Valley question

22 Aug 2008
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United Kingdom
Hello all, I am currently having an extension built and we have come to a bit of a stop with the roof. I won't bore you with the whole story but basically my builders have made a poor job of the valleys where the new roof meets the old roof (at a right angle) the pitch on the existing roof and on the extension is 22.5 degrees and the tiles are a square section large interlocking (designed for the pitch).

The open GRP valleys have been finished very poorly with uneven cuts and very thick uneven mortar beds. The guys have tried to tidy it up a couple of times but it just seems to be getting worse so I have decided to get a local roofing company to re do the valleys and parts of the new and existing roof.

They have recommended the use of a GRP Central up-stand rib trough type 'dry' valley where the cut edge of the tiles sit against the central rib. They also want to strip back the new roof to re gauge the tiles.

The only thing that worries me is that I have a large willow tree close the the house and i'm concerned that these closed type valleys will get blocked up with leafs and tree debris which will compromise the weathering of the roof which is already quite a low pitch. Should I be concerned?

Any advice would be appreciated.
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So nobody knows the answer then! Maybe all the roofers have gone to bed?
We have been using them for years - no prob's.

The look a heck of a lot neater too.
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Many thanks for that, you have put my mind at rest!

Its just that I was reading through this 66.pdf

And also the builders (who did the original job) have said that dry valleys are no good. But I guess they are bound to say that as they don't want to admit they have done a bad job.
We do work for a developer who will only specify dry GRP valleys specifically because they don't get bunged up with tree debris and they do work really well and look very neat
I can't understand why anyone would specify GRP in Valleys. Valleys are the most vulnerable part of the roof as they carry the most water, leaves and other debris.
Why use a material in the valleys that doesn't last nearly as long as the roof covering itself? and the cost saving is peanuts compared with the overall cost of the roof.
It's akin to making nice leather boots with cardboard soles.

I've pulled loads of failed GRP valleys out, and had them re-done properly.
I can't understand why anyone would use anything other than GRP valleys as they are indestuctible and will out-last nuclear waste.

I have lost count of similar products that need replacing due to wear etc.

I have never seen a grp valley wear out.
I have seen damaged leaking grp valleys, anybody who climbs on a roof tv aerial man/gas man fitting a linner they all walk up the valleys and they are not nearly as robust as a lead valley.
I've seen leaking valleys of all descriptions though i have yet to see a GRP one 'wear out'.

Anyone using a the centre of a valley to clamber up a roof is an idiot.
Rather, you would spread you feet somewhat to avoid breaking the fragile valley cuts.
To make roofs more 'idiot' proof all you need to do is use the correct material - it doesn't cost much more overall, no need to worry about it ever again.
I would specify stainless steel in the valley. Never seen one fail yet and don't know of a material that will last longer...... :mrgreen:
I've lost count of the amount of GRP valleys my firm has replaced, lead will always win this argument for me.

At present we are re-roofing a 20 year old house with 6 dormers, every single GRP valley has been leaking like mad. Albeit the roof was done awfully bad and at the incorrect gauge, just an example.

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