How to choose an underlay for a solar roof?

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The primary function of the roof underlay is to ensure long-term watertightness and thereby protect the roof structure.

The primary function of the roof underlay is to ensure long-term watertightness and thereby protect the roof structure. Selecting an appropriate underlay can be challenging due to the variety of options available. The underlay is installed before the roofing material and, in addition to providing ventilation and adding a watertight layer, the underlay also provides protection during roofing work, keeping moisture away. Let us discuss in more detail how to make the right choice and build a long-lasting roof.

Why is a high-quality roof underlay essential?

Regardless of the roofing material (tiles, standing seam metal or other material), an underlay must always be used under the roof covering. Roof underlay standards may vary from country to country. From time to time, one may come across construction sites where underlays of vastly differing quality have been installed. This occurs because building design documentation typically only mentions roof underlay (or roof membrane or just membrane), without specifying the requirements the underlay must meet.

The main function of the underlay is to protect the structure from condensation, rainwater and moisture, while also allowing moisture from the structure to pass through. On the one hand, the roof covering is not fully waterproof (it is precipitation-resistant), and on the other hand, there is no other way to direct the water vapour from the structure into the ventilation gap. This is where the roof underlay plays a key role: it allows vapour to pass through the structure but retains any water that has seeped through the roof covering and also drains away excess moisture.

Cheap underlays can be less sturdy, less durable and prone to tearing. An underlay with poor moisture conductivity can gradually damage the wooden frame, resulting in a shorter lifespan for the roof. Damage resulting from the improper selection of roof underlay is a common issue. All in all, small cost savings can lead to significantly greater losses in the long run.

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What are the different types of roof underlays?

There are two main types of underlays: breathable and non-breathable. In addition to draining excess moisture, a breathable underlay also partially functions as the roof’s ventilation system. The breathable underlay must be installed in direct contact with the insulation to establish the conditions necessary for pressure differentials in the structure. A non-breathable underlay is used when there are no living spaces directly under the roof (attic). The main purpose of a non-breathable underlay is to drain water and moisture. To this end, adequate ventilation must be ensured on both sides of the underlay, where moisture condensation can occur.

The roof underlay comprises three layers. The top and bottom layers serve a protective function, while the middle layer is a functional film that allows water vapour to pass through but is completely waterproof. Vapour permeability is indicated by the SD value, which should not exceed 0.3 m.

How to choose the right underlay?

When selecting a roof underlay, it is essential to know the type of roofing material that will be installed on the roof. The weight of the roof underlay fabric is primarily attributed to the top and bottom protective layers. Therefore, the thinner the protective layers are around the functional layer, the faster the underlay starts to allow water penetration. Thicker protective layers result in a functional film that lasts longer, ensuring that the underlay is watertight for a longer period of time (in other words, the greater the weight of the roof underlay fabric, the more durable it is). It is advisable to use products with a heavier fabric weight rather than those with a lighter weight. The underlay should have a minimum weight of 155 g per square metre. In a northern climate, the underlay must also be capable of withstanding significant temperature fluctuations.

  1. If the roof covering is stone, the installation process is likely to take longer, which means that it is crucial to pay close attention to the UV resistance rating of the roof underlay.

  2. In the case of sheet metal roofing, it is essential to consider the temperature resistance of the roof underlay. Temperatures underneath sheet metal can reach over 80 °C. Therefore, it is recommended to use underlays with a temperature resistance of at least 90 °C or higher, preferably at least 100 °C.

  3. In the case of integrated solar panels, temperatures under the panels are higher than usual, similarly to sheet metal roofing. For this reason, it is advisable to use underlays that have a temperature resistance of at least 120 °C and bear the respective marking of the manufacturer, which allows it to be used under integrated solar panels.

As always, if you lack the expertise, consult a specialist. It is best to consult with the manufacturer of the roofing material.

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