How do I correctly install metal roofing over a workshop with a vaulted/exposed ceiling

Question

I'm in the process of replacing a badly leaking roof over an area that will become my new workshop space once it is dry.

The existing structure is attached to the side of my house, is 14ft wide, and 28ft long, and has a roof that slopes from about 10-11ft where it joins to the house, down to about 8-9ft at the eaves. I measured this as a 2/12 pitch. The construction is 2x6 rafters, 16" OC, with 2x6 blocking halfway down. The sheathing seam lands on the blocking. Prior to my work, the roof was covered with 3 layers of poorly installed asphalt composition shingles, laid over 3/8" plywood sheathing. I have torn everything off down to the plywood, much of which was badly rotted (I stepped through it accidentally to discover this).

For the replacement roof, I am using Fabral Shelterguard 5-rib corrugated metal roofing, and will have an end-lap halfway down the roof because I could not locally purchase 16ft continuous panels of the metal roofing. I also purchased two rolls (4sq total) of 30# ASTM D226 felt. I had intended on putting the felt down under the roofing but I have read tons and tons of conflicting information about whether felt is even necessary under metal roofing. Many people seem concerned that without proper preparation, the metal can sweat on the underside if warm moist air is allowed to contact cold metal panels.

All of this information seems to be specific to house roofs where the underside of the roof is only exposed to a ventilated attic space or similar. I'm not even sure whether I'm going to insulate the ceiling of this workshop. In any case, the ceiling is not going to be isolated from the "living space". I need to know if, and how that changes how I need to install the metal roofing.

Should I tear off all the sheathing, and install the roofing over 1x4 or 2x4 purlins running across the rafters?

If it's okay to put the metal roofing over the existing plywood sheathing, do I need the felt paper under it or can I omit that?

Should I use the foam plug material supplied by the roof manufacturer to seal the eaves underneath the metal roofing (preventing air movement between the sheathing and the metal)?

Answer 1

All metal roofs leak and condensate and therefore need a "moisture barrier". I call this condensation phenomenon, "The Coke bottle effect." When you take a coke bottle out of the refrigerator, it sweats because the inside of the coke bottle is colder than the outside of the bottle sitting in a warm room. So, your metal roofing will be colder on the "outside" compared to the side on the roof sheathing...thus, sweating and condensation.

The type of moisture barrier depends on the slope of the roof. The lower the slope, the better the moisture barrier should be.

For "low-slope" applications, like yours, water is more likely to get blown through joints, up under underlayment (building paper), etc. To protect against this, we use a "peel-and-stick" rubberized underlayment membrane. This membrane is 36" wide and has a sticky backing. It is applied directly to the roof sheathing and lapped 2-3" at all edges. The sticky backing will seal the entire roof. (We turn the peel-and-stick membrane over the edges of the structure about 2-3" to eliminate "blow-back" moisture that could get in under the membrane. )

Then, the metal roofing can be installed directly on the peel-and-stick membrane. When you nail the metal down (hopefully with concealed fasteners) the shank of the nail (or screw) will get sealed by the rubberized membrane.

You didn't ask, but if you insulate the roof, be careful about venting above the insulation. If you don't insulate the roof, be careful to protect the plywood from the dew point or moisture will accumulate under the peel-and-stick underlayment and rot the plywood.

By the way, if the existing plywood is bad enough that you can step through, then it needs to be replaced (or new plywood can be added over the existing plywood if it's dryrot and not insect infestation.) Dryrot will stop if the moisture is stopped, unlike insect infestation. They seem to just keep coming back.