Their picture is terrible so this is a picture from Home Depot :

Although there is not one standard rafter spacing. Common spacing for trusses on this type of building is often between 2 and 4 feet, but trusses built with advanced designs may allow spacing as great as 8 or 12 feet. The same concept holds true for roof trusses. I am using 2'x12' galvanized metal corrugated roofing panels from Home Depot. So the space between rafters can be greater without reducing the maximum span. Use the drawing above to identify the relevant measurements for calculating rafter and verandah beam sizes. I priced materials and I am going with a combination from Lowes and Home Depot. Note: 25 degree maximum pitch, N3, single span, no ceiling and roof load 25kg/m2. I put the joists 4' OC with perpendicular blocking spaced 2' OC.

We burned about 3 1/2 last year (first year in house) so I'll have 2 years stored with a little margin. If I stack 6' high I'll have 9 cords under cover. My shed will have a covered area that is 24'x8' with a 2' overhang for a 28'x12' roof. If this ever happen under a woodshed it wouldn't be a big deal. A larger space between rafters, in addition to resulting in a larger depth in rafters, also results in a larger space between rafters. Under the right moist and cool weather conditions I have seen water condensate on the bottom of bare metal roofing and watched it rain under the roof as it warmed up and the sun hit the metal roof. The industry standard spacing for roof rafters is 12 inches, 16, 19, and 24 inches on center, but structural engineers may also use other spacing. ,ĮDIT For anything but a woodshed I would still put felt paper or some membrane on the strapping directly under the metal. It makes it a lot easier to put the screws in once you get the sheets up on the roof. Just make sure the metal roofing is all stacked exactly on top of each other figure out exactly where the screw holes need to be to line up with the strapping and drill the holes through all the sheets at once. Already having the rise will make it easy to determine the rafter length by loading the numbers into the equation chart. One trick you can use for helping the metal roofing go on way faster and easier is to pre-drill the screw holes while the metal roofing is still in a stack. You'll save some $$$ using 1x4s over 2x4s. I can't see any advantage to using 2x4s for strapping unless your rafters are further apart then 24'. For metal roofing rafters 24' on center with 1x4 strapping (purlins) is pretty standard around here, with the strapping 16' on center.

Rafter spacing for metal roof shed roof