Suggestions for thin joist for loft

[tompajo]

I am building a garage where I want a section with a loft. The area of the loft is 3.2 x 6.2 m. The problem is that I want to maximize the height, meaning I want the floor structure to take up as little space as possible. I need suggestions for solutions. Would, for example, 45x70 beams with cc 300 and 22 floor particleboard suffice?

 

[igelkotten82]

No. Check with KL wood (cross-laminated timber). Should offer the thinnest joist layer and you avoid both laying a floor and cladding the underside. According to the wood guide, you should manage with a total thickness of 100mm for the joist layer. https://www.traguiden.se/konstrukti...kt/5.1.1-plattbjalklag/?previousState=1000000

Alternatively, a load-bearing sheet with the thickest metal sheet. Risk that it may feel a bit unsteady but should help to screw flooring chipboard on the top and bottom. https://www.ruukki.com/swe/b2b/prod...heets-details/load-bearing-sheet-t70-57l-1058

 

[tompajo]

No. Check with KL trä (cross-laminated timber). Should offer the thinnest floor structure, and you avoid both laying flooring and cladding the underside.
According to the wood guide, you can manage with 100mm total thickness on the floor structure.
[link]

Alternatively, a load-bearing sheet with the thickest sheet. Risk that it might feel a bit shaky, but should help to screw flooring chipboard on the upper and underside.
[link]

I've been in contact with Martinsons regarding KL-trä, but they just say they have too much to do, so they can't take on such small projects. Otherwise, I also thought it seemed like a very good option.

I wasn't aware of the profiled sheet. It looks like an interesting alternative. I'll check it out. Thank you very much for your help!

 

[igelkotten82]

Otherwise, check with the hardware store if they might stock KL-trä instead of ordering directly from Martinsons.

Another option might be to buy glulam beams and glue them together yourself into a solid wooden floor. If you buy, for example, 115x495, it should both be quick to lay the joists and can be done manually without a crane, unlike a KL-skiva.
Glulam is available in different quality grades where you could choose a finer quality if you want it as a finished floor on the loft. It is likely possible to order in exactly the length you want since it involves a lot of material.
However, I should say that I don't know if a solid glulam floor has the same load-bearing capacity as a KL floor or would cause unpleasant swaying in the floor, or if you would need to go up a thickness. Or if glulam beams work even when they are loaded flat.
But it might be worth trying by testing a beam and loading it? When it comes to wooden joists, there is usually no risk of it collapsing but rather deflection (which results in it feeling wobbly and uncomfortable to move on) that determines the dimensions.

 

[tompajo]

I have also looked at glulam beams but they build too much in height if they are to handle the sway. Their strength is when used vertically. If you lay them down, they are not nearly as durable. And since it's precisely low height I'm after, that's off the table.

I contacted Ruukki about their load-bearing sheet metal, but unfortunately, I encountered a really unknowledgeable salesperson who couldn't answer what their sheets could be used for and what spans applied. His response was that he didn't deal with free building advice. It was obvious that he didn't have a clue about his own products. I will try to contact someone else at Ruukki who hopefully knows what he's selling

 

[witten]

My proposal is a solid wood floor structure using 45x95 joists with screw-glued 22 mm chipboard flooring. Span of 3.2 m. Deformation in the serviceability limit state is 7 mm, calculated natural frequency and velocity response are also okay (a measure of the acceleration of the floor when someone jumps on it). If deflection is less important, you could probably go down to 45x70, but it will feel a bit wobbly. Regardless, I would test-build a small section to see how it feels.

 

[vectrex]

My suggestion is a solid wood joist floor of 45x95 studs with screw-glued 22 mm chipboard. Span 3.2 m. Deformation in the service limit state case will be 7 mm, calculated natural frequency and velocity response are also ok (a measure of the acceleration of the floor when someone jumps on it). If bounce is less important, you could probably go down to 45x70, but it will feel a bit shaky. In any case, I would test-build a small part to see how it feels.

