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 them down without fixing into the structure.
We’ve covered the ballasted option with our post on Red Sky Solar. 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 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.
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.
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:
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.
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.
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.
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.
SIG Design & Technology takes the risk out of roof design, offering a genuinely independent choice of waterproofing for flat, green, zinc and copper roofs. Designs are covered by our PI insurance and we’re part of SIG plc, a FTSE250 company. Our blog at www.singleply.co.uk/blog explores some of your common problems in roof design and system choice, or book our new RIBA CPDs, ‘A Structured Approach to Roof Specification & Design’ and ‘Specifying Hard Metals: Choosing the Right Product for the Project’.