What's the shallowest allowable slope for a slate roof?

[The Natural Philosopher]

My experience of that was that after a while the felt goes.

And a costly strip and redo is in order.

[Jim]

The other option seems to be to use a modern plastic tanking membrane, hot welded at the joints, rather than felt. But it's unclear whether most contractors would be familiar with this approach, or what the cost implications are.

[The Natural Philosopher]

I think the crappy extension I had failed after about 15-18 years.

It was really the last straw that broke the camels back.

With water coming in in several places, the overhead cable that stopped the roof pitch being decent was undergrounded, the house was demolished, and proper angles used.. ;-)

Thats generally why this is done. I have to say I had a bloody valley join with an existing roof, and they are buggers. If its a leanto or bow window type thing, there isn't the huge volume of water in on spot.

If you have interlocking machine made tiles, then its not so bad. The water can't drive around the interlock easily.

But I still say unless there are overriding reasons to have a low pitch, don't. And know what you are getting into.

[Martin Bonner]

The slate would provide pretty good UV protection for the felt though, so the felt should last longer.

[Bruce]

Oh, absolutely.

But a slate roof at the right pitch should last much longer.

[meow2222]

Sounds like someone did it wrong. If you use fibre cement sarking, overlapping at any joints, the felt etc isn't even needed. The slates act as a windbreak, so there is much less tendency for water to be driven up the sarking overlaps than occurs with the slates. I've experience of such a roof with no felt or membrane, and no trouble at all.

NT

[colinhubs777]

Hey boys dont even bother laying slates any lower than 30degrees Forget th at underlay etc keeping it watertight Wider slates can go slightly lower Do nt try and change whats been tried and tested our years Architects Dont try and reinvent the wheel ?

[tabbypurr]

hat underlay etc keeping it watertight Wider slates can go slightly lower D ont try and change whats been tried and tested our years Architects Dont tr y and reinvent the wheel ?

Not sure why you bring this up now, but fwiw my one very shallow roof has b een doing fine for a long time.

NT

[mechanic]

No probs with snow where you live?

[tabbypurr]

Snow load is not a problem for properly designed roofs.

NT

[newshound]

It does depend entirely on where you live. Also wind loads, and driving rain.

[Robin]

Can you give us a clue why we should believe you rather than eg BS 5534?

[daveriba]

I would never specify a slate roof at that pitch...Normally a min'm is 30 d egrees. At that shallow angle, you would need a waterproof sub layer and th e slates would have to be twice nailed at the head and at mid point otherwi se they'll lift if a strong gust of wind gets underneath. Marley Eternit do a slate appearance tile with a thin Edge called the Edgemere Duo which loo ks like slate. That has a guarantee of going as low at 17.5 degrees.

[RayL12]

I will back that up. 10degrees is low.

In my experience re-slating, there is a noticeable set of water blooms on the slate below where the two upper slates bond. As you look at the slate they appear as an upside down rounded 'V'. At 30 degrees that bloom is as near as an inch from the top of the slate.

The lower the pitch the less the gravity comes into play.

Ray.