Workshop roof help

[bigbadbugga]

I'd be very grateful if some oy you more knowledgable builder types could advise me on my workshop roof.

As it stands currently it is almost a flat roof, it actually has a slope but it's only shallow (5 degrees) over a 4.6M (15') width. It's been fine for nearly ten years apart from a little sagging, until a couple of weeks ago when it started to leak quite badly. The water is coming through the tar felt and seeping straight through the osb boards underneath.

So, I'm planning on taking the roof off and replacing it with box profile metal roofing sheets. I've been in touch with the supplier and he has told me to put the joists on 16" centres. The workshop is 8M long so I will need 20.

The question is what size timber should I use for the 4.6M span?

It currently has 4x2 and it has sagged slightly over the years, should I go to 6x2 or even 8x2?

 

[BrokenBiker]

im by no means a builder but I did replace the roof on my nan's shed a few months ago.

same sort of situation, old roof sagged and then eventually broke through, so I ripped it all off and then put a steel frame inside the shed, flush with the top of the brick work and welded 40x40 box section in as supports.

wooden roof on top, screwed into the box section with self drilling screws and the brick work with rawl plugs and screws, bitumen paint over the wood, felt then more bitumen over the overlapping joins in the felt. all stapled down with those big staples used in the guns.

 

[Water skier 16]

A rule my dad has told me is 2x2 for every foot so I make it 8x2 but that doesn't take into account heavy snow loads as it is based on sea level in Cornwall!
James

 

[cumbriasteve]

Same span as mine and I used 9" x 4"

I doubt you need as many as you mention, my garage is 7.5m in length.

I would suggest the coated sheets to avoid condensation.

 

[Wendelspanswick]

I have bought 175 * 75 mm purlins 5m long for £15 from the local agricultural merchants, it's the standard size for steel framed barns. 16 inch centres seems very close, that's seems more like tiled roof truss spacing.

 

[Wendelspanswick]

Here you go, 4' spacing is more like it:
http://www.slecladding.co.uk/diybearer.htm
And 7*3 purlins.

 

[dan.taylor.1]

box profile sheets are normally about 3/4' centres tbh, or at least all our big farm buildings are, and the purlins are 7"x3" as above which is standard apart from longer than 20' spans where 9"x3" are used, and on the gutter edges of the roof where extra is required for fixing the gutters/side boarding etc.

Most timber frame houses are 16" centres with 8"x2" timber but thats for a slate/tile roof so a lot more weight, as for snow as long as the pitch is steep it isnt as much of an issue as a flat roof as with any luck itll slide off!

Dan