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How to Clear Commercial Roof Drains Before the First Freeze
To get commercial roof drains ready for freezing weather, clear the leaves and debris from a few feet around every drain and scupper. Take off each strainer dome, clean it, and put it back. Pour water into each drain to make sure it flows freely, then check the membrane and clamping ring around the drain for damage. This is best done in late fall, after most leaves have dropped and before nighttime temperatures stay below freezing. Any drain that runs slowly, or has a cracked strainer or loose flashing, should be looked at by a commercial roofer before the first snow.
Why frozen drains are a problem on flat roofs
A pitched roof sheds water by gravity. A flat or low-slope commercial roof depends on its drains, scuppers, and gutters to move every gallon off the building. If one drain is blocked, the water has nowhere to go.
In winter, a blockage builds on itself. Wet leaves packed against a strainer slow the flow. The standing water freezes overnight and seals the drain completely. The next snowmelt or rain then pools behind that plug of ice. Every inch of standing water adds about 5.2 pounds per square foot to the roof. Across a 20-by-20-foot low spot, one inch of water is more than 2,000 pounds the structure was never meant to hold for weeks.
That ponding water also finds weak spots. Seams, the flashing at rooftop units, and the clamping ring at the drain itself are where a winter leak usually starts. Inside, it shows up as stained ceiling tiles, wet insulation, or water dripping onto inventory and equipment. Freeze-thaw cycles widen any opening a little more each night. A roof that made it through fall with a small, slow leak can have a large one by February.
Step by step: clearing commercial roof drains before the freeze
Plan on a dry day with temperatures above freezing, ideally soon after the trees near the building have dropped their leaves. Bring a bucket, trash bags, work gloves, a flashlight, and a phone to take photos.
- Get onto the roof safely. Use the building's roof hatch or a secured ladder. Send two people, never one. Stay well back from roof edges, skylights, and openings without guards. If the building has no fall protection, or anyone involved isn't trained to work on a roof, stop here and call a roofer.
- Walk the whole roof before touching anything. Find every interior drain, scupper, overflow drain, and gutter outlet. Take a photo of each one as you find it. Note any low spots where water is standing or where dirt rings show water has stood before.
- Clear debris from around each drain. Pick up leaves, branches, bottles, and loose gravel within a few feet of the drain and bag them. Anything left close to the drain will wash into the strainer with the next rain.
- Remove and clean the strainer dome. Lift the dome off the drain and clean out what's caught inside it. Check it for cracks or missing pieces. Put it back securely when you're done. A drain without a strainer lets debris into the drain line, where it's much harder to clear.
- Test the flow. Pour a bucket of water into each drain and watch it. Water should disappear quickly. If it backs up or drains slowly, the clog is in the pipe below the roof, and the line needs to be cleared by a plumber or drain service before it freezes.
- Inspect the drain bowl and clamping ring. Look at the membrane where it meets the drain. Cracks, blisters, loose or missing clamping-ring bolts, and membrane pulling away from the drain are leak points. Photograph anything that looks off.
- Clear scuppers, gutters, and downspouts. Scuppers and gutters clog the same way drains do. Check the downspout outlets at ground level as well. An outlet buried in leaves or ice backs water up the whole downspout.
- Check the overflow drains or scuppers. These are the backup if a primary drain freezes. They need to be just as clear, even though they usually sit dry.
Repeat steps 3 through 5 after any big fall storm. Once winter starts, take a look at the drains after major snowfalls and during thaws. That is when a blocked drain does the most damage.
What not to do
Don't chip ice out of a frozen drain with a shovel, ice pick, or hammer. Single-ply membranes like TPO and EPDM puncture easily, and a hole next to a drain sits right where water collects. The same goes for open flame and propane torches, which can melt or ignite roofing materials.
Don't throw rock salt or ice melt on the roof without checking with your roofer first. Some de-icing chemicals can break down roofing membranes and corrode metal flashing and drain parts.
Don't leave a strainer dome off "to let the water through faster." The water does go through faster, along with the leaves that clog the drain line under the building.
Roofer or plumber?
The two trades meet at the drain, and knowing where the line falls saves a wasted service call.
| Problem | Who to call |
|---|---|
| Debris, leaves, or ice on the roof around the drain | Roofer, or your maintenance staff if they're trained to work on roofs |
| Cracked or missing strainer dome | Roofer |
| Damaged membrane, loose clamping ring, or a leak at the drain bowl | Roofer |
| Ponding in the same low spot after every rain | Roofer, who can add tapered insulation or crickets to move water to the drain |
| Water backs up in the drain even though the roof side is clear | Plumber or drain jetting service to clear the line inside the building |
| Drains that freeze every winter | Roofer to evaluate the drain, often with an electrician for heat trace cable |
Commercial roof drain FAQ
How often should commercial roof drains be cleaned?
At least twice a year, in spring and late fall, and after any major storm. Buildings near trees, or with rooftop equipment that drips or sheds debris, may need checks every month or two during leaf season.
What happens if a commercial roof drain freezes?
Water can't leave the roof, so it ponds and adds weight. One inch of standing water weighs about 5.2 pounds per square foot. Ponding also pushes water into seams and flashing, which leads to leaks and, in severe cases with heavy snow load, structural damage.
How do you unfreeze a roof drain safely?
Call a commercial roofer. Chipping at the ice or using a torch can puncture or melt the membrane. A roofer can clear the ice safely, and for drains that freeze every year, can recommend heat trace cable installed by a licensed electrician.
Can I use salt or ice melt on a flat commercial roof?
Check with your roofer first. Some de-icing products can damage roofing membranes and corrode metal drain parts and flashing, so the right choice depends on what your roof is made of.
What's the difference between a primary drain and an overflow drain?
The primary drain carries water off the roof in normal conditions. The overflow drain or scupper sits slightly higher and only takes water if the primary drain is blocked. Both need to be clear before winter, because the overflow is the only backup when a primary drain freezes.
Why call Landmark Roofing before the freeze
A Landmark Roofing commercial drain inspection covers the whole drainage system on the roof, not only the drain you already know is a problem. We clear and check every drain, scupper, and overflow. We flow-test each one, and we inspect the membrane, flashing, and clamping rings where leaks start. You get a photo report showing each drain's condition, along with any repairs to make now and what can wait until spring.
If a section of your roof ponds after every rain, we'll show you where the water is sitting and what it would take to get it to a drain. If a line is clogged below the roof, we'll tell you so you can get a plumber out before the freeze, not after the leak.
The weeks before the first hard freeze are the last good window for this work. Membrane and flashing repairs need dry surfaces and temperatures above freezing to seal properly. Once a drain is iced over, most fixes are temporary until the thaw.
Book a commercial roof drain inspection before the freeze. Ask about adding drain checks to a regular fall and spring maintenance schedule so they're never missed.
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