Ice on the Roof: Remove or Leave Alone?

Deep freeze at -26 Celcius. Next day, a balmy 2 degrees and everything melts. Then it rains. Then freezes again. Then snow. What’s a homeowner to do?

Be prepared. Freezing and melting water can be your home’s biggest enemy. Ice, especially, can cause a lot of damage. But the fact is, removing thick ice from a roof is difficult – at best. Montreal’s 1998 Ice Storm is testament to that. You have to make a decision as to what will be most hazardous – removal or doing nothing. It’s important to realize that people removing ice sometimes cause more damage than the ice itself.

Tools, such as hammers, shovels, scrapers, chain saws, and devices such as shoes with ice spikes can damage roofing materials or the structure underneath the roof.  Chemical de-icers can discolor shingles, break down membranes and corrode flashings and drains. De-icers can also damage plants on the ground.
It’s safe to say that de-icing the roof requires careful consideration, and for most homes most of the time icing on the roof can be prevented. Most often, ice-damming on your roof is likely the result of a faulty ventilation system in your home.

For more on this, read the Mose Report article:  Effective Ventilation System Maintenance Stops Ice Damming.

Ice buildup on the roof is never good. But before taking extreme action, it’s important to observe and evaluate. Before doing anything, questions to ask yourself are:

  • Is the ice damaging the building or putting someone in danger?
  • Can I remove the ice  myself or do I need professional help?
  • Can I afford to leave it alone?

Sometimes, the less you do the better. If there are dangerous overhangs or blockages, then they need attention, but otherwise, you’re often better to observe and react only when necessary.

It is essential to understand what you are doing and to be extremely cautious when dealing with an icy roof. Both the roof and the person clearing it are susceptible to danger in the process. Always call a professional when you’re not sure what to do.

The following information is made available through the CMHC page: Removing Ice from Roofs.

The Basics of Sloped Roof Ice Removal

Most ice collects at roof junctions, behind obstructions such as chimneys or skylights, and at roof edges. Drainage, not removal, solves the problem in most cases.


On a sloped roof, your goal is to make drainage paths through the ice on the lower edge of the roof. That’s where most ice dam and water back-up problems occur. Always shovel off loose snow to expose the ice.

If you have power and electric heating cables, making drainage paths is fairly easy. Attach loops of electrical roof de-icing cables to one or more long boards. With ropes tied to the board and thrown over the roof, pull the board up beyond the ice dam, letting the electrical loops hang slightly off the edge of the roof .

If you want drainage paths higher on the roof, use bundled loops of electrical de-icing cables. They can be drawn high on the roof. Make sure that they hang off the edge of the roof so you get complete water drainage.

You can use chemical de-icers on the edge of the roof. Clear the snow. At about every three feet along the edge of the roof, break the ice crust just above the ice block on the edge of the roof. Put de-icer in each hole above the ice dam and in a vertical line down to the edge of the roof. Use noncorrosive de-icers  and use as little de-icer as possible. Repeat as necessary rather than overdoing it the first time.


Removing ice mechanically from a sloped roof is always dangerous — both for the person doing it and for the roof. Removing ice will probably invalidate your shingle warranty. If ice must be removed, have it done by a professional with proper equipment and training.

Researchers learned a great deal about removing ice from sloped roofs by mechanical means in the winter of 1998. The most important lesson: always start at the top and work down. Starting on the bottom can release ice above you that can slide down and hit you. Small bumps of ice that remain on shingles are caught by ice blocks sliding down. As they slide, they catch and rip off the shingles.

Working from the top down allows you to use the ice on the roof as a slide for the ice that is being freed. Use a sledge hammer rather than an ax.The flexibility of the roof deck will cause the ice to fracture and you will not cut into the shingles.

Freezing Rain

Freezing rain is caused when there is a particular atmospheric “sandwich” of cold and warm air. Precipitation, usually snow, is formed in cold air high up in the atmosphere. As it falls, it travels through a layer of warm air that thaws it into light rain. Just before it hits ground level, it moves into another layer of cold air that brings its temperature to below freezing, but it doesn’t have time or the conditions necessary to crystallize yet. When it hits an object, it immediately freezes.

Snow will collect and then fall off wires and tree branches, and remain relatively light as it accumulates on roofs. Freezing rain compacts into tenacious ice that can weigh almost as much as water. The ice storm of 1998 was in fact a continuous series of small storms, one right after the other, that deposited up to 15 cm (6 in.) of ice on tree twigs, telephone wires, electrical lines and roofs. There is no way to stop freezing rain and it is not generally considered a hazard unless it becomes unusually thick.

