Winter is the hardest season on a flat roof in Western Pennsylvania. A typical Pittsburgh winter brings three to four feet of snow, but the bigger problem is how often the temperature swings across freezing. Every thaw sends water into seams and flashings, and every freeze opens them a little wider. Here is what snow and ice do to flat roofs, and how to get one ready.
Freeze-thaw is the real enemy
Water expands about 9 percent when it freezes. When meltwater gets into a small gap at a seam, a flashing or a crack in the mortar of a parapet, then freezes overnight, it pushes the gap open. A winter with dozens of freeze-thaw swings does that over and over. That is why flashings and brick parapets that looked fine in October are leaking by March, and why spring is the best time for an inspection. See the inspection checklist.
Ice at the drains and edges
On a flat roof, the edge is where water leaves: the rear gutter, the scupper, the drain. Those are also the coldest spots, so meltwater refreezes there first. Once the outlet is iced over, the next thaw has nowhere to go and backs up under the roof edge or over the top of the flashing. Leaves left in the gutter from fall make it worse. On row houses, this is a common late-winter leak at the back of the top floor. See row-house roofs.
Roof leaking right now?
Pittsburgh Superior Flat Roofing repairs and replaces flat roofs on Pittsburgh homes, row houses and businesses, with a written price before any work starts. Request a quote online and we’ll call you back.
When snow weight matters
Most roofs are designed to carry the region's normal snow loads. Concern is warranted when snow is unusually deep or wet, when it drifts against a wall or a higher roof, when rain falls on top of snow and soaks it, or when the roof already sags or has soft decking. FEMA's rough guide: about 5 pounds per square foot for 10 to 12 inches of fresh snow, for 3 to 5 inches of old packed snow, or for 1 inch of ice. Warning signs inside include new cracks in drywall or plaster, doors that suddenly stick, and creaking or visible sag.
If you remove snow
- Use a plastic shovel or a roof rake and leave a few inches on the membrane. Metal edges cut rubber and TPO.
- Never chip ice off the roof. Picks, hatchets and shovels punch holes. Clear a channel to the drain or gutter instead.
- Watch the edges and skylights. Both are hidden under snow. Work from a safe position or hire it out.
- Don't use rock salt on the membrane. Check with the manufacturer before using any de-icer.
Fall checklist before winter
- Clear drains, strainers, scuppers and gutters after the leaves are down.
- Trim branches that hang over the roof.
- Fix open seams, lifted edges and loose flashing while it is still warm enough for materials to bond.
- Check parapet coping and mortar joints.
- Make sure downspouts discharge away from the building and aren't crushed or disconnected.
Repairs in winter
An active leak in January still gets handled. A temporary repair stops the water, and the permanent repair waits for a dry day warm enough for adhesives, primers and coatings to work as designed. See what to do when the roof is leaking now.
The bottom line
Keep the drainage clear going into winter, don't attack ice on the membrane, and get the roof checked in spring for what freeze-thaw opened up. See flat roof repair.