Fixing and Installation Options

There are two fundamental options for fixing a PV system to a flat roof, ballasted or mechanical. A ballasted system adds additional weight to anchor the array to the roof whereas mechanical methods cover two key practises, either they penetrate the roof covering and are fixed to the deck or they do not and leave the waterproofing system intact.

Ballasted Systems

Generic ballasted PV system

Generic Ballasted Systems

Installing a ballasted PV system requires confirmation from a structural engineer that the additional weight and wind load can be accepted into the design. The ballast itself can take different formats and it is important to confirm that the static load created by the designed ballast will be appropriate and sufficient in accordance with the wind load calculation report. 

Where possible, the ballast should allow for a spread of load across the roof rather than any point or line loading.

BioSOLAR combines a green roof with PV

BioSOLAR PV for Green Roofs

The substrate and vegetation provide the ballast to secure the array on the roof. The entire roof area qualifies as a green roof and if a biodiversity finish is specified this can further enhance roof element.  More info ►

In all ballasted applications a suitable protection layer must be allowed for and this should be agreed with the waterproofing warranty supplier.

Mechanically Fixed Systems

Mechanically fixed solutions are used where ballasted systems are not suitable due to the additional imposed load.

BauderSOLAR non-penetrative installation

Non-Penetrative Mechanical Fixing

This method is where the mounting system sits completely separate atop the waterproofing via substructures, that are held onto the roof through mounting plates and welding or bonding overlying membrane sleeves to the uppermost layer of the waterproof covering.  More info ►

These systems typically have large and stable attachment footprints with fixing tolerances that allow for levels of movement to occur without detriment to the entire stability of the array.

Penetrative mechanical fixing of a PV system

Penetrative Mechanical Fixings

These also have two generic forms where the array is installed either via the creation of a plinth or a proprietary fixing post, the size and shape of which can have an impact on the safe waterproofing and thermal continuity of the roof. These forms of attachment can be the only option in pitched membrane applications or where wind loads are particularly high.

These two illustrations show typical detail designs for penetrative mechanical fixings.


Any penetration of the roof structure and its waterproofing increases risk and could invalidate the manufacturer’s guarantee.

Does a penetrative mechanically fixed system really need to be specified or is another installation method possible?