Flat and low-slope roofing

Table 9.26.3.1 of the BC Building Code gives modified bituminous membrane a range of 1 in 50 to 1 in 4, and gravelled asphalt built-up roofing the same. Cold-process built-up runs from 1 in 25 to 1 in 1.33, and smooth or mineral-surfaced roll roofing starts at 1 in 4.

At the top of that range the choice overlaps with metal, which starts at 1 in 4 and carries no maximum. Below 1 in 50 nothing in the table is permitted at all, so a roof measured flatter than that is a re-sloping job rather than a re-covering job.

Measuring the slope is the first task on the roof, and it gets measured in more than one place. A framed roof reading 1 in 40 at the ridge and 1 in 60 at a sagged corner is two different roofs as far as the table is concerned.

A membrane keeps water out. Slope and drainage decide how long it has to. Appendix C lists a one-day design rain of 91 mm for Parksville and 96 mm for Qualicum Beach, which is the volume a roof and its outlets have to move in twenty-four hours without water finding a lap.

The failures are predictable: an outlet set slightly proud of the deck so a puddle sits around it, a scupper cut too high, a downspout discharging onto a lower roof instead of into a leader, a parapet with no overflow at all. Each of those turns a design rain into standing water, and standing water finds the one seam that was rushed.

Every quote sets out where the water leaves, how many outlets there are, and what happens when the primary one blocks.

The Roofing Contractors Association of British Columbia issues the RoofStar Guarantee through RCABC Guarantee Corp, which makes it a third-party guarantee rather than a contractor's own promise. On SBS modified bitumen systems the terms available are 5, 10 and 15 years, and the 15-year term carries materially heavier requirements: a minimum 2 per cent roof slope, moisture surveys with cut tests, insulation overlay, enhanced penetration flashing and 24-gauge metal flashings.

RoofStar coverage is limited to new materials supplied and installed by the contractor, so in-service material left in place sits outside it. Coverage is also conditional on maintenance, and the guarantee can be voided where the roof is not properly maintained.

Two things follow for anyone comparing low-slope quotes. Ask whether the work is RoofStar-guaranteed and at which term, and read what that term requires before assuming the longest one is available on the roof in question.

Most low-slope roofs on houses here are not standalone. A garage roof meets a wall, an addition tucks under an existing shingle slope, a deck membrane runs to a door sill. Those junctions are where the work is.

The membrane has to turn up the wall high enough to stay above standing water and driven rain, terminate into a reglet or under a counterflashing rather than being sealed to the face of the cladding, and pass behind the wall covering rather than butting against it. A door sill needs a threshold detail with a real upstand, not a bead of sealant.

A quote that does not describe the upstand height and the termination at each junction has not priced the part that fails.

• SBS modified bitumen membrane

  Two-ply torch-applied or self-adhered membrane on garages, additions and flat-roofed extensions.

• Built-up and cap-sheet systems

  Multi-ply asphalt systems where the existing assembly or the guarantee term calls for one.

• Tapered insulation and re-sloping

  Fall built into the assembly where the framing does not provide it.

• Drain, scupper and overflow work

  Outlets set flush and sized to the roof area, with an overflow route where a parapet encloses the roof.

• Parapet and upstand detailing

  Membrane carried up and terminated under metal rather than sealed to a cladding face.

• Penetration and curb flashing

  Vents, stacks, skylights and equipment curbs flashed in sequence rather than patched afterwards.

• Roof-to-wall tie-ins

  Junctions where a low-slope roof meets a shingle slope, a wall or a door sill.

• Removal and disposal of old material

  Old membrane and wet insulation stripped, hauled off site, and the property left clear.

Low-slope roofs on this coast spend the winter wet rather than frozen. Appendix C of the BC Building Code lists Parksville at a one-day design rain of 91 mm and an annual rainfall of 1,200 mm, which is water arriving faster and for longer than many flat-roof details were drawn for. Where a garage or addition sits under conifer canopy, needle litter is the other half of the problem: an outlet that clears in September can be blind by December.

Situation

A 1990s addition on a Nanoose Bay property, torch-on membrane over a flat roof tucked under the main shingle slope, water showing on the ceiling below after prolonged rain but never after a light shower.

Complication

The membrane was sound and the outlet was clear. The upstand where the flat roof met the wall had been terminated onto the face of the siding with sealant instead of under a counterflashing, so it held in ordinary rain and failed once water backed up even slightly.

Resolution

The junction was rebuilt with the membrane carried up behind the wall covering and terminated under metal, and the outlet was reset flush with the deck. Appendix C lists a one-day design rain of 91 mm for this coast, and the detail only ever had to survive the days that approach it.