Sorry - long post with lots of piccies
Our architect specified 18mm flooring ply screwed down, for our extension, and we didn't know at the time about the matter, but we are very glad, cost 30 quid a sheet so maybe double that of chip board but you don't really get a peep out of it.
A couple of points here, John - almost anyone in the structural side of construction will tell you that the price of 18mm WBP plywood is (and has for a long time) been more than £30 a sheet (these days try more like £45 to £50 for decent quality 18mm WBP plywood
if and when you can get it) and also that the quality of WBP hardwood plywood has deteriorated markedly over the last 10 or so years, principly since the Chinese came into the market and started to edge out (IMHO better quality) Indonesian, Malaysian and West African supplies. Sadly Brazillian stuff, which was often very high quality, disappeared even further back - possibly because they've more or less clear felled large areas of rain forest and are running out of timber to turn into plywood. You also imply that chipboard material is the problem - well, sorry, but it isn't. The problem is more often than not
poor installation practices combined with lack of adequate supervision, but that also occurs with plywood sub-flooring, although I will admit that with plywood onto traditional solid wood joisting things are slightly less critical and you can get by employing a more slap-dash approach (even if it does result in creaking at a later date because no tongues were installed). One other point, which is becoming far more important - hardwood plywood of the types so often used for sub-flooring in the past is a rain forest product and as such
it is unsustainable in the longer term and is a major part of the ongoing problem of deforestation in the tropics. Chipboard and OSB on the other hand are manufactured from plantation-grown softwoods, a rebewable resource where at least some of the materials used in their production can be locally sourced (becoming increasingly important in our post-Brexit economy, let alone in terms of carbon emitions)
There are one or two tiny creaks in the front bedroom that we retro fitted the same, but those are 100 year old twisted up joists with plenty of notches in them.
Did your installer groove every board edge and glue in a 1/4in plywood tongue? I've rarely seen that done in domestic work, whereas it was often done in commercial work
because it minimises the chances of creaking.
...a better answer would be "whatever the building yard would most commonly have". They certainly looked around that though, similar to insulation boards and OSB sheets in size. I'm not sure what centres but I think 50-60cm - I recall it went through an engineer who signed off on all the structural side if it.
Modern chipboard and OSB sub-flooring products tend to come in 600mm wide "planks" with tongue and groove edges all round. AFAIK wiith the exception of some specialised plywood sub-flooring (e.g. spruce - which comes as 2400 x 1200 mm T & G sheets) full size sheets come with square cut edges - so if you don't want them to creak you need to route out a groove in the edges and install a glued-in tongue. I've pullked up and replaced quite a bit of chipboard in recent years but verey rarely seen this done on non-commercial jobs.
500 to 600mm centres is very wide and when you do see iy it tends to be for composite I-beam joists like these:
or space joists:
or the timber equivalent, the open web flooring truss:
The thing about those, they are generally specified for use with a stressed skin sub-floor (either plywood or chipboard) - which means the sub-floor must be glued and screwed (or nailed) T&G sheets where the boards are glued top the tops of the joists and also at the tongues on the edges. These days S/Es also tend to specify 22 or 25mm flooring on such wide centres. "Just screwing it down" onto these modern joists simply won't work
When a modern floor does creak and you can feel it 'bounce' slightly when someone walks heavily on it, what exactly is going on? Is it the flooring moving in relation to the joists as they bend slightly and getting some play? Is the role of the glue to allow a slight flex without it rattling/creaking?
In a traditional solid timber joist floor structure the bounce is often caused by the joists being marginal for the applied load, assuming that there are no other structural defects such as over notching or over drilling of the joists or problems with their end supports. When you load a joist, if it is properly secured to the sub-floor (by being glued and screwed) the only thing the joist can do if it is heavily loaded is to bow a little and then twist sideways (note that the glue reduces any tendency to bow by supplying a far greater connection area). The way that twist is cured is by adding herringbone or solid strutting between the joists (BTW tyhese are
NOT noggins). Solid strutting should be be the same thickness as the joists and between 80 and 100% of their depth and very tightly fitted to function correctly
Above: Modern metal herringbone strutting
Below: Traditional wooden herringbone strutting
Above: Solid strutting
Below: How strutting transfers side load (twisting)
What the strutting does is to transfer the sideways twisting load into the adjoining joist, and the one adjoining that and so on. This results in a considerably stiffer floor with noticeably less bounce in it.
Whilst herringbone strutting (modern and traditional) is lighter and performs better than solid strutting, it is effectively only possible to install it before the floor is fitted, meaning that solid strutting is the choice for retrofits When installed in new builds it is cheaper and faster than herringbone because it is often possible to use joist end offcuts (where joists have been trimmed to length) to make up the strutting
Please note that the following applies to conventional timber joisting - for composite methods (as illustrated above) input from a structural engineer to cure structural faults may be required
BTW as an aside you might like to know that traditional dance floors were deliberately built with slightly undersized joists and minimal strutting to ensure that the floors actually had "spring" or "bounce" in them., This is taken to extremes in theatre stages where getting the appropriate amount of "spring" in the stage floor can be essential to prevent dancers (e.g. ballet) physically injuring themselves by performing on too "dead" a surface
...as a layman I got the impression you basically just had to securely fasten your boards to the joists, allow a slight expansion gap and that was it
Always interesting to learn more about the proper way in case I ever feel tempted to do this myself. I would be
gutted if I pulled up my pitch-pine boards, spent loads of time on money on new flooring, ages installing it only to find it was worse than the 100-year-old floor I'd replaced!
Even in "the good old days" there was a bit more techique than that. When you get involved with modern lightweight structures it becomes a bit more technical than that and manufacturers'/specifiers' "advice" needs to be obtained and adhered to for best results
At the end of all that my brain hurts
