Metal roofing

Table 9.26.3.1 sets the minimum slope for profiled metal roofing at 1 in 4 and puts no maximum on it, so metal covers everything from a shallow shed roof to a steep gable. Sheet metal shingles sit at the same 1 in 4 floor.

Below 1 in 4 the choice changes. Modified bituminous membrane is permitted from 1 in 50 up to a maximum of 1 in 4, so the two systems meet at exactly one slope and swap places either side of it. A roof measured at 1 in 4 can go either way. At 1 in 5 it cannot be profiled metal, and at 1 in 3 it cannot be membrane. Measuring the slope properly is therefore the first thing that happens, not a formality.

An exposed-fastener panel is screwed through the face of the metal into the structure below, with a gasketed washer at every screw. It is the cheaper system and the faster one to install, and its service life is largely the service life of several thousand washers.

A standing seam panel is held by concealed clips, and the seam is folded or snapped over the fastener line, so nothing penetrates the water plane in the field of the roof. It costs more, moves more freely with temperature, and removes the maintenance item that most often ends an exposed-fastener roof early.

Both are legitimate systems. The difference belongs on the quote as two priced options rather than as an assumption made for the homeowner.

Appendix C of the BC Building Code lists Parksville at a 1-in-50 hourly wind pressure of 0.50 kPa and a driving rain wind pressure of 200 Pa. Qualicum Beach is listed higher again at 0.53 kPa. Those are the numbers the edge details, the fastening pattern and the closure strips are working against.

Metal roofs rarely fail in the middle. They fail at the eave, the rake, the ridge and the transitions — where a closure was left out, a hem was not formed, or a panel was fixed so tight it could not move. On this coast the detail drawings matter more than the panel profile.

Appendix C puts the ground snow load at Parksville at 2.0 kPa with an associated rain load of 0.4 kPa. Snow does not sit long here, but it leaves a metal roof in a sheet rather than melting in place. Where a panel discharges over a doorway, a walkway, a gas meter or an outdoor mechanical unit, snow retention is designed in at quote stage rather than added after the first slide.

Rain noise is the other common question, and it depends almost entirely on what sits underneath. Panels over solid sheathing with an insulated attic below behave very differently from panels fixed straight onto purlins over an open shop or carport.

• Standing seam metal roofing

  Concealed-clip panels with folded or snapped seams, so no fastener penetrates the water plane in the field.

• Exposed-fastener panel roofing

  Through-fastened panels with gasketed washers, priced alongside standing seam rather than assumed.

• Sheet metal shingles

  Profiled metal shingles where the look of a shingle roof is wanted with a metal service life.

• Custom flashing and closure fabrication

  Eave, rake, ridge, valley and transition flashings made to the measured dimensions of the roof.

• Snow retention design

  Retention specified wherever a slope discharges over a door, path, meter or outdoor unit.

• Metal roof refastening and repair

  Failed washers, elongated holes and open closures addressed on sound panels rather than replacing the roof.

• Underlay and deck preparation

  Sheathing checked and underlay laid before panels, on a re-roof or a new build.

• Removal and disposal of old material

  Old covering hauled off site, gutters cleared and the property magnet-swept.

Marine exposure changes the specification more than the appearance. On a lot with open water in front of it, panel coating, fastener metallurgy and the way dissimilar metals meet at flashings and gutters all move up the list, and the windward slope will show wear long before the sheltered one does. Appendix C of the BC Building Code lists a driving rain wind pressure of 200 Pa for Parksville, which is what the closures and hems are holding out.

Situation

A waterfront property at French Creek with an exposed-fastener metal roof around fifteen years old, no leaks reported, but rust weeping from a line of screw heads on the seaward slope.

Complication

The panels were sound. The gasketed washers on the windward face had gone hard and split, and several fastener holes had elongated where the panel was walked on during an earlier satellite dish install.

Resolution

The seaward slope was refastened with oversized gasketed screws into fresh material and the transitions reclosured; the sheltered slope was left alone. Appendix C lists a 1-in-50 hourly wind pressure of 0.50 kPa for this coast, and on a lot with open water in front of it one slope does most of the work.