ᐅ Carport structural calculation for verification – Thank you!
Created on: 18 Mar 2026 05:57
M
Mariopa
Hello dear members,
I have planned a green roof for my carport. Unfortunately, the seller at the trade fair exaggerated a bit and said the saturated dead load is about 40 kg/m² (8.2 lb/ft²). After reading the planning documents, it turns out to be 80 kg/m² (16.4 lb/ft²).
To avoid overloading the carport, I tried to calculate the structural load. Since this is my first time and I don’t want to make any mistakes, I would appreciate it if someone could check my calculations:
Here are the figures I came up with:
EPDM membrane 1.5 kg/m² (0.3 lb/ft²)
Green roof 80 kg/m² (16.4 lb/ft²)
Wooden boards, spruce, 1.2 cm (0.5 inch) thick, 6 kg/m² (1.2 lb/ft²), larch C24
Rafters 6 x 12 cm (2.4 x 4.7 inches), spacing 80 cm (31.5 inches), span 2.84 m (9.3 ft), 4.5 kg/m² (0.9 lb/ft²), larch C24
When I enter this into the well-known online calculator with a snow load of 85 kg/m² (17.4 lb/ft²), the result shows it is just sufficient. To be on the safe side, I would reduce the substrate weight by 25%, lowering the load by 20 kg/m² (4.1 lb/ft²).
Is my calculation correct, or are there major mistakes?
The support beams (7 meters (23 ft)) should be fine, 8 x 24 cm (3.1 x 9.4 inches), larch C24, 12 kg/m² (2.5 lb/ft²).
The posts (8 pieces, 2.5 meters (8.2 ft) high) with 12 x 12 cm (4.7 x 4.7 inches), larch C24, 3 kg/m² (0.6 lb/ft²) as well.
Thanks if someone could take a quick look over a coffee and let me know if my calculations roughly make sense or if there are any significant errors.
Thank you!
I have planned a green roof for my carport. Unfortunately, the seller at the trade fair exaggerated a bit and said the saturated dead load is about 40 kg/m² (8.2 lb/ft²). After reading the planning documents, it turns out to be 80 kg/m² (16.4 lb/ft²).
To avoid overloading the carport, I tried to calculate the structural load. Since this is my first time and I don’t want to make any mistakes, I would appreciate it if someone could check my calculations:
Here are the figures I came up with:
EPDM membrane 1.5 kg/m² (0.3 lb/ft²)
Green roof 80 kg/m² (16.4 lb/ft²)
Wooden boards, spruce, 1.2 cm (0.5 inch) thick, 6 kg/m² (1.2 lb/ft²), larch C24
Rafters 6 x 12 cm (2.4 x 4.7 inches), spacing 80 cm (31.5 inches), span 2.84 m (9.3 ft), 4.5 kg/m² (0.9 lb/ft²), larch C24
When I enter this into the well-known online calculator with a snow load of 85 kg/m² (17.4 lb/ft²), the result shows it is just sufficient. To be on the safe side, I would reduce the substrate weight by 25%, lowering the load by 20 kg/m² (4.1 lb/ft²).
Is my calculation correct, or are there major mistakes?
The support beams (7 meters (23 ft)) should be fine, 8 x 24 cm (3.1 x 9.4 inches), larch C24, 12 kg/m² (2.5 lb/ft²).
The posts (8 pieces, 2.5 meters (8.2 ft) high) with 12 x 12 cm (4.7 x 4.7 inches), larch C24, 3 kg/m² (0.6 lb/ft²) as well.
Thanks if someone could take a quick look over a coffee and let me know if my calculations roughly make sense or if there are any significant errors.
Thank you!
M
MachsSelbst18 Mar 2026 08:27What is your carport's roof load capacity according to the calculator, datasheet, or planning documents? 85 kg/m² (17.4 lb/ft²)? 300 kg/m² (61.7 lb/ft²)?
You should have this value, at least for the building permit / planning permission.
You should have this value, at least for the building permit / planning permission.
N
nordanney18 Mar 2026 08:28Mariopa schrieb:
According to the calculation, I’m just within the limits, and the deflection is 1.5cm (0.6 inches). I just ran this through our calculation AI. It gives me a (visible) deflection of 1.65–1.8cm (0.65–0.7 inches) under load, which would cause sagging that could lead to issues. Additionally, the 1.2cm (0.5 inches) boards would bend like a hammock.
