Which compressibility class for insulation must I choose on flat roofs to install solar panels?


Any roof whose slope is less than 15%, and which is then considered a flat roof, must have at least class C insulation to accommodate the installation of solar panels (photovoltaic or hybrid).

There is only one exception for expanded polystyrene which can be class B only.

It is also important to check that the load-bearing element of your roof (concrete, wood, metal frame) is compatible with the fastening system that will hold the panels.

This can be verified in the certifications between the fixing system and the panels: we have provided you with all of our certifications here so that you can carry out your project with complete peace of mind.

As for the waterproofing system, it must be approved by Technique advice in accordance with DTU 43.1 “Waterproofing of flat roofs and pitched roofs with load-bearing masonry elements in lowland climates”.

Here we remind you of the different classes of compressibility of an insulator according to the UEAtc guide:


Deformation (%) 

Temperature (°C)

Load (kPa = 100 kg/m²)


(Roof only accessible for maintenance. According to the assessment of the institute)

≤ 15 %


20 kpa


(Roof only accessible for maintenance. Can be used without restrictions)

≤ 5 %


20 kpa


(Roof accessible to pedestrians. Can be used for frequent maintenance of equipment)

≤ 5 %


40 kpa


(Roof accessible to light vehicles. Can only be used if the waterproofing coating is protected by a concrete slab or other)

≤ 5 %


80 kpa



> To go further : Are Dualsun panels compatible with SOPRASOLAR FIX EVO and FIX EVO TILT mounting system?