Corrugated roofing - recommended fixing method seems "wrong"

hello
The manufacturers of roofing materials such as Onduline show how to fix their product:
http://www.ondulinebuildingproducts.net/Onduline%20Fixing%20Guide%20Grey.pdf
( http://bit.ly/eaI9Rb if mangled )
They show that you nail through the "crest" or top of the "wave" profile rather than through the trough or valley. To my mind (I appreciate that I must be wrong) that approach seems flimsy; trapping the Onduline between the head of the nail and the structure you're fitting it to would be much firmer if there wasn't a void inbetween, wouldn't it?
I presume the reason for the manufacturer's approach is because the peak stands a smaller chance of any standing water, whereas the valley is where the water will flow when it rains. Minimising the chances of water ingress through the roof is a primary concern, fair enough.
Given that you are told to use "waterproof nails" (or screws) c/o Onduline PP or Safetop fittings, is it really such a risk to screw/nail down via the valleys rather than the peaks?
DDS
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DDS wrote:

As I understand it, fastening through the peaks uses the elasticity of the corrugated material to absorb movement, while allowing it to spring back. Nailing through the bottom of the trough would mean you have a rigid fixing between two rigid items, so when things move, the joint would fail earlier.
--
Tciao for Now!

John.

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This seems eminently sensible to me and is what I did with the screws I used. I didn't want to use nails because the piles of junk and things hanging from the roof inside would have fallen off when I nailed. I used screws and washers, and think I achieved a sound, consistent fixing. When I climb up there for the damsons, it sometimes gives a bit, but has been fine. It has also survived the recent gales, torrential rain, ice and snow and last summer's sun as well as the previous years. If I were re-roofing somewhere else, I'd definitely use this method (screws through the peaks) again. The only slight difficulty was sourcing the nails and washers rather than using the Onduline/Wickes nail kits. I had to go to two different suppliers.
--
Bill

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That's one reason. Thermal expansion is another. Nailing through the crests with long nails allows a bit of lateral movement with temperature changes.
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The peak will have a few square inches of rain catchment above the screws, while the valley may have water from a lot of the roof area coming down.
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On 04/03/2011 23:39, DDS wrote:

Thanks for all of the replies - there are several extra points which hadn't occurred to me. I knew my thoughts were wrong and now I am confident that when I fit Onduline, I'll do it properly.
DDS
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Yes - I've seen it done by a Doubting Thomas like you - and it leaked like a sieve.
--
Skipweasel - never knowingly understood.

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The fixing nails supplied by Onduline have a screwer thread on the shank and the cap on the head is shaped on the underside to fit the contour of the crown of the sheet. If theses are put in the down of the roll the sides of the nail cap dig into the side of the roll and will not form a seal at the bottom of the roll.
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Interestingly it's not rain - except for really disastrous rain, that is the problem from my experience. I did a recycle of some of the small corrugation clear plastic roofing - typical used on lean-to's. One sheet went on upside down (silly me!) and it was the snow that was the real finder of the holes; once it started to thaw, the water collected at the bottom of the corrugations and really pee'd through as the holes were the easiest route downwards. Rob
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