Thank you again for your opinions:
@MachsSelbst: I don’t have any planning documents for the carport. A building permit / planning permission is not required in Bavaria. It is designed for a snow load of 85kg, but that doesn’t say anything about the overall load-bearing capacity. Therefore, I used the online calculator and arrived at the figures in my initial post, which suggest it will work, but I wanted to double-check here for reassurance.
@nordanney: Thanks for recalculating and for the note about ponding, since it is a flat roof carport.
By now, I am planning to add 6 additional 12cm (5 inch) rafters, which will halve the rafter spacing to 40cm (16 inches). That should be sufficient, or does the risk of sagging still remain due to the span between the beams of 2.84 meters (9 feet 4 inches)?
And like a broken record, I want to ask again: Is my calculation correct at all, or are there any major mistakes?
Thanks for the active participation.
@MachsSelbst: I don’t have any planning documents for the carport. A building permit / planning permission is not required in Bavaria. It is designed for a snow load of 85kg, but that doesn’t say anything about the overall load-bearing capacity. Therefore, I used the online calculator and arrived at the figures in my initial post, which suggest it will work, but I wanted to double-check here for reassurance.
@nordanney: Thanks for recalculating and for the note about ponding, since it is a flat roof carport.
By now, I am planning to add 6 additional 12cm (5 inch) rafters, which will halve the rafter spacing to 40cm (16 inches). That should be sufficient, or does the risk of sagging still remain due to the span between the beams of 2.84 meters (9 feet 4 inches)?
And like a broken record, I want to ask again: Is my calculation correct at all, or are there any major mistakes?
Thanks for the active participation.
M
MachsSelbst18 Mar 2026 08:41Well, yes, that says exactly something about the load-bearing capacity...
A snow load of 85 kg means that not much more than 85 kg should be applied there...
There are carports rated for 300 kg snow load; if you want to green the roof, you should choose something like that.
Imagine standing on a board that bends 2 cm (0.8 inches) under load... would you really want to park your car under it and spend time underneath?
A snow load of 85 kg means that not much more than 85 kg should be applied there...
There are carports rated for 300 kg snow load; if you want to green the roof, you should choose something like that.
Imagine standing on a board that bends 2 cm (0.8 inches) under load... would you really want to park your car under it and spend time underneath?
N
nordanney18 Mar 2026 08:56Mariopa schrieb:
I’m now at the point where I’m adding an additional 6 x 12 rafters, which halves the rafter spacing to 40cm (16 inches). That should be enough, or is there still a risk of sagging due to the 2.84m (9.3 ft) span between the beams? No, that will definitely be safe. But you still have the 1.2cm (0.5 inch) boards as a weak point. If they ever get exposed to moisture over a longer period (which can happen with a DIY green roof), you’re in trouble...
@MachSelbst: Thanks for the clarification, but if I understood the terms correctly, snow load is an additional factor and does not indicate anything about the load-bearing capacity of permanent loads. That’s why I used the calculator and worked “backwards” based on my rafters, concluding that they can support a permanent load of about 120kg/m² (25 lbs/ft²) plus a temporary snow load of 85kg/m² (17 lbs/ft²). So it’s borderline but still feasible overall.
I realize that being in a borderline area calls for improvements, which is why I suggested adding extra rafters, and whether that would be sufficient (which nordanney has now confirmed).
@nordanney: Thanks again to you as well, and of course, the risk of moisture is always there. Firstly, it should be noticeable from the inside, and secondly, if needed, the roof would have to be removed and repaired. But that would apply even without the green roof, if there are defects.
Still, I want to note that my calculations seem to be correct, which I take as a positive point for myself. However, I hadn’t considered your advice about sagging, so I’m glad I asked the question here.
One last question: You do consider the posts and support beams uncritical as well, right? (My calculations have indicated this as well).
Best regards
I realize that being in a borderline area calls for improvements, which is why I suggested adding extra rafters, and whether that would be sufficient (which nordanney has now confirmed).
@nordanney: Thanks again to you as well, and of course, the risk of moisture is always there. Firstly, it should be noticeable from the inside, and secondly, if needed, the roof would have to be removed and repaired. But that would apply even without the green roof, if there are defects.
Still, I want to note that my calculations seem to be correct, which I take as a positive point for myself. However, I hadn’t considered your advice about sagging, so I’m glad I asked the question here.
One last question: You do consider the posts and support beams uncritical as well, right? (My calculations have indicated this as well).
Best regards
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