Noggins needed under plywood?

[Kaymo]

I have just built a new suspended timber subfloor, 3m length 95x45mm joists at 450mm centres. I am about to put 18mm plywood on top, perpendicular to the joists. The short edges have been cut so they meet on a joist, but although I have a fitted a couple of noggins across each joist they are not under the short edges. I maybe should have planned this better, but when putting together the subfloor it felt like the noggins worked better a bit nearer the joist ends rather than 1.2m in.

I will be refitting 18mm solid wood flooring that I lifted, on top of the ply, with tongue-tite screws and these will run perpendicular to the unsupported long edges of the ply, so I am thinking than noggins under these edges is probably unnecessary as the wood flooring should pull it all together?

 

[JohnD]

Put them under all unsupported edges.

If you don't, there is enough deflection on the boards that if you are standing on one, it will be slightly lower than the board next to it.

 

[freddiemercurystwin]

Why have you used such slender joists? Well undersized.

 

[Kaymo]

Put them under all unsupported edges.

If you don't, there is enough deflection on the boards that if you are standing on one, it will be slightly lower than the board next to it.

Ok, thanks. There isn't very much flex at the moment on those edges and I had hoped the solid boards running across them would give enough support. I presume I could get away with less deep noggins, given the only purpose is to support the edges.

 

[Kaymo]

Why have you used such slender joists? Well undersized.

Not undersized. I consulted the span tables beforehand and they are well with tolerances. Granted they are thinner than is typical but I had a very shallow void and the previous joists were resting on the ground in places.

 

[aveatry]

Not undersized. I consulted the span tables beforehand and they are well with tolerances. Granted they are thinner than is typical but I had a very shallow void and the previous joists were resting on the ground in places.

Are you saying that the void is only 95mm and the "joists are touching the floor - then there is no airflow under there. And at 95mm you would be better off filling with concrete. But at 95mm you would not even have room for any insulation slabs.

 

[Kaymo]

Void is about 150mm now in most places. Will get 90mm insulation between joists.

Coxncrete would not be a good idea, no dpm in walls and some issues with ground levels so the approach I am taking is probably the best of a number of bad options.

 

[Kaymo]

Are you saying that the void is only 95mm and the "joists are touching the floor - then there is no airflow under there. And at 95mm you would be better off filling with concrete. But at 95mm you would not even have room for any insulation slabs.

Think you misread my post. The old joists were touching floor not the new ones

 

[freddiemercurystwin]

So the joists are spanning 3m?

 

[aveatry]

So the joists are spanning 3m?

I think I see where this is going - I am not a builder but I dont think those joists are big enough, maybe but some blocks under every X? distance. 95x45 -- Those are smaller than you would use in your garden for some decking.

 

[Kaymo]

So the joists are spanning 3m?

Ah, sorry, I see the concern. No, the joists are supported at a couple of points in the middle. I think the span tables allowed between 1.7 and 1.5m depending on the loading. I have a max clear span of 1m.

This was the tradeoff I was working with. Its a long story but the floor at the front of the house appears to have been lowered in the 1880's and its left a suspended floor with very shallow void and as the Victorians dug down to lower the house, there is no scope to dig down any further or the footings would be compromised. The floor I took up had deep joists touching the floor in places, blocking airflow etc and rotted wood and fungus all over. Shallow joists with shorter span allowed me to get just about the minimum gap under the joists for proper ventilation.

 

[Kaymo]

I think I see where this is going - I am not a builder but I dont think those joists are big enough, maybe but some blocks under every X? distance. 95x45 -- Those are smaller than you would use in your garden for some decking.

See my other post, I have supports.

 

[foxhole]

T&g ply?

 

[Kaymo]

T&g ply?

No, regular square edge

 

[Kaymo]

Just thought I would post the following alternative opinion on this (although I do still appreciate the advice above). This is from an American manufacturer, but the principles are the same, and they say blocking (noggins) not required if solid wood floor going on top. They don't explain why, but it's obviously because the perpendicular T&G finish floor is spanning all the ply joints. I knew I had seen this before but just stumbled on it again last night.

I tested the theory this morning and if I step on the long edge of one plywood board, at the point between two joists I can definitely see the slight flex that John mentioned. However, if I place even one of the shortest sections of the wood flooring across the joint and stand in the same place, I can no longer see any flex. Even if there is a slight amount of deflection that I can't see, I would imagine that by the time all of the boards are interlocked in the T&G and screwed into to the plywood it should be fairly rigid with the load spread out rather than point loaded on the join. I can totally see why supporting all joints is standard practice though, as trades wouldn't always know what kind of flooring is going to go on top and this clearly wouldn't be appropriate for tiles or carpet.