- 1 Aug 2008
- Reaction score
You won't like the answer woody.
More here http://www.monier.co.uk/bs-5534/underlays.html3. UNDERLAYS.
The primary purpose of a roofing underlay is to reduce the wind load acting on the roof tiles by taking a significant proportion of the wind load itself. To do this successfully, it must not stretch to the point where it can touch the underside of the roof tiles when subjected to wind pressure.
If a roofing underlay does stretch, or “balloon”, so that it touches the back of the roof tiles, then it ceases to perform this primary function with the full wind load transferred onto the roof tiles.
The tile manufacturers’ fixing specifications do not allow for this additional load and roof tiles can be blown off the roof even if they are nailed and clipped.
A related problem concerns the bursting open of unsealed underlay laps when subjected to wind pressure which can also cause the removal of roof tiles. Both the old and new Code of Practice recommend an additional timber batten be installed over the horizontal lap between courses of underlay to prevent the risk of this happening. While this is recommended practice many roofing contractors do not like installing these additional battens and they can cause a trip hazard.
So, Redland will guarantee their interlocking tiles to keep the roof below waterproof without an underlay below then. Did you ask Marley that on the phone too?
Oddly, there are more buildings in the UK without an underlay than with, and yet I don't see or hear of those tiles being deposited on the streets and gardens below very often.
Could it be that the introduction of underlays being recommended by the tile manufacturers came in at the same time as the introduction of concrete interlocking tiles. I wonder. I also wonder why. Do you know why AJ?
BTW, Redland's advice you quoted there contradicts with the advice from the manufacturers of breathable membranes. Come to think of it, how does a breathable membrane resist wind pressure anyway?
Hmmm, too much knowledge can be a bad thing eh? SMH
Ok guys, am I missing something here
Marley Eternit plain tiles, interlocking tiles, slates
and fibre cement slates are designed and
tested to minimum rafter pitches, below which
a build-up of surface water may cause the
product to leak.
If the design rafter pitch is less than the
minimum recommended rafter pitch for the
particular tile or slate, then they can only be
considered to have an aesthetic function.
In such cases, the true weatherproofing of the
roof system must rely on a fully supported
waterproof membrane with an uninterrupted
drainage path between counter-battens to the
eaves gutter. Details of the full specification for
such a roof construction can be obtained from
the Technical Advisory Service.
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