Installation of Solar Collectors

Britain and Ireland pose a greater challenge to the safe installation of panels to roofs and other supporting structures than is faced in most of Central Europe.. Our western coasts in particular are much more exposed to storms with strong winds rolling in of the Atlantic. Collectors fastened to roofs must remain structurally sound over a timescale of at least the collector design lifetime, which many manufacturers consider to be 30 years. Building Regulations Section D requires an Agrement Cert, which in turn looks for a 60 year design lifetime for any components integrated into the roof. In addition to the penetrations of the roof to fasten the collectors, the installation also requires the collector pipework to be returned through the roof in a manner that must not compromise the weathertightness of the roof. As house construction techniques are changing, in the future there may be more 'warm-roofs' encountered with the insulation installed on top of the rafters, there will be far more interest in maintaining the structure's airtightness.

Wind loading of collectors

The assessment of wind loadings is a complex process, involving not just a knowledge of windspeeds and airflow but also of the type and strength of collector, fastenings and mounting rails and roof structure. The collectors on the roof are subject to force that create uplift or down lift at right angles to the roof as well as forces along the roof. (Diagram?) Although some work has been done in this area to develop some national standards by TNO and BRE, there are no European wide standards at present. The EU ActiveRoofer project is currently trying to extend the BRE and TNO standards into a prototype . BRE Digest 489 gives details of the calculations required for solar PV panels, which could be modified for solar thermal flat-plates and may yield an approximation of lmiting values for tube-based collectors. At an installer level, manufacturers advice should be sought on the suitability of the collectors and mounting systems for the exposure of the site. as can be seen from the maps below, the assessment of maximum windspeed does depend on the timescale chosen. A once in fifty year value by the Met Eireann assessment is almost twice that assumed by BRE for Ireland. Once in fifty years is within the design lifetime of the roof and is probably a more representative value to use in assessments.

General Guidance on minimising collector loads

The windspeeds in the maps above are measured at sea level and will increase further with altitude or due to the concentrating effect of other geographical features that will channel wind. A site assessment must take these additional factors into account.

Windspeed will also increase on taller buildings or due to the presence of other tall buildings (that may shield the building from wind from certain directions, but increase the wind speed from other directions).

Trees, hedges often provide a softer shielding effect and tend to reduce windspeeds over a horizontal distance of up to ten times their height on the lee side.

Collectors in general should not be positioned topwards the ridge or towards the eaves of the roof, where vortices can form and increase the uplift forces on the collectors.

The collectors at the edge of a bank of collectors will experience the largest forces. In exposed locations, it is advisable to increase the number of fixings (reducing their separation) and also by doubling-up the fixings at each end.

• Keep Collectors (particularily Flat Plate) ½ meter away from the ridge and eves.
• Keep Collectors as low to the roof as possible, to impede wind blowing underneath.
• Provide 200kg of fixing strenght per meter² of panel.
• Double up outside panels fixings if deemed appropiate.
• Flat roof installations, must be especially carefully assessed. One manufacturer recommends 3 x 1,500 kg concrete sleepers for a 10 meter² panel.
• Vacuum Tube Collectors without reflectors will generally have a much lower wind load, because of the round profile of the tubes. The gaps between the tubes will help prevent suction forces on the panel.
• Use certified Roof fixing products.

Structural Strength of the Roof

An assessment should be made during the site survey of the nature of the roof to assess its structural strength. The weight of the collectors is less of a concern for in-roof systems as the weight per metre squared of the collectors are generally of the same order as the roof covering they are replacing. For on-roof systems and systems on flat roofs, the static loading on the roof will increase and this will increase much more when potential wind loadiing is taken into account. Conventional roof tend either to be rafter and purlin or truss constructions. Rafter and pulin constructions tend to use heavier timbers and to have been designed more conservatively with regard to additional roof loads. Modern truss constructions are computer designed to minimise the number of trusses and the size of the timbers required. They tend to have less capacity to take an additional load and more care should be exercised when assessing loading. Roofs that have special constructions e.g. hip and valley or that have rafters already cut for rooflight windows should be treated with even greater care and the location of the collectors kept away from potentially weaker areas.

