Weatherproofing of in-plane roof lights

Recently we have had a number of enquiries pertaining to the weatherproofing of in-plane roof lights and in particular those comprising polycarbonate material. In-plane roof lights are those where the translucent material is made to the same profile as the roof cladding material and effectively replace a section of the cladding at the point where a roof light is required as opposed to out of plane roof lights where the translucent component is fixed to a supporting frame, etc. which projects above the plane of the roof. In-plane roof lights are by far the preferred form of roof light used in the RSA.

The most common in plane roof light configurations are;
Ridge to eaves
Mid-slope (a hybrid of chequerboard and ridge to eaves with portions of metal cladding at the ridge and eaves).

Whilst the chequerboard configuration is considered to provide the most even distribution of light it is the most difficult to weatherproof. A ridge to eaves configuration eliminates the problem of upslope metal to translucent lap joints but exposes the translucent cladding to the high wind loading at the eaves and to a lesser extent at the ridge. As translucent cladding is in the main non-trafficable the ridge to eaves configuration will inhibit access across a roof. Of the three configurations the mid-slope is the most practical.

The two main factors that make it so difficult to weatherproof in-plane roof lights are differential thermal expansion and thickness of the translucent materials.
GRP and polycarbonate cladding have longitudinal coefficients of expansion of two comma five and five comma six times respectively of that of steel. Surface temperature on a roof is considerably higher than ambient temperature. Surface temperatures of 60°C are common during the summer months. Heated through 60°C a 3.6m long metal sheet will expand 2.6mm whereas a GRP sheet will expand 6.5mm and a polycarbonate sheet 14.6mm. Assuming an operational range from 0° to 60°C and the cladding is installed at a temperature of 15°C the steel will expand/contract 1.9/0.6mm, the GRP 4.9/1.6mm and polycarbonate 10.9/3.6mm. The differential movement at the ends will be +1.5 -0.5mm for GRP and +4.5 -1.5mm for polycarbonate. GRP will require a hole 3mm larger in diameter than the fastener and polycarbonate a 10mm slot. If the differential is not provided for the translucent cladding will buckle between fasteners or may even crack. It is important to remember that the weatherproof seal on the underside of the metal washer of the primary fastener also has to accommodate this differential movement as does the sealer strip inserted in the end laps. With sealants transvers movement is directly related to thickness. A bead of sealant squashed to 2/3mm simply won’t cope. Based on this data we recommend that the lengths of GRP and polycarbonate profiled cladding be limited to 8.0m and 3.6m respectively.
Profiled translucent cladding is designed to fit over metal cladding, not under it. This is the reason it is almost impossible to weatherproof a metal over translucent end lap. A more practical solution is to fix the translucent cladding over the metal cladding on all four sides and then back flash the upslope lap to the ridge.

It is our considered opinion that in plane rooflights are not an efficient and safe solution. We recommend that profiled translucent cladding be installed, in short lengths, transversely to the metal cladding on raised supports which run parallel to the metal cladding. This will eliminate the need for end laps as well as being more readily identifiable as non-trafficable areas of a roof.

Most manufacturers of profiled cladding do not support the use of end laps in their products on flat roofs i.e. roofs with a slope of five degrees or less.