Floor joists, which spacing?

[LyckeboPetter]

Can anyone here determine if I can manage with cc 600, or if I need to go with 300 between the floor joists? And what class should I choose? C24?

Span of 4200, width of 4400. 45x195.
There will be tongue and groove wooden flooring on top. Living room (no pianos or safes will be placed on it).

Old house, so it doesn't need to meet modern building standards. It just needs to feel solid!

 

[justusandersson]

Denser c/c allows the floor structure to handle a larger load. However, it affects the deflection only marginally. Limiting the deflection is primarily a comfort issue that affects the experience. With a span of 4.2 meters, even 45x220 C24 + screw-glued chipboard is too weak with regard to deflection. 45x195 is not an option. There are a few alternatives. The simplest is to use glulam. For example, 66x225 c/c 600 with screw-glued chipboard. Or, you can place 90x225 at c/c 1200 and insert (with joist hangers) 45x95 c/c 600 across. Then you can lay tongue-and-groove wood directly on the beams. If you're inclined, you can make your own wooden I-beams from 45x120 C24 by screw-gluing two horizontal studs to a vertical one. This will be very sturdy and not particularly expensive.

 

[LyckeboPetter]

Closer c/c allows the joist structure to handle a larger load. However, it affects the deflection only marginally. Limiting deflection is primarily a comfort issue that affects the experience. With a span of 4.2 meters, even 45x220 C24 + screwed and glued particleboard is too weak regarding deflection. 45x195 is not an option. There are a few alternatives. The simplest is to use glued laminated timber, e.g., 66x225 c/c 600 with screwed and glued particleboard. Or you can place 90x225 on c/c 1200 and embed (with joist hangers) 45x95 c/c 600 crosswise. Then you can lay tongue-and-groove wood directly on the beams. If you are inclined to be crafty, you can make your own wooden H-beams from 45x120 C24 by screwing and gluing two horizontal joists on a standing one. It's very sturdy and not particularly expensive.

Oh dear... The floor we just removed had a c/c of about 650 and 65x180 joists. We didn't notice any deflection.

How do noggins affect the situation?

 

[justusandersson]

Why did you throw it out then? It was probably 3x7 inches, which was almost standard at one point. Blocking helps distribute point loads to surrounding beams to reduce deflection, with marginal effect unless you have a lot placed with great precision. However, blocking does not increase the load-bearing capacity of the floor structure. I think it's a simpler strategy to reinforce the beams rather than using blocking.

 

[LyckeboPetter]

Why did you throw it out then? It was probably 3x7 inches which was almost standard at one point. Blocking helps to distribute point loads to surrounding joists so that the floor’s deflection decreases, with marginal effect unless you have very many set with great precision. However, blocking does not increase the bearing capacity of the floor framing. I think it’s a simpler strategy to reinforce the joists rather than using blocking.

Threw it out due to fungal attack and rot. It's from sometime in the 1920s, but some genius redid the foundation work and covered the ventilation openings.

The beams are not particularly high, so if I go with 225 they'll come down quite far into the foundation.

Gluing H-beams from 45x120 sounds simple. But tricky to fit insulation on the underside! Or do you lay them flat then, so they become 135 high (3x45)?

 

[LyckeboPetter]

And on which c/c do I place the H-beam then?

 

[justusandersson]

They should be positioned like an uppercase i. The outer measurement will be 120 in width and 210 in height. You can experiment with the composition of the parts according to the same principle. It's only your imagination that sets the limits. Why should you insert insulation from below? c/c is determined by the dimension of the floorboards. 600 is probably suitable if you don't plan to use extra thick boards.