My architect has specified slate on a roof with a slope of 16.8 degrees. Is that too shallow? Googling implies that 4:12 is the shallowest you can go with slate. That's 20 degrees.
PS: does anyone have a digital copy of BS 5534 they could... 'make available for download'?
Thanks.
I've never used/lived with it but I was once told that slates (natural and artificial) can be used on lower pitches with a waterproof sub-roof - design to depend on exposure, area etc. And that seems to be the case from eg
I gather its too shallow to be watertight, but you can put a watertight layer underneath the slate to allow its use at any angle. Fibre cement sheet makes a good durable sarking.
NT
Check your local library website - you may find that library membership includes online subscriptions. For example, miine includes the online OED, Times Digital Archive, Groves music and art online and BSI reports.
The relevant part of BS 5534 is
5.3.4.3 Roof pitch The recommended roof pitch and minimum head-laps for double-lap, natural, fibre-cement and other artificial slates, nail-fixed or hook-fixed, should be obtained from Table 4 and Table 5. The roof pitch should be not less than 20°.
We had a low angle slate roof on a bit of our house. Didn't work, people couldn't make it work. Gave up, had the entire thing rebuilt with a proper roof.
Indeed. I had similar problems with IIRC pantiles. Slope (30 dgress IIRC)too shallow to b reliable in heavy weather.
If at all possible go for a more sensible angle. This is asking for trouble (though it CAN work)
=A0I
Hmmm. That doesn't sound like what he's specified. The spec mentions an underlay of 'Tyvek Supro'.
You're lucky! My council libraries dont provide them at all - you have to go to the National Library. They have to retrieve the hard copy from off-site location. I'm trapped in the stone-age apparently.
Problem is it is an extension of an existing roof - the existing is a cruddy felt construction and will get replaced. I the pitch of the roof could be steepened by taking one or two rows of bricks off the top of the wall to at least achieve 20degs...
If it were my roof and I couldn't make it more than 16.8deg I would increase the overlap of the slates.
mark
Surely, within limits, what matters is the overlap? The greater the overlap the more shallow the roof can be made.
Do you not think fibre cement sarking would be cheaper than 50% more slate? It would also look conventional.
The other problem with adjusting slates from 2 layers to 3 (cant think how better to explain it) is that it changes their angle for the worse.
NT
They words here are "within limits".
Even with greater than 50% overlap, you won't get a watertight slate roof shallower than 20 degrees.
I am not so sure
Given a certain windspeed, what matters is how high the water can be driven. if its more than the rise of a tile, it will climb up the tile.
The uber flat roof stuff is laid with interlocking joints I seem to remember
I think I'll be ensuring that the pitch is at least 20degrees by raising the high end of the roof further up the wall of the main building (it's a lean-to). At that pitch at least there will be specified acceptable head-laps etc given in BS5534. You'd think it specifies a minimum pitch for a reason, and upping the headlap by a 'finger in the air' amount at lower pitches has no guarantee of success. I looked at the interlocking 'slates' such as Redlands Cambrian, but if plain real slate is possible, it'd be preferable, since it looks better. Apparently the interlocking joints on these single-lap systems work like miniature gutters which direct water down onto the lower course of slates.
The architect has been saying things like 'I think 17 is within the tolerances' and 'remember the underlay membrane is a second line of defence' both of which I think are... well, he's not a roofing contractor.
I couldnt find anything about using fibre cement sarking on the internet - is the idea that the roof would be waterproof even with no slates, making the slate decorative?
This thread is quite interesting since we've just been having an exactly similar conversation with our architect, who has drawn a roof at 22 degrees inclination and has specified it as a flat roof with slates for decoration.
I emailed Marley and they sent me a "Below Pitch" spec which describes laying sarking, two layers of felt, and a membrane under a normal slate roof at these kinds of angles. It's a bit belt-and-braces but sounds like the correct thing to be doing.
I haven't had a roofing contractor's opinion yet on it.
We had a new single-storey extension roofed with slate at 20 degrees about ten years ago, and despite being in an exposed location in a wet part of the country, it hasn't given any trouble at all.
We were constrained by the height of the first-floor windowsills, and in order to get as much as 20 degrees slope and still have adequate headroom at the low end of the roof, we sloped the floor. It's about three inches lower at the far end, which in practice is virtually unnoticeable.
Yes. The slate also has one more function: its the robust layer, it stops falling objects breaking the fibre cement. So it takes both layers working together to provide all the required functions for a long lived functional roof.
NT
I'm not a roofing contractor but that roofing system is clearly designed to makes the slates, or tiles, redundant.
Good to know.
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