Updated September 2026
What 1,000 millimetres of rain a year does to a roof
Nanaimo Roofers builds every roof here to 1,000 mm of rain a year, a one-day rainfall of 91 mm on the 1-in-50 event, a driving-rain wind pressure of 200 Pa and a ground snow load of 2.1 kPa. Those 4 design figures for Nanaimo decide the fastening, the underlay and the flashing on every roof we build in the Regional District of Nanaimo.
Roofs here spend much of the year wet and shaded. That is what wears them out.
Driving-rain wind pressure is the figure for how hard the wind pushes rain sideways into a roof. We set our flashing heights from it, because a roof that sheds a straight-down drip can still leak in a storm.
Maintenance keeps a roof going longer, but it cannot rebuild a deck that has already failed. Where a roof is past repair, the next step is replacing it outright.
The figures a roof here is built against
We build every roof against the design values for the place it sits. The ones below decide fastening pattern, underlay, flashing height and how much venting your roof needs.
| Design value | Nanaimo | What it decides |
|---|---|---|
| Elevation | 15 m | Nanaimo's height above sea level |
| Annual rainfall | 1,000 mm | How long the assembly spends wet each year |
| One-day rainfall, 1-in-50 | 91 mm | Drainage capacity at valleys and gutters |
| Driving-rain wind pressure | 200 Pa | Flashing height and lap detail |
| Hourly wind pressure, 1-in-50 | 0.50 kPa | Fastening pattern and edge detail |
| Ground snow load Ss / rain load Sr | 2.1 / 0.4 kPa | Structural capacity of the deck |
| Degree-days below 18°C | 3,000 | Whether eave protection is required |
Design values for Nanaimo under the 2024 BC Building Code.
What does moss actually do to a roof?
Moss holds water against your covering instead of letting it run off, and as a mat thickens at a course edge it lifts the tab above it. We often find a north-facing slope under fir cover stays damp most of the year, so the same roof can be solid on one plane and failing on another.
Venting is what dries the assembly
We vent to not less than 1/300 of the insulated ceiling area, and 1/150 below a 1-in-6 slope or where your roof is framed with roof joists. We split it across opposite sides, with at least 25% at the top and at least 25% at the bottom. Air has to enter low and leave high or it does not move at all.
What happens at the eave?
We carry eave protection well past the wall line. We do not get ice damming here, but we do get about 1,000 mm of rain a year, and that is just as hard on an eave.
What wind does that rain does not
The 1-in-50 wind here is 0.50 kPa, which is well short of what it takes to remove a roof. It is enough to lift the edge of a covering that was fastened short, and enough to push water uphill under a course. Wind damage here usually reaches you as a leak months later rather than as missing shingles, so we check the edges and laps after a storm.
Spring and fall, and why both
The spring visit shows us what winter did, and we make repairs in the best weather of the year. The fall visit gets your roof ready for the winter coming.
The maintenance that changes the outcome
- 1
Clear the surface — Needles, leaves and moss off the field, out of the valleys, and out of the gutters and downspouts. We clear every edge so water keeps moving instead of sitting there.
- 2
Cut the cover back — Branches overhanging a roof keep it shaded and slow to dry. We trim them back to help a north slope dry out.
- 3
Check the details, not the field — Flashings at walls and chimneys, valleys, and every penetration. These are where a 200 Pa driving rain finds its way in, and they are almost never where you're looking.
- 4
Fix small things in spring — We treat a lifted tab, a lifted flashing or a blocked valley as a short visit in April.
A roof on this coast is worth looking at before winter. Send the address to request a free quote, and we'll get someone on the roof to see what it actually needs.