Wind driven rain coming in dormer roof. *PIC*

[Anonymous]

I thought I had fixed this problem but obviously not.

We have 4 big dormer windows all of which are a bit knackered. They have plastic windows which gives you some idea of how well made they are.

Three of them are showing no signs of leaking yet, but one is letting in water.

They are a bit unusual in that they arent flat roofed but the roof is on a very slight pitch (about 3 or 4 degrees at a guess)

These flatish roofs are coverd in artificial slates, and I think that they have roofing felt under the tiles.

The one that leaks is the most exposed and what I think happens is that wind drives the rain up under the tiles until it drips down inside.

The edge of the felt (which looks like thick plastic sheet) sticks out over the gutter. I had thought that the wind was driving the water up the underside of this so I hung out the window and crimped on lead weights to bend it down to help it drip into the gutter but this hasnt worked...

Any ideas?

I am pretty much resigned to having to strip all this#####off and re-roof but with what? The other dormers will still be tiled so it would be nice if we could keep the tiles somehow...

George

 

[Anonymous]

Exactly where does the water come in? Mike.

 

[Anonymous]

It appears to run down the top side of the plaster ceiling and emerge in from the join at the top of the plastic window frame...

 

[Anonymous]

Sounds like a detailing problem at the change of roof angles. Possibly the least destructive thing would be to take off the plasterboard and hope that you can see how the water is getting in. It is often very difficult to work out leaks when you can stand and look at the job but even more difficult over the air waves! Good luck. Mike.

 

[Anonymous]

If the slates/tiles are free laid in a common double-lapped manner, as they would be for a traditionaly pitched roof, then a 3-4 degree pitch is entirely unsuitable I'm afraid.

Bryan

 

[Anonymous]

Bryan is a roofing expert, so I should pay attention to what he says.

I must say that tiling a 3 - 4 degree pitch sounds ludicrous to me. As you say, rain and/or snow can blow straight up under the tiles. And if the membrane underneath happens to be in two overlapped pieces, then I guess it could blow under this joint too, leaving it free to leak into your room.
I reckon this (to all intents and purposes) flat roof should at least be properly felted with torched joints. Better still it should be a lead roof. If you are unable to do either of these things, then at the very least I would take off the tiles and have a look at the membrane. If it has an overlap joint then I would replace it with a one-piece covering. At the same time I would ensure that it runs a good distance up underneath the pitched roof tiles at the back of the dormer.

 

[Anonymous]

Thanks guys,

The main roof is very steeply pitched so I think there is little danger of water blowing far up that one.

The tiles looked ludicrously shallow on the pitch to me too, but as I said I wanted to keep them if possible.

Currently I am considering stripping off the tiles to the ridge, replacing the felt/membrane with a single sheet that feeds right back under the main roof for a good distance, rebattoning and re-tiling. But I cant say I like the idea of nailing the battons through the membrane.

Another option would be to use an EPDM membrane and no tiles, maybe turning it into a green roof like the one I did for my parents garage.

Or as you say a big sheet of lead, though this will be damn heavy and expensive.

The only other possibility would be to do it in say copper....

George

 

[Anonymous]

Yes you are right George, a very shallow pitch, far too shallow for slates or tiles, rainwater wont just blow back under it will probably run back quite freally on its own accord.

Treat it as a flat roof.

Bryan

 

[Anonymous]

Whilst there is no question that the pitch is too shallow I have seen this sort of thing working assuming that the detailing is correct. A quick look under the ceiling may show you what is wrong and save a considerable amount of money and effort. Mike.

 

[Anonymous]

What might you expect to find - precisely what would he be looking for Mike?

As you probably know, double-lap roofing material is not meant to be put on virtually 'flat' surfaces, at 3 degrees some pieces will at best be laying dead 'level' (by design), in fact the eaves course could actually be running backwards.

Capillary attraction is bad enough as it is with some over uniform, man-made, mass-produced roofing materials, without laying the stuff flat.

Bryan

 

[Anonymous]

Merely that I have seen onduline (sp.) sheeting used as a sarking on ludicrously low pitches with the slates/tiles being purely cosmetic. Aesthetically it was awful but it worked. Mike.

 

[Anonymous]

Yes I see what you mean Mike, the 'Onduline' (or whatever), would have worked, but the slates/tiles could not have been much more use than ornament.

Bryan

 

[Anonymous]

Yep - the slates are just ornamental. Probably the wrong word though cos' they look ridiculous at that sort of pitch!
Mike.

 

[Anonymous]

Here's a photo of one of the other dormers. This one doesnt leak (yet) but is the same pitch and general design of detailing.

The one in question is about twice the width and 20% deeper and taller.

Suggestions on how to detail them all long term (that would look reasonable, be long lasting etc)would be appreciated.

 

[Anonymous]

The only thing I would suggest is board the dormers out and cover them as per flat roofs using felt or lead, etc.

It’s ‘horses for courses’ really as there are numerous systems on the market these days. Built up layers of single or double ply membranes would be worth looking into. It looks like it would be a reasonably simple job.

Nothing with a profile though, otherwise you would have difficulty stitching into the main roof.

Bryan