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!
H
hanghaus202318 Mar 2026 11:48Have you ever considered lightweight green roof systems?
Lightweight green roof systems with a substrate layer thickness of about 4 to 5 cm (1.5 to 2 inches) weigh only around 60 to 70 kg/m² (12 to 14 pounds/sq ft).
Before adding rafters, I would rather screw OSB boards on top. That way, you can even save on wind bracing.
Lightweight green roof systems with a substrate layer thickness of about 4 to 5 cm (1.5 to 2 inches) weigh only around 60 to 70 kg/m² (12 to 14 pounds/sq ft).
Before adding rafters, I would rather screw OSB boards on top. That way, you can even save on wind bracing.
@hanghaus: Yes, of course. As I said, at the trade fair the salesperson confidently told me it was around 40 kg/m² (8.2 lb/ft²). That’s why I was so surprised when the datasheet showed 80 kg/m² (16.4 lb/ft²). I’m not a fan of returning things, so I wanted to calculate whether my roof can still support it -> see first post.
By now, I’m considering several measures.
1. Reinforcing or doubling the rafters so that their spacing is around 40–42 cm (16–17 inches).
2. Using less substrate (6 cm instead of 8 cm (2.4 inches instead of 3.1 inches)) and installing a 40 cm (16 inches) gravel strip all around the edge, which weighs about 1/4 less than the substrate.
I’m avoiding returning it because (if I understood the others correctly) even 40 kg/m² (8.2 lb/ft²) wouldn’t be without issues, and I would have to double the rafters anyway.
By now, I’m considering several measures.
1. Reinforcing or doubling the rafters so that their spacing is around 40–42 cm (16–17 inches).
2. Using less substrate (6 cm instead of 8 cm (2.4 inches instead of 3.1 inches)) and installing a 40 cm (16 inches) gravel strip all around the edge, which weighs about 1/4 less than the substrate.
I’m avoiding returning it because (if I understood the others correctly) even 40 kg/m² (8.2 lb/ft²) wouldn’t be without issues, and I would have to double the rafters anyway.
N
nordanney18 Mar 2026 14:11Mariopa schrieb:
Could you please comment again regarding the deflection? That should resolve the issue.
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