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Bonded-in roof lights

Uncle Bulgaria

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A warning for those of us who have roof lights bonded into the cab roof of our motorhomes.


The roof light in my Hobby is bonded into the cab roof and so does not open. I have heard reports and come across instances where owners of motorhome similar to mine have suffered a failure of the bond and water ingress or even total loss of the roof light.


I've been talking about this potential problem with my dealer and he has experienced three failures of which two were associated with noticeable distortion of the acrylic material of the roof light, the outer panel in one case and the inner panel in another. These roof lights are typically fitted with a concertina blind that can be drawn across the roof light. A theory that appears to fit the observations is that if the blind is fully closed in a hot, sunny climate the roof light might get so hot that it reaches the softening point of the acrylic. Furthermore the sideways expansion of the roof light will be resisted by the bonding between the roof light and the cab roof (assuming the roof light expands more than the cab roof). So if the roof light is at high temperature and is restrained then either the roof light material might distort or the force due to expansion might be enough to break the bond between the roof light and the cab roof, or both.


I note that on my roof light Polyplastic has put a small (and rather confusing label) suggesting that in sunshine the blind should be closed no more than half way. This suggests that the manufacturer is warning about possible excessive heat build-up in the window material if the blind is routinely kept fully closed in bright, hot sunlight, so I have normally left my blind fully open in sunlight. Careful inspection of my roof light shows no distortion or any evidence of partial failure of the seal, but I am looking into ways of reinforcing the bonding of the roof light into the cab roof.


So if you do have a bonded-in roof light particularly if made of plastic material, you may want to look carefully at any advice regarding not closing the blind in strong sunshine. Naturally, those with opening roof lights do not face the possible failure of the bond as the roof light is free to move as it expands, but the material could still be prone to distortion if a closed blind caused the temperature of the roof light to become too high.


If anyone has experience that may contribute to understanding the failure mechanism that has been experience by some owners it may be helpful to circulate it.


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All of my Rapido’s roof-lights can be opened, but the Rapido User Manual for the vehicle warns (for each different type/make of roof-light) that in strong sunlight (and presumably when the rooflight is closed!) the concertina blind must be left at least one-third open to avoid the risk of overheating and resultant damage to the seals. This repeats advice in the operating instructions for Heki rooflights and I believe that (for several years) Polyplastic windows have carried the warning-label you mention.
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