Tiles, Slates and Sarking

In the UK and Ireland the weatherproof layer of the roof consists of a sarking layer of felt or membrane over the rafters (trusses), held in place with battens that are used to support the tiles or slates and allow fixings to be fastened. Concrete pantiles are generally single lapped and are often only fastened at the ridges, gables and eaves and around any roof opening. they permit easy installation of roof hooks since they can be pushed up underneath the tile above or with care removed for modification. A series of pushed-up tiles are used as means of moving around the roof by roofers as long as care is taken not to damage the underlying sarking. Double lapped slates provide a more difficult installation, since they are all fastened in place and form a more fragile roof covering that may break when walked upon, particularly in the case of natural slate that has weathered on an older roof. In assessing roofs, guidelines should be adopted as to when the roof work is beyond the competence of the installer and is best left to roofing professionals.

On-roof Solar mounting systems

Roofhooks are most easily used on roofs where the tiles can be slid up underneath one another to expose the sarking and the rafters. Roofhooks are best constructed from stainless steel. The roofhooks available from continental suppliers often have a wide spacing of the fixing holes, since rafters there are commonly between two or three inches wide. Rafters of this width can easily take two staggered coachbolt fasteners without splitting. The narrower roof timbers found in Ireland and the UK would be easily damaged by fastening in this way or by the use of benchscrews of thicker diameter. Roofhooks may have to be modified by drilling an additional hole and perhaps more of them used to distribute the load over more trusses, as well as using finer fixings.

Solar Bolts provide an alternative to roofhooks particularly for double-lapped tiles. To use these the the roof covering is drilled from outside, through the tile or slate and sarking into the centre of the rafter or a noggin. The alignment of the hole is critical if drilling into a rafter as it is possible to split the rafter, if the hole is too close to the edge.



Various techniques can be used to identify the right point to drill. These include the placing of a rare-earth magnet on the inside of the rof on the centre of the upper of the rafter and then using a second magnet outside on the roof to accurately line-up with this. Alternative some installers drill a small pilot hole from the inside of the roof outwards using the rafter inside as a guide to ensuring it is centred and then drilling the full-sized hole from the outside in. Neither of these techniques are advocated if the rafters are narrow. There is too much risk of causing structural damage to the roof by splitting the rafters. It is best practice here to fit noggins between the rafters of at least 50mm thickness, using either joist hangers or skew-driven screw nails to attach them to the rafters. This can simplify the benchscrew alignment as the outer hole in the roof can be drilled before the noggin is fixed and the noggin then aligned to the outer hole (a two-man operation).

The screw is attached at the highest point in the tile profile, so that rain will have a natural tendency to flow away from the seal. Do not mount the screw in the tile "valley" as this will block rain water flowing down the roof.

A 8.0 mm hole is drilled through the roof tile, normally with a diamond drill bit. Because the highest point in the tile profile will only occasionally match up with a rafter below, a noggin can be attached securely between adjacent rafters in the attic to provide a strong anchor point for the screw. We recommend (& SEI insist) that the solar bolt is screwed into the wood/rafter a depth of at least 2 inches or 50mm. A batton with a typical depth of 25mm is therefore not suitable as a fixing point.

In-roof Solar mounting systems

At present only flat-plate collectors can be integrated into the roof in this way. The collector essentially becomes part of the weatherproof layer of the roof. This requires a high standard of materials and workmanship to provide durability comparable to standard roofing such as tiles and slates. Usually collectors will only be installed in this fashion on a new-build site, as the degree of disturbance to an existing roof and the cost of the extra labour is not justifiable in a retro-fit application. On a new build roof, the collector will be installed on top of the tile battens and then flashed in situ in a similar manner to a roof light. Generally a header flashing will divert the flow of water coming down the roof over the top of the collector or around it onto the side flashings. The bottom edge is finished with a flashing to redirect the flow of the water onto the slates or tiles underneath. All hydraulic connections are concealed behind the flashing and this gives the collector a clean appearance on the roof. In addition to minimising the wind load on the collector, this method of mounting also reduces the cooling of the rear of the collector and yields a slight increase in efficiency.

Flat Roof Solar mounting systems

Flat roof mounting systems can comprise either aluminium frame systems or plastic containers. The frames may be either ballasted onto the roof or mechanically fixed to the roof without breaching the weathertightness. The plastic containers are ballasted only. Ballast systems will greatly increase the static load on the roof and requires careful professional assessment of the strength of the roof. Wind loading also requires professional assessment of the exposure of the site, of the angle, orientation and size of the collectors and of the holding power of the ballast or mechanical fixings. Most larger solar companies will be able to provide the required technical advice.