Interlocking concrete tiles for 15 degree pitch?

[RomanAbramovich]

Have a proposed single storey side extension and due to the span (wall plate to wall plate 10350mm) the truss company drawing shows a pitch of 15 degrees, in order to keep the roof at/below 4m height (enabling permitted development). Found a couple of products suitable for this pitch but on further investigation noticed 15 degrees cannot be achieved where valleys are in place (they will be).

There are several tiles on the market that are a good match to existing covering, but tech details state 'down to 17.5 degrees'.

If I install concrete interlocking tiles for 17.5 degrees on a 15 degree pitch, with a 100mm+ headlap, am I asking for trouble? In my experience much of this tech advice is over engineered and in real world conditions there are never any issues. Will also have inspection from bco at some point - do they usually ask to see tech details for roof coverings? (I've never known then to physically measure the depth of a footing, for example)

 

[big-all]

i dont have a clue
but increasing the overlap will flatten the tiles a bit more further reducing the angle

 

[noseall]

I have commented on low pitch roofs many times. Add any complexities to the flow of water on low pitched roofs and you are going to be at risk.

Everything needs to be done expertly and with diligence if you are going for a low pitch with valleys etc. Instances where the odd tile pitch will be flattened out, i.e. at the bottoms of valleys or on the fascia kicker will need special attention.

Double felt with the joints staggered will help.

 

[RomanAbramovich]

Thanks for replies.

Existing roof is 22.5 degrees at best, the concrete interlocking tiles have only a two and a half inch lap. Reading tile manufacturers websites there is no way this configuration would be recommended nowadays, yet the roof covering has been in place without any issues since late 60's.

To my mind the specifications for building products are often under stated / over engineered (which isn't a bad thing) - how many two feet deep footings have held perfectly adequately for decades, yet extending the same property, single storey, now demands one metre deep footings. etc etc.

Just trying to assess the real world risk of using a tile at 15 degree pitch where manufacturer has specced for use down to 17.5 minimum. As noseall suggests, techniques like double felting must surely reduce risk of issues arising.