For roofing a storage shed or other small utility building, the most cost-effective choice is good old galvanized corrugated sheet metal. Though maybe not the most attractive option, this product gives the best combination of low cost and durability. Readily available panels are approx. 26" wide (for 24" o.c. rafter spacing) x 120" long. I found them at Home Depot for around $14 each. It's harder, however, to find accessory pieces and installation instructions.
On-line sources advise that, to avoid leaks, exposed panel fasteners should be installed at panel ridges - never in the valleys. But what about the space under the ridge? Some kind of support must be provided to avoid deformation of the metal. The cross-section detail below illustrates one easy solution.
The 1/4 round wood molding strips are perfect for this job for many reasons:
- They're cheap and readily available,
- They provide solid support for the roof panels along the entire length of the panel overlap,
- The shape of the panel holds the strips in place during installation,
- Splitting is not a problem because the screws pass between the two wood pieces.
Panels with deeper corrugations may need molding strips with a larger radius. The drive screws shown are another standard item. The neoprene washers provide a weather seal around the penetration. For extra leak protection, run a fat bead of silicone caulk between the panel overlap surfaces. Shaped neoprene strips are available to seal panel ends under the eaves above exterior walls. Because the metal roof is lightweight, 2x4 rafters at 24" spacing are sufficient for spans up to eight feet.
