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.

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.

We measure your roof slope in more than one place before we pick a membrane. 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.

Your roof's slope and drainage decide how long the membrane must hold water. 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 we write sets out where the water leaves, how many outlets there are, and what happens when the primary one blocks.

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.

We run the membrane up the wall above standing water and driven rain, tuck it behind the wall covering, and terminate it into a reglet or under counterflashing instead of sealing it to cladding. A door sill needs a threshold detail with a real upstand, not a bead of sealant.

• 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 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.

• 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. 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

An addition with 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 is sound and the outlet is clear. The upstand where the flat roof meets the wall is finished with sealant at the face of the siding rather than under a counterflashing, so it holds in ordinary rain and fails once water backs up even slightly.

Resolution

We rebuild the junction by carrying the membrane up behind the wall covering, terminating it under metal, and resetting the outlet flush with the deck.