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Slate roof - minimum pitch


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#1 diydie

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Posted 07 January 2011 - 09:43 AM

I have a lean-to extension on the rear of my house that is in two parts - one part living accommodation and the other a covered walkway. Two photos are attached.

The roof is of traditional slate tile, with quite a shallow pitch. The pitch on the covered walkway is more shallow than the pitch on the accommodation section. I can't tell what the headlap is but I think it's pretty substantial.

The problem I am having is that during heavy rain, both roofs leak. In each case, a rainwater downpipe feeds onto the pitch from the substantial main house roof above (visible in both images), so in a storm both roofs get drenched with the downflow, plus whatever water hits the pitch directly.

What I am trying to establish is:

1. Will a traditional slate roof take such a shallow pitch?

2. Is it bad practice for the downpipe from the main roof to feed onto these shallow pitched slate roofs?

3. Is it likely that feeding the main downpipe directly to storm drain (ie not onto the pitched roof) will solve the leak issue?

Thanks in advance for any advice.

Slate roof

#2 joiner

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Posted 07 January 2011 - 12:23 PM

The answer to all your questions is 'yes'.

22.5 degrees is the general rule of thumb for minimum pitch, although in practice it's determined by a number of factors, common to all materials: snow loading, rainfall, and of course the elevation of the roof relative to prevailing winds.

Generally, the steeper the pitch the better it is at throwing off the weather. One of the main disadvantages of a shallow pitch is that it is highly susceptible to wind driving rain and snow under the slates.

Looking at your pics, that's a hell of a shallow roof to be dumping so much water onto. Slates will only take so much hammering and it pays to remember that they ARE porous. OK, not a lot, but it depends on what you're dumping on them.

This might help you to understand them a little better and be a bit more considerate to them. They are, after all, doing their best for you...

http://www.athyecoslate.com/naturalslate.html

#3 saddened1

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Posted 09 January 2011 - 06:56 PM

I would say that the monopitch slate roof shown is at too shallow an angle to stop water being blown under the slates and making the roof leak. I have the same scenario with an extension I did some time ago. I have sarking felt under the extension slates and the downpipe from the main roof is extended across the single story monopitch roof into the gutter, thus keeping the rainwater pipe flow off the monopitch roof. This works fine and there has never been a problem. I would advise putting a membrane under the slates and extending the downpipe across the single storey roof. You will still get a certain amount of rain blown under the slates, so it is important that your slating battens are preservative treated.