Existing contruction of roof .
5 x 2 inch rafters at 16 inch centres down length of 32 foot garage roof which are supported across 10 foot width by 3 of 8 x 2.5 inches beams. Distance between beams is approx 110 inches. Existing boarding chipboard topped with hot laid and grano chips and not maintained by previous owners so roof in bad state in places. I want to overlay with OSB on 2 x 1 inch battens would extra supports be required between the existing beams or would this extra weight not effect the 5 inch rafters on the 110 inch unsupported gap. Hope I've explained ok. let me know if it's as clear as 'MUD'. Detached from bungalow but attached to next doors so that present a problem stripping I've done a small bit and it was a s...d to get it off.

Bob, May 2009

Hi John appreciate your reply.
The 8 x 2.5 beams are supports across the 10 foot width and sit on brick piers. The rafers are 144 inch long start on wall plate at back end wall of garage to first beam and so on down length. I agree they should have been 6 x 2 whatever direction laid. Chipboard covering well the planners must have had dark glasses on to allow chipboard even though a detatched garage. Cause of edge board ending up like wet wheatabix slplits in covering of uptand arris angles on outside edge [the otherside joins next doors garage and it's all in one roof, another planning mistake to my mind. Should have been seperated by continuing the dividing wall up and flashed, if you get my meaning.
Had no control over the build if I did I'd kick arse.
Bounce is slight but I put that down to the 5x 2 and chipboard, ply and 6 x 2 would have eliminated bounce.

Bob, May 2009

Its a fairly curious one this - 32' is a long span for 5x2 rafters, yet the beams coming across the span are 8x2.5" so its really those that are taking the load. Does the roof bounce if you walk on it?
Ideally I'd like to see 6x2 beams at 16" centres across the 10' span, but if this isn't possible then ideally the old chipboard should be removed (it'll be soaking wet and swollen anyway) and replaced with 22mm WBP plywood, ready for another coat of mineralised felt or better still a full coat of glass fibre matting. On my own garage roof I took away the original ruined boarding and replaced it with salvaged corrugated steel panels, plastic coated. Expansion problems solved!

John, May 2009