Metal Roofing Guide for DIYers in the Pacific Northwest
This is a technical learning page. Use it to understand how water behaves, why overlap rules exist, and how to plan a roof that stays dry in wind-driven rain.
- Seams and laps (overlap choices and alignment)
- Ridge and eave transitions (closures, ridge cap, drip strategy)
- Fasteners (wrong placement, over-driven washers, missed framing)
1) Water control fundamentals (how metal roofs stay dry)
Metal roofing is a water management system. Your goal is not to “seal everything.” Your goal is to create predictable shedding and drainage so water never gets a chance to travel uphill, sideways, or into seams.
The three water behaviors you plan for
- Gravity flow: water runs downhill
- Wind-driven rain: water is pushed sideways and uphill at laps and edges
- Capillary action: water wicks through tight seams without an air break
2) Measuring and layout (start square, plan waste)
Minimum measurements you need
- Roof run (eave to ridge) and roof length (end to end)
- Pitch (affects detailing and end lap risk)
- Overhangs (eave and rake) plus transitions
- Decide: single-length panels or end laps
- Start the first course square at the eave
- If the first course is off, ribs will “walk” and laps become inconsistent
- Inconsistent laps are where wind-driven rain finds a path
3) Roof types and direction (shed vs gable)
Roof shape influences where water concentrates and where details fail. Direction and transitions matter as much as the panel itself.
4) Coverage and overlap (this feeds the pillar page)
Usable coverage is panel width after overlap is applied. Raw sheet width is not usable coverage. This is the number one reason DIY orders come up short.
| Panel type | Where used | Overlap logic | Why it matters in PNW |
|---|---|---|---|
| Corrugated metal | Roof | Roof overlap is stricter | Wind-driven rain and capillary action target corrugation seams |
| Corrugated metal | Wall | Wall overlap can differ | Walls shed differently; seams still need blow-in protection |
| Rib panels (Ag style) | Roof and wall | Engineered seam | Seam geometry helps water control when installed correctly |
| Polycarbonate corrugated | Skylight and daylighting | Match profile overlap | Expansion changes drilling and fastening rules |
Visuals (planning view)
- Use this page to understand why overlap exists
- Use the pillar page to apply the exact overlap rules and coverage numbers
- Then run the calculator to estimate quantities correctly
5) Pitch and end laps (common leak point)
Panels are strongest and most watertight when they run in one piece from ridge to eave. If you cannot do that, you create a horizontal end lap where water must cross a seam.
- Lap direction fights water flow
- Seam is placed where water slows down or ponds
- Sealant is used as a substitute for correct lap direction
6) Fasteners (what fails and why)
Fasteners are both a structural connection and a water seal. Most leaks that “appear later” start as an over-driven washer, a missed framing member, or a screw placed in the wrong part of the profile.
Washer compression rule
Tighten until the EPDM washer seals and slightly bulges. Do not crush it flat. Over-driving damages the seal and invites water.
7) Closures, ridge, and trim (finish the water system)
Corrugated profiles leave natural gaps. Closures, ridge caps, and trim are functional weather components, not decorative pieces. They control blow-in rain, insects, debris, and the pressure changes that push water where it should not go.
The ridge combo (common leak prevention)
- Profile-matched closures fill gaps and block blow-in
- Ridge cap sheds water over the ridge transition
- Correct overlaps keep water flowing on the outside surface
8) Planning and ordering (apply knowledge to your build)
Best order flow
- Use this page to understand seams and water behavior
- Use the pillar page to apply the exact overlap rules and coverage planning
- Run the calculator for quantities
- Add accessories (screws, closures, ridge and trim)