When specifying a photovoltaic array for your flat roof, there is the option of either mechanically fixing the array, or alternatively using ballast to weigh them down without fixing into the structure. This article will look at the options for mechanically fixing solar panels to a flat roof, and make the case for an engineered solution.
The Membrane Covered Batten Approach
PV arrays are a great addition to a flat roof, and SIG are often asked to include them. However many PV installers send them proposals for which uses a membrane covered softwood batten. SIG say they can understand why a PV installer might want to use such a detail – it appears to be quite a simple, cheap solution. However SIG do not recommend it for the following reasons.
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 difficult to guarantee a roof design using this method.
Lifespan – The life span of a timber batten, particularly in an exposed situation, 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.
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.
SIG 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
SIG recommend looking 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. 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 and no risks
4. No reliance on roof-applied perishable mastic or adhesives.
5. Approval can be obtained from your chosen membrane manufacturer.
6. Protects your roofing system warrantee.
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 SIG work with, and recommend. Both these systems allow the PVs to be top fixed to the roof and are therefore suitable for retrofitting of solar PVs.
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 fixed 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.
• 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 univerally accepted M10 anchor points
Solar Panel Support Post™ by Latchways
Developed from 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.
• 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 12 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.
With both these solutions the manufacturers 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.