Why not the above suggestion but screw-glue chipboard on both sides. It will be much more stable than just on the top side because the tensile loads end up on the underside of the joist.

 

[witten]

Why not the above suggestion but screw-glue floor particle board on both sides. It will be much sturdier than just on the top side because the tensile loads are on the underside of the joist.

If a solution has been calculated, there is usually no reason to make it even better at the expense of more work and cost. In this case, it won't be much sturdier either, since the cross-section is already saturated with material that can handle tensile loads, unlike a cassette floor where there is air between the beams. In that case, a board on the underside adds a lot.

Note that the solution I proposed is very stiff, the natural frequency is at 18 Hz, which is good.

 

[tompajo]

My suggestion is a massive timber floor structure of 45x95 joists with screw-glued 22 mm floor particle board. Span of 3.2 m. Deformation in the service limit state case will be 7 mm, calculated natural frequency and velocity response are also ok (a measure of the floor's acceleration when someone jumps on it). If deflection is less important, you might be able to go down to 45x70, but it will feel a bit wobbly. Regardless, I would test build a small part to see how it feels.

Thank you for your suggestion. It is a good and sensible alternative. Unfortunately, it builds about 135 in height, which is too much when every mm counts.

Ruukki's sheet metal was ruled out, but there are others who manufacture roughly the same thing. Plannja70/1mm is a flooring that can be used and builds a total of 88 mm in height including floor coverings. So that's probably what I'll use. Another advantage of Plannja's variant is that it is easy to run electricity etc. in the grooves.

 

[frma71]

Hi, did you go with a sheet metal floor? I'm having the same thoughts.

Thanks for your suggestion. It's a good and sensible option. Unfortunately, it builds about 135 in height, which is too much when every mm counts.

Ruukkis sheet metal was out, but there are others that manufacture roughly the same thing. Plannja70/1mm is a floor that can be used and it builds a total of 88 mm in height including floor covering. So it will probably be what I use. Another advantage with Plannja's variant is that it's easy to run electricity, etc., in the grooves.

 

[tompajo]

Hi, did you go with a sheet metal floor structure? I have the same thoughts.

It became a steel floor structure. I used square profiles on which I laid a pine floor directly and plaster underneath. You can't make it any lower than that unless you use carbon fiber or some other super material.

 

[frma71]

Ok, thanks for the response. For me, it was double 45x70 with cc30, screw-glued chipboard on top and screw-glued OSB and drywall underneath (115.5mm). What dimension and cc did you have on the square profiles?

 

[tompajo]

The profiles were 40x70 mm and 5 mm wall thickness.

 

[Ingenjörssnickarn]

Reviving the thread... I see that member Witten, among others, seems to be calculating things. I have built a sleeping loft with a minimal 45x70 cc30 with screw-glued floor chipboard. Rock steady, though only a span of 4m and a supporting partition wall underneath (i.e., practically only a 2m span).

Now I'm building something completely different that I've been contemplating for several years - the sauna. I've set myself the goal of building the thinnest sauna benches in the country. After much more advanced considerations involving vertical strips of K-plywood, metal profiles, etc., I've finally settled on the following:

A span of 1.8m with a 45x45 screw-glued frame, 2 crosswise short beams at each third of the length, asp planks 21x90 screw-glued on top and at the front. Lastly, and perhaps most importantly - screw-glued perforated straps 1x20mm under each 45x45.

Spontaneously, this should hold for both standing and sitting without excessive flex, what do you think?

 

[Ingenjörssnickarn]

Once a solution has been calculated, there is usually no reason to make it even better at the expense of additional work and cost. In this case, it also won't be much more stable because the cross-section is already saturated with material that can handle tensile loads, unlike a cassette floor where there is air between the joists. In that case, an underside board adds a lot.

Note that the solution I suggested is very rigid, with a natural frequency of 18 Hz, which is good.

Witten, curious about your view on my revived question below! Thanks in advance.