ᐅ Carport structural calculation for verification – Thank you!
Created on: 18 Mar 2026 05:57
M
MariopaHello 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 07:44And what do you plan to do when winter hits and you actually get 50 to 60kg of snow per m² (square meter) on the roof? Climb up a ladder twice a day to brush it off?
N
nordanney18 Mar 2026 08:091.2cm (0.5 inch) sheathing will bend significantly – please use 20mm (0.8 inch) or thicker.
The rafters for this span look very questionable as well. They probably won’t break immediately but will also bend, especially under winter loads.
Personal assessment of the design: I wouldn’t do it this way and wouldn’t have long-term confidence in the stability.
The rafters for this span look very questionable as well. They probably won’t break immediately but will also bend, especially under winter loads.
Personal assessment of the design: I wouldn’t do it this way and wouldn’t have long-term confidence in the stability.
Thanks @nordanney for your assessment.
I need to clarify something for all other readers:
The carport has already been built and has been standing for several years.
The green roof is now optionally planned to be added, and what I really want to know is whether my structural calculation is correct or if I have made a (calculation) error.
According to my calculation, it just works, and the deflection amounts to 1.5cm (0.6 inches).
Therefore, it would be helpful if someone with experience could tell me whether I have done the math correctly.
From my own assessment, I also tend to think it won’t work. But I would like to rely on the mathematics and am only afraid that I am calculating “wrong” or using incorrect assumptions.
Thank you.
I need to clarify something for all other readers:
The carport has already been built and has been standing for several years.
The green roof is now optionally planned to be added, and what I really want to know is whether my structural calculation is correct or if I have made a (calculation) error.
According to my calculation, it just works, and the deflection amounts to 1.5cm (0.6 inches).
Therefore, it would be helpful if someone with experience could tell me whether I have done the math correctly.
From my own assessment, I also tend to think it won’t work. But I would like to rely on the mathematics and am only afraid that I am calculating “wrong” or using incorrect assumptions.
Thank you.
If the carport has been standing for a while, check or measure the deflection. The constant load deforms the wood, the deflection adds up, and you can easily have 5cm (2 inches) or more of sagging. You can’t just twist your roof like a shelf board. Before greening the roof, I would strongly reinforce the structural support.
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