Here's another thread about timber sizing for a pergola, but I can't find anything previously that exactly matches my question, so I thought I'd take a chance and create a thread here.
Anyway, this is roughly what I'm thinking. Apologies for the rough sketch 😅
I will want 3 posts (possibly 115x115 impregnated) at the front, and then attach the entire structure to the facade. The depth will be roughly 4.5 meters, and the width 4.3 meters; it's primarily the 4.5-meter part I'd like some input on.
Option 1:
Screw-laminate two pieces of 45x170 and use that for the entire "frame" and then single 45x170 as joists in between.
Option 2:
Same as above but 45x145 to reduce the weight of the whole thing a bit?
Option 3:
Glulam beams? However, it feels like they'd be much more expensive and bulky?
To make it all a bit more complicated, I would like (if possible) to size it so that a roof can be added later without needing to reinforce things when the time comes for that.
How does the deflection compare with 45x170 versus 45x145?
There was never any pergola when the thread was started, but now the urge is coming back when one wants something to protect from the sun, so I'm making another attempt here. 😉
I'm still a bit inclined towards option 1, but does it work with the long span of 4.5 that it will have?
I have also looked a little at glulam and k-rauta has glulam in the dimensions 56x225, but it gets quite expensive compared to "regular" timber.
Glued laminated beams are generally available as "standard" up to 6 m in hardware stores, and longer ones are not a problem to order (transportation might be more expensive). For the calculations, use Svenskt Trä's tool, and then you'll have to consider whether you want to deviate from the safety margin.
How much in the red can one tolerate? It's not a precision build in that sense, I think. I was thinking of having about 60cc between the roof rafters so I'll probably fit about 7 of them.
If I use these measurements which match reasonably well, I get the following result, but what surprises me is that it's the rafters that are the weak point; I thought it would be the beam itself that is most important. The roof rafters are just supposed to lie there without any roofing on top..?
I was thinking of using 45x220 for the beam and 45x170 for the roof rafters but this service doesn't agree with me..
It sounds reasonable that the rafters are more robust than the joists. The rafters have a span of 4500 mm while the joists' span is only 2150 mm, and they are also continuous over three supports.
The recommended requirement for maximum deflection in the rafters in your case is 4500/300 = 15 mm.
Note that with those dimensions, the bärlinan is most stressed, achieving a utilization rate of 99% compared to the takbalkarna which have 79%. However, what becomes the determining factor is the deflection, which is greatest for the takbalkarna due to their span.
If I go up to 45x220 for everything, I get a utilization rate of 79% on the roof beams and 78% on the bearer. It's the issue of max deflection that I need to consider how important it is. 21mm is quite a bit, of course.
If I increase to 45x245 for everything, I'm almost there, then I get 16mm deflection and a utilization rate of 63% on the roof beams. However, it's hardly possible to find reasonable pressers for those dimensions, usually max 220, right? To reduce deflection further, I'd have to look at glulam, which would be a significant price difference.
How would you evaluate this? How much does the deflection on the roof beams of a pergola really matter?
You have an overdeflection of 6 mm in the middle of the beam (one usually refers to L/300, which in your case becomes 4500/300 = 15 mm, and currently, you get 21 mm). This should be weighed against the cost of increasing the dimension as well as other beam types.
Note that L/300 is not a requirement; it will not affect load-bearing capacity but is more aesthetic.
If I go up to 45x220 on everything, I get a utilization rate of 79% on the roof beams and 78% on the carrier beam. It's just that thing with max deformation I need to think about how important it is. 21mm is quite a bit, of course.
How much does that deflection on the roof beams affect a pergola?
The lumber is only meant to support itself and two by nine planed is more than enough for that.
You have an overrun of 6 mm in the middle of the beam (usually compared to L/300, which in your case is 4500/300 = 15 mm, and currently, you get 21 mm). This should be weighed against the cost of increasing the dimension, as well as other types of beams.
Note that L/300 is not a requirement, it won't affect load-bearing capability; it's more about aesthetics.
Ok, thanks! If it’s just aesthetic, I think I dare to chance on 45x220. 21 mm deflection on 4500 mm shouldn't be too visible to the eye unless you hold something flat against it to check, I guess. It's hard to relate to that number without seeing it directly, but it doesn't sound too bad.
Adding noggings between the ceiling beams doesn’t do anything for the deflection, I guess? It only increases dead weight without providing any real improvement.
Reduced risk of bending, but it will look much worse. My advice is to skip kortling.
You're not planning to hang any swing chair or similar? If not, I would settle with 45x220 and be careful with the mounting on each side.
Good, I'll follow your advice I'm planning to notch the support beam and recess the roof beams a few centimeters, that should make it as sturdy as possible and you avoid a lot of angle brackets and such.
Well, maybe a swing chair/hammock in the front and in the front beam resting on the posts. Otherwise, nothing heavy will be hung from the ceiling, just some simple lighting and such.
Good, then I'll follow your advice I was thinking of detaching the bearer line and recessing the roof beams a few centimeters, that way it will be as steady as it can get and one avoids a lot of angle brackets and the like.
Well, maybe a hanging chair/swing, but then at the front and on the front rule resting on the posts. Otherwise, nothing heavy will hang from the ceiling; it will just be some simple lighting and the like.
It will be great!
Feel free to update with a picture when you're done.
Good, then I'll follow your advice I'm planning to notch the beam and recess the roof beams a few centimeters, it will be as sturdy as it can get and you avoid a lot of angle irons and the like.
Well, possibly some hanging chair/swing but then in the front and in the front beam resting on the posts. Otherwise, nothing heavy will be hung from the ceiling, just some simple lighting and the like.
Hope you're not planning to notch the beam so that it becomes less than 45x170.
45x170 has a 99% utilization rate, (without lighting and the like).
Hope you're not planning to notch the carrying beam so that it becomes less than 45x170.
45x170 has a 99% utilization rate, (without lighting and the like).
I was thinking just a couple of centimeters only, so I'll keep an eye on not taking too much then. Thanks for the input, good to keep in mind. 👍
Vi vill skicka notiser för ämnen du bevakar och händelser som berör dig.
AXS said:
I'm planning to notch the support beam and recess the roof beams a few centimeters, that should make it as sturdy as possible and you avoid a lot of angle brackets and such.berisen said:
Good, then I'll follow your advice