The roof goes on top.

Choosing metal

Compared with asphalt shingles, metal has a lot of advantages as a roofing material:

• Durability: Metal roofs are much more durable than asphalt shingles, and should last over 50 years.
• Energy Efficiency: Metal roofs, and their PVDF resin finish, absorb a lot less heat from the sun than shingles, so they’re more efficient during the summer months. You want a roof with high reflectivity (how much of the sun’s energy gets bounced away instead of absorbed) and high emissivity (how quickly thermal energy gets re-radiated back into the sky after it’s been absorbed). Both of these characteristics are combined into one number called the Solar Reflective Index. The range is 0-100, where 100 is better. An asphalt roof has an SRI around 25, while the steel system I installed is rated 45.
• Weight: Metal roofing is lighter than asphalt shingle. Since the house needs to weigh in at under 10K pounds to stay within the trailer’s load rating, any weight saving is a plus.
• Green: Steel panels are 100% recyclable (so they claim), and are good for solar panel installations.

It boils down to upfront installation cost vs. lifetime maintenance costs. Metal costs more up front but pays for itself over time by lasting and saving energy.

I bought 24 gauge steel A.B. Martin ABSeam Panels because they were locally available, had the best SRI ratings among products I looked into, and especially because all their product info was available online. The panels and installation kit cost $1060.

Design considerations for longevity

It feels like every detail of planning a roof installation is a slight variation on the same theme: keep water out. Life will be bad if water gets in, and stays in, between the roofing material and the top of the structure (my SIP panels’ OSB). Mold, rot, and ice damage. It’s the same principle that made the siding rainscreen gap necessary, but it’s even more important here because the roof is more exposed and is low-slope so water doesn’t drain as easily.

Preventing moisture damage

Air gap

I replicated the siding rainscreen gap under my steel roofing panels. Just as in the rainscreen, the gap allows any water that infiltrates the system to drain out while allowing air to move through via the stack effect and dry things out quickly.

Some additional benefits of the gap in this situation:

• Prevent ice dams: Ice dams occur in cold weather when heat from inside the house warms the bottom layer of snow sitting atop the roof. This layer melts, runs down to a colder area of the roof (usually an eave overhang), and refreezes. This process repeats and destructive ice buildups form. They’re a big problem when you have shingles, extended eave overhangs, or gutters. My design has none of those but I felt it’s probably safest to guard against the problem.
• Safety valve for water vapor: If my SIP assembly wasn’t perfect (quite possible), some amount of warm, relatively-humid air from inside will escape out between through the roof panels. If it’s cold outside, moisture from this air could condense under the roofing panels. The air gap provides ventilation to keep things dry.
• Less heat absorption during summer: The air gap prevents direct transfer of heat from the sun into the house via conduction. This means less stress on the SIP OSB, and lower cooling bills in the summer.

Peak and eave details

Here are diagrams of how the roof panels and trim all interface to allow air into the gap while keeping water out.

Roof pitch

Full disclosure: I installed the roof panels at a slightly lower slope than is recommended by the steel panel manufacturer.

There’s a balance to strike between maximizing interior space of the structure and maximizing drainage off the roof. A completely flat roof would create the most space inside (very important to create a usable loft space within the max road-legal height constraint of 13.5’) but would provide no gravity drainage. Also the longer your roof, and the more valleys it has, the longer it will take to shed water, so a steeper slope is necessary to prevent water from rising to the top of the panel seams and get inside.

The recommended slope for most standing seam systems (including mine from AB Martin) is 3/12 (3” down for every 12” over). I designed my roof at a slope of 2/12 to get an extra 7” of interior head space on the low side in the loft. I decided this would not slow drainage enough risk water rising above my panel seems because:

1. The length of my downhill slope is extremely short compared to a normal roof: 8’ 7”.
2. The roof’s profile has no valleys where water running down two different slopes collides.
3. There are no gutters which could get clogged or otherwise impede drainage speed.
4. There is an air gap as an extra safety mechanism to ensure any water that makes it inside can dry out.

Installation

Installation took me about 4 days. See my installation vid, or check out the manual for the gory details.