Mark the Roofer came round to replace a broken tile on our roof late last week, and he told us that the Velux windows we’d had installed were fitted incorrectly. Apparently, up until two years ago, Velux windows needed to be fitted to the rafters, but now they should be installed onto a batten.
The consequence of fitting the newer style windows using the old method is they aren’t set high enough on the roof. The result is rainwater doesn’t drain freely, and is only held back by the surrounding felt. As the felt degrades over time (he said it’s usually two to three years), water starts to drip through into the room below.
The Velux installation guides and videos are actually very clear and well written, so it seems the reason why some builders seem to be installing the windows incorrectly is because they haven’t read the instructions in the last two years.
This roofer has read them, and makes a habit of checking any Velux windows he sees on the roofs he’s working on. It means he is able follow up many of his small £20 tile replacement jobs with larger £400 Velux re-installation projects. Sometimes, reading the manual pays.