Laying wood floor - insulation, T&G direction & ply flex

[AWC79]

Hi guys - hoping for some advice - sometimes internet provides too much to know whats right!

Putting new floor in hall and lounge - t&g solid oak boards, 20mm x 185mm and 1200-1800 lengths, decent stuff. Due to floor heights with adjoining rooms and age, i'm replacing old boards with ply - thickest i can manage is 12mm and that still means 3-5mm diff with other rooms. Joists are 40mm centres.

When heating was replaced, plumber attached new pipes to the underside of the joists which makes me reluctant to insulate between joists in fear of freezing pipes under solid floor - they're push fit, foam lagged (tho not corners), am i right to worry (live in Surrey so not exactly Iceland, but...)?

I'd like to run new boards in line with joists due to doors, light etc. Given width of boards, 12mm ply and 40mm joists... is this wrong? Am worried not insulating might mean more moisture so promote flex - will obv be using underlay with vapour barrier and some thermal resistance.

Don't mind chucking in noggins (seems good practice anyway?) but if i do, how far apart should they be? Lounge is about 24m2, so at eg 100mm i'm looking at 50 noggins which feels a lot? If its all a bad idea given thickness of ply then do say.

Last question - i'm assuming expansion gap at door between hall/lounge is a must - would just rather not for continuous look?

Sorry for for all the questions, any help appreciated.
Andy

[Job and Knock]

When heating was replaced, plumber attached new pipes to the underside of the joists which makes me reluctant to insulate between joists in fear of freezing pipes under solid floor - they're push fit, foam lagged (tho not corners), am i right to worry (live in Surrey so not exactly Iceland, but...)?

I understand your concerns, but if you are worried why not finish off the lagging by doing the elbows ("corners") using mitre joints on the lagging (and taping where the pieces join using aluminium foil tape)?

I'd like to run new boards in line with joists due to doors, light etc. Given width of boards, 12mm ply and 40mm joists... is this wrong? Am worried not insulating might mean more moisture so promote flex - will obv be using underlay with vapour barrier and some thermal resistance.

Running in-line with the joists is not really the right way at all as you will get flex as a result of the boards not being adequately supported. Whilst adding solid strutting ("what "noggins" between floor joists are referred to as) will stiffen the floor structure you'll lose some of the advantage by laying the flooring in line with the joists (as opposed to at right angles) because even with additional strutting there may still be sufficient movement between the joists to cause cracking in the long joints of the new floor should adjacent joists deflect differently under (uneven) loading. If at all possible I'd consider ply boarding the floor with at least 12mm (1/2in), preferably 18mm (3/4in), hardwood ply laid at right angles to the joists and cut to fit so that all the board ends land "half and half) on joists and are fully supported before laying down your new flooring. That will even out loading on the existing joists somewhat better (especially if glued and screwed down). I understand why it may not be feasible to do this. Doing things your way will require solid strutting wherever there are end joints in the new flooring (possibly restrictive as you won't end up with a random pattern of plank ends which can look a bit odd), together with 400mm centres (or the same centres as the original joists) to even approach the same level of stiffness, I'd say.

Don't mind chucking in noggins (seems good practice anyway?) but if i do, how far apart should they be?

See above. Normally strutting is employed in larger floor structures to stiffen them and make them less bouncy. For a 2.5 to 4.5 metre span a single row of strutting is recommended at centre span, whilst for 4.5 metres and above strutting at 1/3 and 2/3 span is the recommendation (Source: NHBC Standards, 2011). Solid strutting would be the same thickness as the joists and at least 75% of the depth. In your case, though, the strutting will need to support the joints, hence these rules don't really apply. This may mean a lot of solid struts to support your joists which is probably not what you want to hear

[AWC79]

Thanks for taking the time Job and Knock, really appreciated.

I think I'll take your advice, do the corners and insulate properly between the joists.

I was planning on using 12mm ply in addition to the boards anyway. If you think it'll be ok, I think I'll go to the effort of putting in a good few struts (maybe 3 or 4 per 4.5 span), lay the ply and then see if there's any flex.