I-Joist centre distance

[Kerlaan]

Hello

I intend to construct a cross joist loft floor for lightweight storage in my single storey home in France that has a trussed rafter roof (pls see attached figure). 1979 construction with no rot or damage. No water tanks creating additional loads.

The existing joists are 35wx95d on 600mm centres. The building is 8.4m wide, the maximum unsupported span is 4.3m in the region where the flooring will reach but this will stop at where the truss supports which are, a 2.80m central span.

I intend to make I-joists by grooving 10mm deep on 38wx47dx2400mm treated timber and 2400x120dx9mm OSB to obtain a joist depth of 175mm to provide a 270mm insulation void.

The floor will be Kronospan OSB3 16mm thick grooved and screwed to the I-joist.

What centres should my I-joists be please? 600mm seems appropriate, with diagonal cross bracing.

Thanks in advance.

 

[John D v2.0]

My concern, apart from the obvious site gluing of structural parts, would be the excessive depth to width ratio. Are they going to be restrained? They'll need a lot of noggings to stop them twisting.

 

[Kerlaan]

Thanks. I find it hard to believe I am working with excessive depth to width ratio. The numbers I offered are close that shown at https://www.jamesjones.co.uk/jji-joists/ where their ratio is even greater on depth. I understood OSB to be stable too. I agree the need of noggings; as I said, diagonal cross bracing. That is for lateral stability with the further benefit of preventing twist. The final element is the 2400x1200 OSB boarding screwed down to prevent buckling and creaks.

It is the spacing of the beams I need help with please.

 

[John D v2.0]

For that flooring you'd normally need about 400 spacing, but if it's lightweight storage then you won't be sitting on a sofa up there, so 600 would be OK. But normally you'd consult the manufacturer of the joists also to confirm their suitability, or check the span tables for the grade of timber.

 

[John D v2.0]

What glue are you using on the housing joints?

 

[Kerlaan]

Cool. I am happy to perform all the stress analysis if I were working in linear metals (I design wings and understand bending, shear, torsion etc). Timber is outside of my experience and I never assume nowt. 400mm to 600mm sounds good to me but I shall consider the position more. I will ponder further and see if there is other perspectives. For what it is worth, I am seeking to not use those floor spacing posts. They seem to possess inherent instability and rely on the shear of the screws to the joist and floor!

Glue has yet to be defined. Suggestions welcome.

 

[John D v2.0]

My suggestion would be pu glue. Expensive but it sticks well. However you have to carefully control the gap between the thing you're gluing to be the minimum, otherwise you don't really get the strength you expended.
I just used about 4 tubes of it (40 quid) for plating the sides of notched joists. I screwed them down hard with 5x60mm screws to get a good clamping force for the glue, which seemed to work reasonably.
I should have asked here first, as I didn't quite do everything perfectly, but it's better than it was (and plating notches is not structurally critical)

 

[Kerlaan]

Thanks. PU is on the list. I was also thinking Cascamite (also known as Polymite). It is especially formulated for exterior joinery and boat building since it has weather exposure ability. A synthetic resin, has good gap filling properties and suitable for load bearing and laminating. It is mould resistant too which could be useful. The wet of weather exposure is not something I expect in my dry attic space!

 

[John D v2.0]

You also have to make sure the glue in your osb meets the same standards for structural stability as the ones you're planning to use.

 

[Kerlaan]

Kronospan board is for load bearing structures per: - http://kronospan-express.com/en/pro...b/osb-3-superfinish/osb-3-superfinish-22#c=39. I am reasonably happy with the structure in that regard (have used it for wind-tunnel work too).

 

[chappers]

cascamite would be over the top, something like caberfix D4, it's about a third the price of say evostick PU.

 

[Kerlaan]

Good steer chappers. Thank you.