The Mad Surveyor

I was called out over the weekend to attend a leaking roof. With all the torrential rain we have had this week I suspect we will be getting quite a few more calls yet. Anyway, this roof visually looked in good order, all tiles were on, none were cracked and all looked correctly positioned. So how could this roof leak?

Well, first thing I noticed was the relatively low pitch of the roof, about 25 degrees. Second thing I noticed was that the tile head cover was only 40 to 50 mm. This is the amount of tile overlap between the rows of tiles. Third thing I noticed from inspecting the roof from inside the loft was the fact that the felt underlay had largely perished and was hanging down in places, clearly passed its sell by date. The felt underlay on shallow pitch roofs and roofs with small tile overlay is pretty much essential.

So how was the roof leaking then? Well, get a bit of wind to blow the rain up through and between the tiles, particularly if the overlay between tiles is only 40 mm, then the rain will clearly enter the loft space if the felt, the barrier of last resort, is in poor condition. Not only that, once water starts to "track" it can continue to run uphill across short distances on relatively shallow pitched roofs without wind assistance. This was what had happened here, a roof that looks apparently watertight actually leaking like a sieve because the water is running up hill due to the tracking.

Try this experiment, hold a tile under a running tap at quite a steep angle and you will notice that the water runs cleanly off and does not run around to the back of the tile but "drips" consistently off from the very end of the tile. Now decrease the angle you hold the tile to the water and low and behold the water runs down the tile but then creeps around under the tile before finally dripping off. This is how water can track uphill and why felt underlay is essential on very shallow pitched roofs and this becomes the waterproof layer, not the tile.

The solution? Well, I have to admit that spraying with polyurethane foam might appear to be an answer but this would not be a good solution. To enable the roof timbers to breath, the polyurethane sprayed has to be semi permeable type and the type used is typically 95% closed cell, loosely meaning that it is 95% waterproof. So, in a continuous and sustained soaking of the polyurethane it will gradually absorb water and with a shallow pitched roof of say 25 degrees or less and with less than say 100 mm tile head overlap this is more than likely, absorption to the point it leaks again. One solution would be to strip and re-felt the roof and then relay the tiles. A better solution would be to retile with greater head overlap and an even better solution would be to redo the roof timber carcass and increase the pitch to 30 degrees or more - more is better.