Flat roof repair in Kamloops

The dry-climate trap

Kamloops gets very little rain. That fact, which is true, is quietly responsible for a lot of badly performing flat roofs in this city — because the number that governs a drainage design is not the annual total. It is the worst single day.

Set those two figures against an annual rainfall measured in the low hundreds of millimetres and the shape of the problem appears. A substantial fraction of the year’s rain can arrive in a single day, and a meaningful part of that can arrive inside a quarter of an hour.

A sloped roof does not especially care. Water leaves a pitched surface because gravity makes it, and the gutters are the only thing that has to keep up. A flat roof has no such luxury: it holds water until a drain takes it away. Size that drainage for the average and it will be overwhelmed by the event — and the event is the only condition that ever matters.

Ponding is the diagnosis, not the symptom

Flat roofs are not flat. They are built with a deliberate fall toward drains, and the working definition of ponding is water still standing more than forty-eight hours after the rain stopped. Past that point it is not draining; it is sitting.

Standing water does four separate kinds of damage, and they compound:

1. It concentrates the ultraviolet attackWater magnifies and holds heat, and in a high-sun climate a ponded area ages faster than the membrane around it. On most Kamloops flat roofs the failure is a distinct patch rather than a general decline, and the patch is where the water sat.
2. It freezesWater expands as it turns to ice. A pond that freezes and thaws repeatedly works at every seam and flashing inside its footprint, and the daily freeze-thaw rhythm here means that happens many times a winter rather than once.
3. It is weight the roof is carryingWater is heavy, and a shallow pond spread over a wide area adds up. A deflection that collects water gets deeper as it collects it, which is a self-reinforcing problem rather than a stable one.
4. It hides what is underneathDebris settles into standing water and mats over the drain, and the silt line left behind after evaporation is often the only visible evidence that a pond was ever there. Inspect a flat roof after rain, not in August.

Two drains, and the second one is the one that saves you

The most common serious fault on a low-slope roof is not a failed membrane. It is drainage that had no margin.

Primary drainage is the route water is supposed to take — internal drains, or scuppers through the parapet. It is sized for the design event and it works until it blocks. Which it will, because the same wind that brings the storm brings the debris, and the debris arrives at the drain first.

Secondary or overflow drainage is the one that exists for the day the primary is plugged. It is set slightly higher than the primary, so it does nothing at all in normal use, and it is the difference between a nuisance and water coming through the ceiling. On a roof with a parapet it is often an overflow scupper. Its other job is diagnostic: if water is discharging from the overflow, the primary drain is blocked right now and somebody should be on the roof today.

Worth asking directly about your building: how many primary drains, where are they, is there an overflow, and what is its discharge point. If nobody can answer, that is the finding.

The membrane almost never fails in the middle

Whatever the system — a single-ply sheet, a modified bitumen torch-on, a built-up assembly — the open field of the roof is the part doing the least work and lasting the longest. Failures cluster at the interruptions.

Seams are where two sheets join, and they are only as good as the day they were made. Terminations are where the membrane stops — up a parapet, into a reglet, under a cap flashing — and they carry the movement of two different assemblies pulling against each other. Penetrations are every vent, drain, pipe, curb and mounting foot, each one a hole in the weather layer with its own detail. And changes of plane, the inside corner where the deck turns up a wall, take the most strain in the assembly.

The practical consequence is that a flat roof in reasonable condition with three bad details is a repair, not a replacement — and a flat roof whose field membrane has gone chalky and brittle across the whole surface is the reverse. Mechanically fastened equipment mounted after the fact is the single most common source of trouble, because it was rarely detailed by whoever installed the roof.

What winter does differently up here

A sloped roof sheds snow. A flat roof accumulates it, holds it for the season, and then has to get rid of the meltwater — often through a drain that is still frozen. That is the specific failure sequence to watch for: a warm afternoon melts the surface of the snowpack, the water runs to a drain that has not thawed, and it ponds on a roof already carrying the weight of the snow that produced it.

It is also why the City does not apply a single ground snow load across Kamloops. Its building by-law sets out two zones rather than one, and elevation is the reason. A low-slope roof up the hill is not carrying the same design case as one on the valley floor. Confirm the design values for your address with the City of Kamloops or a structural engineer — that is an engineering figure, not something to take off a website.

If water is already coming through the ceiling, finding where it entered is its own discipline and it is rarely above the stain — see roof leak repair. If you need the condition of the whole roof established in writing before deciding anything, start with an inspection. And if the field membrane is done rather than the details, that is a replacement.