Why is it important?
When you decide to install solar panels, it is important to know their weight as it can impact the integrity of the roof structure. Solar panels are planned to be placed on the roof for a period of around 25 years. So, before installing it, there is a need to ensure that the roof structure has the capacity to withstand the panel weights.
How much do they weigh?
A typical solar panel weighs around 1-2 kg per square foot with the mounting structures. This is usually within acceptable limits of roof loading. On a wattage level, a 250 watt crystalline panel typically has a weight of 20Kg.
What are the different roof loading scenarios to consider?
The weight impact of solar panels and mounting systems on the roof structure requires an evaluation of the condition and type of material used for its construction. It is essential to do a structural analysis of the roof before installing solar panels over it. With respect to weight considerations for solar panels, the following points have to be considered.
1. Load-bearing capacity of roof material
The weight per square footing (100 square feet) for some materials are given below:
| Roofing material | Weight per 100 sq.ft (lb) | Weight per 100 sq.ft (Kg) |
| Asphalt shingle | 190-250 | 85-110 |
| Metal | 80-150 | 35-65 |
| Plastic polymer | 70-300 | 30-135 |
| Clay tile | 600-1800 | 270-815 |
| Concrete tile | 550-1000 | 250-450 |
| Slate | 800-1000 | 365-450 |
| Wood | 200-350 | 90-165 |
Source : http://www.todayshomeowner.com/choosing-a-roof/
With reference to the table above and other available information, the following conclusion can be made:
- Concrete tiles, albeit their high material weight can typically supports high loads and are known to be durable
- Asbestos or plastic roofs, which are commonly found on top of warehouses, are lightweight. Activities such as drilling could affect the load-bearing capacity of asbestos.
- `Metal roofs have one-fifth the weight of concrete tiles and are better to withstand the additional weight of solar panels. They are especially good for roof installations due to the fact that they have high strength-to-weight ratio.
2. Distribution of solar panel weight
A key point to note here is to reduce the distributed weight of solar panels on the roof. The point load for different mounting (or fastening) locations have to be distributed in such a way that it reduces the overall load on the roof and minimize roof leaks.
Note that although this technique may reduce roof leaks, this can be a reason for large point loads. Hence there is a lot of optimization to be carried out in this regard.
3. Balancing solar panel weight
Very often the solar panels are going to be mounted on one side which could have the adverse effect of unbalancing the structure if it is insecure.
Conclusion
Whether the roof be concrete, asphalt shingle, metal, wooden shingle or clay tile, they can all handle the weight of a typical solar power system. Therefore, it can be concluded that most roofs are designed to withstand the weight of solar panels.
There have been cases where weight of crystalline panels were too much for some roofs to handle. In such a scenario, it is preferred to go with thin film cells as they provide significant savings in weight albeit a reduction in efficiency.
However, the factors that have to be looked into while choosing the roof are the age of the roof and appropriate installer for that particular roof type.
In order to ensure the integrity of the roof structure, it is advised to consult a structural engineer.
Related Articles
2015-10-31
Dear Mr. Narasimhan Santhanam:
I am an electronics engineer involved in a apartment building project in Chennai in which we are considering rooftop solar power, co-existing with HVAC, clothes drying and solar hot water heating.
I believe that one approach could be to use hybrid solar PV and hot water heating units for both electricity generation and hot water heating.
Would you be able to help in this approach, to optimise the use of the available roof space for this dual purpose.
I look forward to your reply. Thank you.
Regards
Khader Hussain Sayeed