Screws and glue for subfloor?

[deadfire19]

I'm installing tounge and groove chipboard subfloor with channels for underfloor heating to replace old floorboards.

I've added 150mm PIR between the (170 x 50 mm) joists, level with the top of the joist.

The joists aren't particularly even and I've added plastic spacers in some places where some joists were too low.

I'm wondering whether it's worth using wood glue in addition to spax screws to hold the sub floor down. I'm hesitant against using wood glue since it will make it impossible to ever replace the subfloor but on the other side I'm worried about squeaks if I don't use wood glue.

I'm wondering if using good quality screws every 10cm along the joist removes the need for glue.

Any advice would be appreciated.

 

[Deleted member 174758]

The joists aren't particularly even and I've added plastic spacers in some places where some joists were too low.

Plastic packers aren't designed for that and I think there will be a major risk that you'll end up knocking some of the packers out of the way (leaving unsupported areas) as you attempt to manouvre each sheet into position. If your joists are really that uneven, more than 2 to 3mm out, it is better to sister them using something like 4 x 2in softwood screwed to the sides to provide a level fixing surface than attempt to use packers.

I'm wondering whether it's worth using wood glue in addition to spax screws to hold the sub floor down. I'm hesitant against using wood glue since it will make it impossible to ever replace the subfloor but on the other side I'm worried about squeaks if I don't use wood glue.

TBH Spax screws for sub floors are an unnecessary waste of money - ordinary flooring screws will do just as well and cost a lot less. As to glue, manufacturers like Egger specifically state that T&G chipboard flooring should be glued with D4 glue (and not just ordinary PVA) at the joints and to the tops of the joists to avoid squeaking. In fact using plastic packers may well propagate squeaks unless you manage to put a screw through every packer you use.

In terms of access, I have to ask just how often you think you'll ever need to lift the floor, and just how you think you'll manage that if the tongues are glued? This question comes up time and again and the answer is always the same - you rarely need to lift floors because there is rarely anything beneath them which can have problems - maintainable components such as CH manifolds and the like should be somewhere like on a wall inside a cupboard, so should any maintainable electrical components, etc. If they aren't, then a removeable access hatch can be provided above them, but the same would be true of a planked softwood floor, tiled floor, laminate floor, etc.

A little tip: when installing T&G chipboard flooring to get the joints to pull up tight I use my "patent fine adjuster" - a 7lb or 14lb sledgehammer and a metre long (or longer) piece of 4 x 2 CLS placed alongside the T&G chip. Because you always start with tongues to the wall the 4 x 2 is used against the grooved edge and does not damage it. You heel and toe the timber - legs apart, heels on the chipboard, toes on the timber when you swing the sledgehammer (actually fairly gentle taps are normally enough to pull the joints up nice and tight - no need to do the Tarzan thing).

Incidentally, the need to apply a bit of force to pull the joints up should inform you as to why packers don't work well beneath T&G chipboard flooring