Hello,
We currently have a deck that is 1 x 5 m and will extend it 2 meters onto the lawn so it becomes 3 x 5 m. The existing deck has joists with the dimension 45x120 which I plan to continue using. I plan to place the joists on slabs that are 30x30 cm. What I'm wondering about is whether the length of the joists is okay or if additional support is needed.
According to the description on page 22, the maximum free length for the joists I intend to use can be 1.81 m, which is shorter than the 2 m I planned to extend the deck. However, since the slabs are 30 cm, if I deduct that, I'm at 1.7 m which should then be okay. So the question really is if I can calculate it that way?
https://www.svenskttra.se/siteassets/6-om-oss/publikationer/pdfer/svt_lathunden_2018.pdf
If anything is unclear, I'm happy to answer questions.
We currently have a deck that is 1 x 5 m and will extend it 2 meters onto the lawn so it becomes 3 x 5 m. The existing deck has joists with the dimension 45x120 which I plan to continue using. I plan to place the joists on slabs that are 30x30 cm. What I'm wondering about is whether the length of the joists is okay or if additional support is needed.
According to the description on page 22, the maximum free length for the joists I intend to use can be 1.81 m, which is shorter than the 2 m I planned to extend the deck. However, since the slabs are 30 cm, if I deduct that, I'm at 1.7 m which should then be okay. So the question really is if I can calculate it that way?
https://www.svenskttra.se/siteassets/6-om-oss/publikationer/pdfer/svt_lathunden_2018.pdf
If anything is unclear, I'm happy to answer questions.
Best answer
I had similar considerations before my patio construction this past spring. I had joists measuring 45x145 and also rested them on slabs of 300x300. The distances between my support beams were 2.2 meters, but Swedish wood standards recommend 2 meters if I recall correctly. Adding an extra support beam seemed unnecessarily difficult, so I justified to myself that Svenskt Trä probably has some leeway in their guidelines since they have an economic interest in the matter, so a few centimeters here or there don't matter. And if it turned out to be too weak, you'd only fall a maximum of 145 mm to the ground. I had my uncle, who teaches construction engineering at a university, visit during the build. He laughed and said the patio can now support greater weights than the floor structure in his house.
The finished patio is stable. No sagging, and I am very pleased.
I think your reasoning holds up well!
The finished patio is stable. No sagging, and I am very pleased.
I think your reasoning holds up well!
What governs the recommendations is the flex in the deck. It's not something that will collapse if you slightly adjust the distances by a few percent.
What could happen is that you might feel the floor flex a bit when you walk.
What could happen is that you might feel the floor flex a bit when you walk.
I agree with your reasoning and Tungbetong's. In this type of design tools, there are large safety factors according to current standards. Nothing will collapse either, provided that it is reasonably solved in terms of construction technology. What often determines dimensions is deflection, thus the only risk is that you experience flexing due to this minor deviation from the recommendations.
Thank you so much for all the quick responses. Now I feel more confident with the construction. Have a great summer everyone.
I want to emphasize what @Nyfniken writes that deflection is the most important criterion when assessing the appropriate length of beams. It is primarily a comfort issue even though it belongs to construction rules. The risk of breakage is minimal. Decks are particularly sensitive because the decking boards on top of the beams are not tongued and grooved and therefore do not work together. I would pull in the last row of the slab by a few decimeters and instead let the beams cantilever the corresponding distance.
One thing I've thought about afterward is whether the tiles should have been laid rotated 45 degrees so that the joists span a longer distance than if the tiles are laid perpendicular to the joists. In other words, the joists form a diagonal X over the tiles. That would gain a few cm in shorter span.