I think, so I want to make sure nothing can get through again. as for joist ends I guess I will find out what state they are in when I take it up .

this stuff? still 18mm, is there 19 available elsewhere? https://www.wickes.co.uk/Wickes-P5-T+G-Chipboard-Flooring---18mm-x-600mm-x-2400mm/p/164516

or this, but doesn't seem to be t&g https://www.jewson.co.uk/timber/she...nt-caberfloor-p5-chipboard-18-x-600-x-2400mm/

Got the egger board, it seems heavily geared to using their joint and joist adhesive. Do people generally do that or use mechanical fixing?

Thanks

The Egger board is far better than the Wickes stuff. Egger do push you towards their adhesive, but I've had no problems with literally miles of the stuff installed using D4 PVA for the joints. I tend to screw down to the joists because I have access to collated screw guns (they're far faster). For a small area I'd probably just go for Floor-Tite screws. Glueing to the joists will further reduce any tendency to squeak (the hallmark of a badly installed chipboard floor). If there is any thickness difference just use 1mm shim packers as required on top of the joists

Thanks, presumably using shims makes glue on the joists pointless? I will be using 1mm shims. I think I'll screw down and glue the t&g together, see below for existing subfloor in all its glory. Seems a mix of 33 and 49 cm boards ?

Ordered a vat of PVA, is it worth sealing the surface of the boards with a dilute mix? Plan to put vinyl sheet on

rjm2k said:

...is it worth sealing the surface of the boards with a dilute mix? Plan to put vinyl sheet on

Click to expand...

I don't really think so. Are you going to glue the vinyl down?

Yes

Thanks

The result of a little pressure with a screwdriver, at least it shouldn't be difficult to take up ! The plans for the house say this is 19mm board, doesn't it look more like loft board than kitchen flooring?