On Tue, 4 Aug 2020 11:10:13 +0100, TimW wrote:

This is an ordinary timber garden shed 8' x 10' (2.4m x 3m), the roof is
felt on treated sw boards on 3x2 purlins, a central ridge and a shallow
pitch of maybe 15deg. Very standard cheap wooden shed. Over 20 yrs I
have refelted and half re felted and it's leaking again, and the soffit
boards are totally rotted.

Soffits on a shed, there's posh! B-)

Or do you mean the roof boards?

I was thinking of covering the roof with something better, hoping to
extend its life and usefulness - what would be good? Onduline?

Refelted our shed roof at least twice but it never lasts more than
about 5 years before it starts to fail. Bog standard shed roofing
felt, clout nail it down stuff.

Last time it needed doing I put coroline(*) on it that was Sep '16
and it's still water tight and looks almost new. The hard bit was
working out the cutting list to minimise wastage as the ridge to
gutter length was a couple of feet less than a sheet. So every third
lenght you had enough "wastage" to do another length. I think the
shed is 6x8 a 8x10 might not be quite so bad.

(*) The name of the smaller corrogation corrogated bitumen sheet.

--
Cheers
Dave.