Hi John,


John Rumm coughed up some electrons that declared:

How about something like:


http://www.screwfix.com/prods/50089/...Adaptor-225mm#

augmented with a fly screen glued to the back of it?

(chop your way through with a multimaster)


http://www.screwfix.com/prods/45372/...ate-205-x-61mm

might cover the edges

and


http://www.screwfix.com/prods/78560/...-90-Bend-225mm

could be used on the inside (pointing up obviously ;-) to make sure
water can't blow in


Just wanted to say - I've tweaked your idea a fraction - there's a 45 degree
bend in System-225 here, which I think will give a good airflow and still
retard rain:

http://www.i-sells.co.uk/system-degr...end-p-416.html

Now, according to that web site, the actual duct dimensions are 234mm x 29mm
which sounds ideal. There's a 300-odd mm wide one in the same depth, but I
suspect that will weaken the board too much...

I think this idea will totally work. It might be possible to do the work
in-situ - it's a balance between the doing it from the rear or up a ladder
versus taking the board down, and more importantly, putting it up again in
a weakened state (it's 3.4m long). It will be stable enough once it's
nailed of course.

At the moment, from a ladder in spring is winning in my mind.

Anyway, all the parts are in the spreadsheet and cost of bits is about 95
quid plus flyscreen.

Again, many thanks for an excellent suggestion.

Cheers

Tim