Slate roof below 25 degrees

[transfix8]

I am currently building an extension and am almost up to roof level. The roof will be around 10 degrees but one of the conditions of the planning approval is that it must be a slate roof. Cambrian slates will only go down to 15 degrees.

I was wondering, is there any way to seal the slates together and place 2 layers of tyvek below and tri-iso insulation at the bottom, would this be an acceptable way around the problem. Alternatively, should I ply the roof the same way as a flat roof then use the above method.

There is no way of increasing the pitch as headroom would be too low. Planning dept won't budge on the use of slates.


This is the first time I have undertaken such a large DIY project and I realize I have made a rather large cock-up which I now need to sort out. Hope someone can help.

 

[mattysupra]

im confused! is your angle of degres working the correct way?

 

[transfix8]

Yes. The roof is almost a flat roof (I think technically anything below 10 degrees is classed as a flat roof).

 

[SurreyRoofingEstimator]

We had to do something similar once (though not as low as 10 degree's) using Eternit fibre cement slates and the suggested way (and the way it was done in the end) was to fix a timber deck over the rafters and treat it like a flat roof with bonded felt then batten over and finishing with the slates, the slates will only be a decorative feature like this and will very likely let water through to the felted roof below. Basically as you thought yourself.

Another avenue to try would be the Onduline corrugted undertile system which is a corrugated bitumen sheet that acts as the weathrproofing where the pitch is below the recommended.

One thing I can't see is why would increasing the pitch lessen the headroom, surely it would increase it?

 

[^woody^]

Yes, board it in 12mm WBP ply, then flat roof felt, then 25mm counterbattens, then the lath and slate.

The slates will be the weathering cover and the felt roof beneath is only to catch any driven rain that may be blown under the tiles on the odd occasion, if at all.

With this roof, just remember to allow for the extra thickness of the roof as a whole and try not to have an odd looking deep fascia - cut the rafter tails to form a 'normal' fascia depth, then it does not look out of place

 

[SurreyRoofingEstimator]

oh, yes and as woody says, counter-batten it (ooops, forgot that bit lol).

 

[^woody^]

oh, yes and as woody says, counter-batten it (ooops, forgot that bit lol).

Lol - check all your quotes ... NOW!

 

[redtonybbb]

Tb-Ex Roofer /Adv. Craft C&G/Chartered Surveyor.Mrics.Mciob.Mior.
Hi-Many roof pitches on extensions on residential property are governed by the height of upstairs windows.A pitched roof may be increased slightly by using timber firings which for example go from 100mm down to nothing over a distance of say 3m(The firings can be ordered from timber saw mills generally -some are still around/wood is still being cut).My brother once had a problem as this in North London and I increased the pitch using 2no firings on top of each other which saved stripping all the roof rafters off,as the pitch he had installed was not sufficient for the building inspector.A low pitched roof may be treated as a flat roof and completed in for example built- up felt.If you then fixed counter battens(roof batten carrying up the roof pitch thereby reducing any punctures to the felted surface/you may further seal the sides of the counter batten,and then batten the roof as normal,to carry the slated roof.The best low pitched slate is an Eternit(man made fixed with copper nails and rivets),which can be in various colour.ie black/brown.The building dept. are generally ok.they just need to cover themselves,and not forgetting you have a secondary roof.It may be good to discuss this with them as a higher spec. normally is ok.The firings as mentioned are the same firings which are manufactured to give flat roofs a slight pitch,which is generally 1:25.(Good Luck).

 

[justlead1]

Hi A lot of my work has been associated with roofs of the type you describe.
where people have tried to save money and ending up spending a bundle sorting out the leaks. There is no way a slated or tiled roof will keep water out with this pitch.
My advise would be (if slates are a must) ply the roof, apply hot felt or torch on. making a water tight job. Then worry about how you fix the cosmetic slates. Or see if the planning people will suffer a copper, lead or zinc roof.
Good luck

 

[transfix8]

One thing I can't see is why would increasing the pitch lessen the headroom, surely it would increase it?

The ridge is a fixed height so the only way to increase the pitch would be to drop the rafters at the opposite end therefore decreasing head height inside.



Thanks for all the advice and the speedy replies. It seems the consensus is to treat as a flat roof and fix slates as decorative (with counter batons then batons), will also call the Onduline people tomorrow to see what they say regarding the low pitch. You have all put my mind at rest now. Thanks once again, greatly appreciated.

 

[^woody^]

I would not bother with an Onduline solution.

It will be more expensive and much more involved for this situation, than you need.

 

[transfix8]

Spoke to BCO today and he is happy with a double layer of 6mm WBP with staggered joints followed by 2 layers of tyvek with staggered joints then counter baton, baton, cambrian slate.

Will also seal the joints between the ply before the tyvek (any advice what to use? Was thinking window sealer but will this bond to unpainted timber?).

Think this should stop all weather problems plus added bonus of sound insulation.

Any thoughts anyone?