how far is the joist on the other side, from that edge of the partition wall?
Standard NMP is only 6mm thick & as you’re over boarding just a 16mm thick T&G timber floor (they may have been power sanded at some time), I would be concerned about providing enough rigidity. NMP also comes in 9mm I believe which would be better but, personally, I’d go for 12mm Hardiebacker or, as your floorboards are only 16mm thick, 15mm WBP.
The BS & adhesive manufacturers’ minimum spec for over boarding existing floors (usually 18mm) is 15mm but it’s generally recognised that 12mm is sufficient in most cases. For tiling over new suspended floors, 25mm WBP is specified but, again, you can get away with as little as 18mm in low use/load areas but it depends on the floor structure. What size/pitch/span are your floor joists?
The depth is OK but 1½” is a bit slim, they are usually 2”; it’s just as important to know the joist span between supports.The joist seem to be 8.5 x1.5 with a 40cm gap. I'm not sure on the span.
You must ensure the original boards are not loose, fix with additional screws as necessary. Screw through into the joists not just into the top of the original floorboards; no need to use grip adhesive.If i did go for 15mm ply would that help if the floor boards were a bit on the loose side? Also, should i be screwing the ply into the floor board and/or the joists and is there any need to use grip adhesive?
Standard NMP is only 6mm thick & as you’re over boarding just a 16mm thick T&G timber floor (they may have been power sanded at some time), I would be concerned about providing enough rigidity. NMP also comes in 9mm I believe which would be better but, personally, I’d go for 12mm Hardiebacker or, as your floorboards are only 16mm thick, 15mm WBP.
How are the joists supported either side of the room? Why cant you just stick in another full with joist supported on joist hangers instead of all the noggins & 1 x 1 bearer which won’t offer much support anyway. As it’s so close to the wall, it will only be lightly loaded unless it’s where one of the bath feet ends up so doesn’t necessarily have to be the same size as the rest of the joists.
You could fit a noggin between the joists at the unsupported end to take a joist hanger, fit a joist hanger to the support on the left hand side of the room & fit a new joist.
I know it’s an inconvenience but sometimes you have to undertake additional work to get the right solution. Lift floorboards to gain access if you have to, move a pipe if it’s in the way; you can’t make an omelet without breaking a few eggs.
Don’t use nails for the floorboards or over boarding, use screws.
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