Moss in the Frames, Not on the Glass

Most people picture window moss as a green haze on the glass. It rarely is. Glass is smooth, non-porous and gets wiped by every rainstorm, so almost nothing establishes on it.

The growth that matters is a few centimetres away, in the frame. It sits in the channels the sashes slide in, along the exterior sill, in the corners where the frame meets the trim, and in the small drainage openings at the bottom of the frame. Those are damp, sheltered, full of grit and dust, and shaded by the frame itself. As far as moss is concerned, they are ideal.

What Weep Holes Do

Modern windows are not built to keep every drop of water out of the frame. They are built to let water that gets in drain back out. Those small slots or holes along the bottom of the exterior frame are weep holes, and they are the drain.

When a weep hole blocks, the frame stops draining and starts holding. Water sits in the bottom channel with nowhere to go, and it sits there through a Cowichan Valley winter where the wet stretch runs from roughly October into March. Everything that follows comes from that standing water.

Moss is very good at blocking weep holes, because a weep hole is exactly the kind of small, permanently damp cavity it likes. Add needle debris off a Douglas fir or cedar overhead and the blockage packs in tight.

What Standing Water Does to Each Frame Type

The consequence depends on what your windows are made of.

Wood frames rot. Water held against the bottom of a wood frame keeps it saturated, paint or stain fails from behind, and the wood goes soft at the corners and along the sill. By the time you can push a thumbnail into it, the repair is carpentry rather than cleaning.

Vinyl frames do not rot, but they are hollow and welded, and water trapped in the chambers has to go somewhere. It works into the sealed unit at the glazing edge, and once that seal fails you get the permanent fog between the panes that no amount of cleaning removes. The whole sealed unit gets replaced at that point.

Aluminium frames handle water better but conduct cold, so they run with condensation on the inside during our damp months. That condensation drains to the same channels that are already blocked, and where dissimilar metals and fixings sit in constant moisture, corrosion follows and sashes stop running properly.

The Signs Worth Looking For

You can check this yourself from outside, at ground level, on any window you can reach.

• Green or dark growth on the exterior sill, especially at the outer corners.
• A soft mat of moss in the channel the sash slides along.
• Water sitting in the bottom of the frame a day after rain rather than draining away.
• Paint blistering or lifting on the lower rail of a wood frame.
• Fog or moisture between the panes of a sealed unit.
• A sash that has become stiff to slide or no longer seats cleanly when closed.

North-facing windows and anything under a wide eave, a covered porch or overhanging branches will show it first, because those are the ones that never get direct sun or a drying breeze.

A Maintenance Job Now or a Replacement Later

This is the argument for dealing with it early, and it is a straightforward one. Clearing moss out of a channel and reopening a weep hole is routine work on a ladder. Replacing a rotted wood sill or a fogged sealed unit is a different scale of job entirely, and it involves a joiner or a glazier rather than a cleaning visit.

The frustrating part is that the early stage is invisible from inside the house. Nobody notices a blocked weep hole. They notice the soft sill three winters later, and by then the window has been sitting full of water every wet season since.

How the Work Is Done

Frame work is detail work, and it is done by hand. Growth comes out of the channels and off the sills with brushes and picks sized for the profile, not with a pressure washer that would drive water past the seals and straight into the wall assembly.

Weep holes are cleared individually and then checked to confirm they are actually draining, because a hole that looks open at the face can still be packed solid behind. Tracks and channels get flushed gently and wiped out so grit does not go back into a seal or under a sash. Exterior stops, corners and the underside of the sill get the same attention as the visible face, since that is where growth hides.

Wood frames get looked over for soft spots while we are there and you get told what we find, whether that is good news or not. Vinyl and aluminium frames get checked for seal condition and drainage the same way.

It is unglamorous work on a small scale, and it is one of the few maintenance jobs on a house where an hour of attention genuinely decides whether a window lasts.