replacing lean-to roof with twin-wall polycarbonate

This house has a lean-to conservatory roofed with corrugated plastic. The roof has been there since we moved in and is coming to the end of its life. I'm looking to replace it when we have the guttering above replaced. The wooden cross-supports are perpendicular to the house wall and at irregular intervals (67,62,61,62,61,65,57,58,22cm centre-to-centre). Reading C&A Supplies site <www.casupply.co.uk> suggests to me that the cross-supports should be at regular 1m intervals and to my reading that the roofing is screwed down through the glazing bars.
Just to confuse me further, one of the guys we had come to quote for the guttering told me that the supports should have been placed parallel to the wall and suggested I put battens across the existing supports.
What do the panel suggest? And what would you suggest is the most sensible compromise between cost and 'u' value etc.
tia,
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dave @ stejonda

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dave @ stejonda wrote:

The battens your guy mentioned are called purlins. Whether these are needed depends on the length of the rafters which you haven't told us. C&A have charts to calculate size of rafters and if purlins are needed. Normally they aren't used. Glazing bars merely clamp the edges of the polycarb to allow for expansion. The polycarb is not screwed down. So the rafters are set at the width of the sheets allowing for width of glazing bar and expansion. Your irregular spacing would make this difficult, hence perhaps the purlin suggestion. The glazing bars then could be put on a regular pitch on top of the purlins
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Length of existing roof 215cm. Existing rafter dimensions - 3"x2" which Table A suggest should be 4"x2". :(

Ooops - I was looking at an older catalogue which didn't have these charts.

Except for Sealing Buttons in the centre of sheets. ?

...and without changing the rafters I would end with a mess of rafters and glazing bars occluding the light. :(

Thanks for your response Bill. You've explained a lot. It seems like the real solution is going to be to rip the existing frame down and start again - something I really don't want to spend time and money on doing.
Methinks I'll just go for replacing like for like, for now.
(btw, are you really the postmaster @ hotmail.com)
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dave @ stejonda

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dave @ stejonda wrote:

An alternative would be to make the sheets custom fit your rafter spacings by trimming. > (btw, are you really the postmaster @ hotmail.com) No, but I get so much spam from them they can have some back :-)
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Heh, where did you learn this joined up thinking? I wish I could do that.
(I'll look into the measurements and see if that makes sense with the sheet widths.)
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dave @ stejonda

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