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.

rooftop-solar-advisor-banner

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.

Subscribe to FREE Solar Mango Newsletter - News and Opinions on Implementing Solar

Related Articles