Thin flooring

[frma71]

Hi,

I'm planning to build a joist for a loft and of course want it to be as thin as possible. I would like to get it down to 90-100 mm with a span of 2.7 meters. Any tips on how to do this?

- Cross-laminated timber (CLT)? If so, where can I buy something like 3000x900x90?
- Self-supporting profile sheet (like Planja 70) with floor chipboard on top and gypsum/ceiling tiles underneath?
- Any other ideas?

 

[larsbj]

Hi,

I'm going to build a joist for a loft and of course, I want it to be as thin as possible. I would like to get down to 90-100 mm with a span of 2.7 meters. Any tips on how to do that?

- CLT? If so, where can I buy something like 3000x900x90?
- Self-supporting profile sheet (like Planja 70) with floor chipboard on top and plasterboard/ceiling tiles underneath?
- Any other ideas?

Glued laminated timber, e.g., 90*315, placed next to each other. Finished surface on both sides. No need to lay a floor or ceiling then. And easy to obtain.

 

[frma71]

Thanks, good idea, how do you best join the long sides?

/Fredrik

Glulam beams, for example 90*315, which you place next to each other. Finished surface on both sides. No need to lay flooring or ceiling afterwards. And easy to find.

 

[larsbj]

Thank you, good idea, how do you splice the long sides most appropriately?

/Fredrik

Depending on how wide it gets, one option is wood glue. But then all movement will be concentrated in one place.
Alternatively, if you have or rent a router and mill a groove on the sides with a loose tenon made of, for example, 10 mm plywood or a wooden strip.

 

[frma71]

Would it become less rigid or worse to screw and glue two layers of 45x220 in a half-lap joint?

 

[larsbj]

Would it be less rigid or worse to screw-glue two layers of 45x220 in half bond?

Yes, it becomes a bit "softer"
The bending value for 45*220 c24 is 24 MPa
And the equivalent for glulam is 30 MPa

The value is slightly increased by the gluing. But still a difference.
And perhaps not as finished surfaces when there are screw holes etc.

 

[bossespecial]

Just to interject that it is the modulus of elasticity you look at when considering bending stiffness, not bending strength. With the mentioned dimensions, the difference is 13GPa compared to 11GPa

 

[bossespecial]

An alternative can also be to glue plywood layer by layer.

 

[frma71]

Thanks for the suggestion, can't find anyone selling K-plywood that is 3000mm long, feels tricky/fussy to splice in all directions.

/Fredrik

An alternative might also be to glue plywood layer by layer.

 

[frma71]

Thank you, checked in the CLT Handbook. (https://www.svenskttra.se/siteassets/5-publikationer/pdfer/svt-kl-trahandbok-2017.pdf)

- Table 3.7 on page 38 states that CLT with C24 has a modulus of elasticity of 11GPa (the same as construction timber).
- Figure 2.6 on page 27 shows that 90mm is more than enough for a 3 meter span and requirement L/300.

Is it correct to conclude that screwing and gluing 45x220 C24 joists in a half-lap joint would work?

/Fredrik

Just to point out that it is the modulus of elasticity you look at when checking bending stiffness, not bending strength. With the mentioned dimensions the difference is 13GPa compared to 11GPa

 

[bossespecial]

I would probably say that it will be difficult to get it right since the joists are not perfectly straight, which results in poor contact and interaction.
What are you planning to store in the loft?

 

[frma71]

Ok, don't you think it can be pulled together with clamps and screws? Or do you have to plane everything? I can probably accept that it won't be aesthetically perfect.

One is a sleeping loft for my daughter, and what will be stored on the other one only my wife knows but I don't guess any particularly heavy items.

I would probably say that it will be difficult to get it right since the studs are not perfectly straight, which gives poor contact and cooperation. What will you store in the loft?

 

[frma71]

Want to follow up here in case someone else is searching. Will be screw-glued 22mm particle board on top of 70mm joists and screw-glued plywood (~12mm) and gypsum (6mm) on the underside. ~110mm