November 27, 2025
The “Freeze-Thaw” Trap: Why Winter Destroys Flashing Faster Than Rain
This is a dangerous misconception. While summer storms are loud and obvious, winter damage is silent and mechanical. It doesn’t rely on wind speed; it relies on physics.
The “9% Rule”: When water freezes, it expands by roughly 9%. If that water is trapped inside a tiny crack in your mortar or behind a piece of loose flashing, that expansion acts like a hydraulic jack, prying your roof assembly apart from the inside out.
Here is why the “Freeze-Thaw” cycle is the ultimate enemy of the urban row home.
1. The “Snow Fence” Effect
A sloped suburban roof sheds snow fairly easily. But flat city roofs with parapet walls act like “snow fences.” They trap wind-blown snow against the corners where the roof meets the wall.
The Risk: During a rainstorm, water rushes over your flashing. But snow piles against it. As that snow slowly melts from the heat of your house, it turns into standing water that sits on your flashing seams for days. If there is even a pinhole opening, that water finds it.
2. Masonry is a Sponge
Many St. Louis homes have brick chimneys and parapets. Brick is porous—it absorbs moisture.
The Risk: In winter, your brick absorbs meltwater during the day. At night, temps drop below freezing. The water inside the brick freezes and expands, causing the face of the brick to pop off (spalling) and the mortar joints to crumble. This loosens the counter-flashing embedded in that mortar, breaking the waterproof seal.
3. The Hidden Ice Wedge
Metal flashing overlaps metal flashing. In summer, these overlaps are tight.
The Risk: In winter, water can wick between these metal sheets via capillary action. When that trapped water freezes, it forces the metal sheets apart, creating a permanent gap. The next time it rains, you have a leak that didn’t exist in November.
Don’t Wait for the Thaw
The worst time to fix a roof is when it is covered in ice. The best time is right now, before the deep freeze sets in.