Hello all gathered knowledge!
I am going to build a cabin in loose timber (the slab was cast last fall and now I will be constructing wall panels) and I'm at a crossroads. Due to the limited building height stipulated in the detailed plan, I want to maximize the height of the loft by having the thinnest possible intermediate floor. Initially, the loft will have no rooms and will be used for storage, additional sleeping space, and as a play area for the kids. The wall behind the kitchen will extend all the way up to the outer roof but otherwise, there will be a railing at the loft edge.
A traditional intermediate floor would mean at least 170mm joists at c/c 600 + chipboard flooring + laminate/parquet. (approx. 200mm total) if I calculated correctly.
With CLT, I've been recommended 150mm, but I think it’s possible to reduce the dimension to 100mm or less since the useful load will be relatively low on a loft. Here I figure I can manage entirely without any additional flooring.
For electrical installations, I expect to need battening and to install some form of ceiling board underneath regardless of what I choose. The image below shows approximately what is planned; you can see the supports from the interior walls shining through the loft floor. The total area is about 35sqm. The concrete slab measures 10x7.8m.
CLT will be more expensive, but I believe I can gain about ten centimeters in ceiling height (up or down depending on what height I choose to place the intermediate floor at).
Am I making a mistake in terms of soundproofing, natural frequency, or fire protection requirements? Has anyone built something similar?
Can you indicate which walls can be considered load-bearing? Is the square furthest in under the window on the ridge gable a void?
All walls are at least 95mm and should then be able to be considered load-bearing. Along the exterior walls, the support will rest on a beam or iron of some kind. The square in the middle of the gable is not a void. See the image below for approximate measurements (sorry for the messy measurements).
A fun but complex problem. The question is what load capacity on the beam layer to account for, as it significantly matters. 0.5 kN/sqm as a light storage, 1 kN/sqm as an intermediate option, or 2 kN/sqm as a normal living space? By choosing too low a load, you limit future usability. Personally, I think 1 kN/sqm is a minimum based on your description. The determining factor for the entire loft (if you plan to have the same beam height throughout) is the section that the staircase leads up to.
CLT solutions are exciting, but I wonder if they are justifiable in your case. I don't think you gain much in height. An alternative is to use (solid) glulam that you split in half. Then, glulam beams measuring 90x157 mm on c/c 600 might suffice. If you plan to use ordinary construction timber, it involves 45x220 C 24 beams. That's likely what you should compare with.
I think that a maximum of 1kN/sqm is quite reasonable to consider, as more than that won't be utilized given the ceiling height is already limited towards the walls. Beds and possibly a TV + sofa are what will realistically go up there.
For the area by the stairs, it might be necessary to insert a glued laminated beam or joist. With CLT, it should be possible to glue-screw a glulam beam under the edge to reinforce that section.
Hmm... if it's 45x220 I should compare with, then CLT becomes even more relevant, I had hoped to manage with 170mm with lowered demands on deflection and self resonance.
So you mean splitting 90x315 glue-laminated timber, right?
Yes, of the homogeneous type, i.e., the same timber quality throughout. I forgot to mention that I only considered deflection L/300, but not any bounce requirement. I've looked at what's mentioned about KL-virke in the wood guide and suspect that it will be difficult to go below 15 cm. Getting it into a mountain cabin must also be a problem. It certainly won't be cheap. Note that I assumed you have the same joist height all the way. You gain some in stiffness if the joists run from outer wall to outer wall. On the underside of the joist, you don't need to add anything that increases the height.
You really put effort into the task justusandersson! Cross-laminated timber is definitely more complicated in every way, which I've also realized after talking to suppliers... just the design and drawing materials for cross-laminated timber cost a lot of money.
Your suggestion with glulam beams seems to be the most cost-effective. A rough estimate shows that it's about 4 times more expensive than C24 studs but still 4 times cheaper than cross-laminated timber. With a screwed plank floor, I would gain a few centimeters compared to chipboard + parquet and still get a decent floor.
Adding an extra support beam should be doable as long as it's not so low that you hit your head, but it should be manageable.
Cross-laminated timber is definitely more complicated in every way, which I've also realized after talking to suppliers... just the design and drawing materials for cross-laminated timber cost a lot of money.