Fixing Solar Panels to Flat Roofs – why we recommend an Engineered Solution

When you specify a photovoltaic array for your flat roof, there is the option of either mechanically fixing the array, or alternatively using ballast to weigh it down without fixing into the structure. 

In this article we will look at the options for mechanically fixing solar panels to a flat roof and make the case for an engineered solution. 

Not sure whether you should fix or ballast your array? The choice of mechanically fixed or ballast mounted PV System will depend on the roof structure. Contact us for advice on which would be suitable for your project.

The Membrane Covered Batten Approach

PV arrays are a great addition to a flat roof, and we’re often asked to include them. However many PV installers send us proposals for fixing similar to this sample detail, which uses a membrane covered softwood batten:

We can understand why a PV installer might want to use such a detail – it appears to be quite a simple, cheap solution. However we don’t recommend it for the following reasons.

Poor Quality

This type of detail is difficult to install in a reliable way. It would be difficult to prevent leaks through the point at which the membranes are pierced, and contractors may resort to sealants. Leaks will lead to moisture penetration, dramatically shortening the life of the battens. If the battens are to be fixed into the roof deck, there is a risk of cold bridging unless thermally broken fixings can be used. The batten layout may also interrupt the drainage of the roof, leading to ponding and a build up of detritus. All these installation variables would affect the ability to design to predetermined maximum loads, and therefore make it diffichttps://www.singleply.co.ukult to guarantee a roof design using this method.

Short Lifespan

The life span of a timber batten, particularly in an exposed situation such as this, is going to be considerably less than the 25 years life span of many PV Arrays. This means that at some point the building owner will have to arrange and pay for the fixing system to be replaced, at considerable cost, and if this is ignored or avoided, damage to the roof may well ensue, leading to additional cost and inconvenience. Better to use a fixing system with a similar lifespan to the array.

Deceptive Cost

Whilst the batten solution appears to be cheap, it is only the materials that are cheap compared to an engineered solution. Installation will be labour intensive, and given the requirement to reinstall after the battens deteriorate, on-going maintenance and reinstallation costs will swiftly make it a costly decision.

Our field technicians are often called out to roofs that have had PVs retrofitted without thought to the waterproofing. These can take the form of battens in some cases, but also metal angles bolted through the membrane such as in the picture on the right. Installing a PV system in this way will breach the waterproofing, with the subsequent damage and cost entailed, but will also invalidate the guarantee, exposing your client to significant additional cost and inconvenience.

The case for an Engineered PV Fixing System

As always, we recommend that you look at the whole life cost of the roof and its ancillaries, in order to get a good idea of the actual cost to the building owner. With a little consideration, it isn’t difficult to see why an engineered solution offers considerable benefits:

1. Designed to ensure a secure, waterproof fixing
2. Negligible resistance to water movement, preventing build up of detritus and ponding
3. Loadings can be accurately calculated, producing an efficient layout of fixings
4. No reliance on roof-applied perishable mastic or adhesives.
5. Prior approval can be obtained from your chosen membrane manufacturer.
6. Protects the roofing system warranty.

Two Engineered Solutions for fixing solar panels to flat roofs

Here are two engineered solutions for fixing your PV Arrays. Leading membrane manufacturers approve these two systems, including many of the membrane manufacturers we work with, and we’re happy to recommend them. Both these systems allow the PVs to be top fixed to the roof and are therefore suitable for retrofitting of solar PVs. 

ROOFTRAK™ IFP (Integrated Fixing Point) by Nicholson

The IFP from Nicholson is a low-profile puck-shaped stainless steel fixing point, which is supplied with a flange of membrane factory fixed to it. The type of membrane required is specified when the fixings are specified. The fixing is then normally attached to the structural roof deck through the insulation. The only point of penetration is therefore factory produced, meaning the roofing contractor only needs to weld the flange of membrane in the same way as the remaining roof, using their existing skills. 

Benefits of the Nicholson IFP: 

• Suitable for bituminous and single ply membranes including PVC, TPO, EPDM and PIB 
• Low profile fixing point standing 45mm above the finished roof surface 
• Can be used with either direct or thermally broken fixings. 
• Direct fixing to the roof structure with a calculable pull out value, enabling accurate design of fixing layouts. 
• Fixing points are all underneath the membrane. 
• Where approved, roofing system warrantee covers the IFP 
• Provides simple to use universally accepted M10 anchor points 

Solar Panel Support Post by Latchways

Developed from MSA Latchways’ Constant Force® post technology used in their award winning ManSafe® fall protection system, the Solar Panel Support Post consists of a coated aluminium baseplate and body, and an electro polished stainless steel dome with stainless steel fixings. The specifier chooses an approved membrane, and the baseplate and body are specialist coated in the factory for use with the chosen membrane, which can then be hot air welded on site after the fixing is installed.

Benefits of Latchways Solar Panel Support Post:

• Suitable for bituminous, PVC and selected FPO/TPE single ply membranes, as well as standing seam and secret fix roofing. 
• Innovative Constant Force post technology with over 20 years in use. 
• Each post is capable of resisting a maximum tensile, shear and imposed load of 2.5 kN 
• 20+ years design life. 
• Latchways Overload feature indicates if the PV array has suffered excessive uplift and protects roof membrane from damage during an uplift incident. 
• Can be installed and inspected at the same time as a ManSafe fall protection system if both systems are installed together. 

For the Solar Panel Support posts, MSA Latchways recommend that a structural engineer calculate how many fixings are required and their locations. Training in installation can be provided or a registered installer can be used. 

Ballast with a Green Roof?

Mechanically fixing solar PVs needs to be done right, but you might also be able to ballast the system instead. In recent years a new alternative to conventional ballast has been developed. This is generically called BioSolar and combines PVs with green roofing to provide the benefits of both. 

SolarNature Biosolar Roofing System

SolarNature is a complete roofing system, including the waterproof layer, green roof build-up and integrated photovoltaic installation. This solution is driven by the need to achieve both carbon net-zero and biodiversity net gain. Particularly in urban locations and on public projects, such as schools, where existing green spaces are often built over to provide additional facilities.  Find out more about SolarNature.

The System Agnostic Roofing Consultants

When your business needs a supplier for construction products who can offer you impartial advice and support, you need a distributor who you can trust.  

Solar PV has become increasingly popular in the last decade, yet the fixing of PVs to roofs is probably not considered seriously enough. What are your experiences of using Solar PV in your projects? We’d like to hear your comments.