The 1998 ice storm created two problems: direct weight and blockage of the natural flow of rain and melting ice. The freezing rain stuck all over the roof, not just on the bottom edge, and created ice dams. The dams backed up run-off water just about anywhere on the roof. Flat roofs suffered serious weight problems, while sloped roofs tended to suffer more water-penetration damage.

Common Winter Ice Dams

Under normal winter conditions, many houses in Canada form ice on the edge of sloped roofs or over part of flat roofs.

This is very different from freezing rain. It is caused by heat from the attic melting the bottom of the snow on the roof. When outside temperatures are just below freezing (0 to – 10°C), water flows down the roof under the snow and freezes when it reaches an unheated portion of the roof. This can create an ice dam on the lower edge of a pitched roof. Water can then back up under the shingles and into the roof space.

The first line of defence against ice dams is to reduce the attic temperature by stopping air leaks from the house below and adding sufficient insulation to the attic floor. Heating cables and other de-icing techniques are a last resort to minimize ice build-up and prevent water damage. For full details on dealing with common ice dams, see the CMHC’s Attic Venting, Attic Moisture, and Ice Dams.

Signs of stress

Water leaks showing up inside the house are troublesome and expensive to repair, but don’t necessarily mean that there is a structural problem requiring total clearing of the roof. Opening drainage paths may stop or minimize the leaks and avoid the expense and danger of clearing the roof. Structural stress shows up first at internal doors. They begin to jam.

New cracks show up in drywall and plaster. Jammed doors and cracks in drywall and plaster are usually near the centre of the house, not on outside walls.Watch carefully for these signs of stress. If there is significant change as an ice storm continues, take action. If signs of stress appear but do not change from day to day, the structure is holding solid.

On sloped roofs, another indicator is excessive sagging of the ridge line. If in doubt, arrange for an inspection by a professional, although during a crisis, that is easier said than done.

Recommended Procedures — Flat roofs with central drains

When is it a problem?

In most areas, flat roofs are built to safely hold a maximum of 17 to 20 cm (7 to 8 in.) of solid ice, or 38 to 43 cm (15 to 17 in.) of hardened snow, or 70 to 80 cm (about 30 in.) of fresh snow.

If there is more than 15 cm (6 in. ) of hard ice on your roof, you will have to lighten the load. Freezing rain accumulation can often resemble a hard snow more than a solid block of ice. Testing and judgment is useful. Pour hot water from a thermos in one spot. If it melts a small bowl and holds water, it is probably hard ice. If it cuts through to the roof, the accumulation is more likely hardened snow.

There may have been significant renovations below the roof to many older dwellings with flat or basin roofs. If walls have been removed or modified without full structural compensation, the roof may not even support 15 cm (6 in.) of ice. If signs of stress (see above) are significant, reduce the weight on the roof no matter how much ice is on the roof. You may also have to build temporary bracing inside the house.

Under certain freeze-thaw-freeze conditions, ice can exert strong lateral pressure on the parapet and other roof flashings.The pressure can cause roof leaks. It is a good idea to use one of the drainage techniques described below to separate the ice field from all flashings, leaving room for expansion of the ice field.

Electrical Cables

If electrical power and wires are available, this is the easiest and most effective method of creating and maintaining drainage paths on flat roofs.

Shovel off loose snow. Clear about 60 cm (2 ft.) all around the drain. The safest way to do this is to use non-corrosive de-icers or hot water — a hammer or shovel may cause the drain to leak.

Lay electrical de-icing cables from near the drain to each corner of the roof. (Do not put the electrical cables inside the drain — the drain pipe may contain inflammable gases). Run a loop around obstructions, such as skylights and ventilation hoods. If you can work safely near the edge of the roof, run a cable around the inside perimeter.

The cable will melt its way to the roof surface and keep drainage paths open. It will not penetrate the ice until it is warmer than -10°C and, of course, will not work if there is no electricity.

De-icers for cutting into ice

Pour a 6-mm thick by 75-mm wide (1/4 in.-deep by 3-in. wide) path of de-icer from the drain to each corner of the roof and circle obstacles such as ventilators and skylights. Use the same drainage pattern as you would for electrical cables. See Chemical De-icers for details on products. You may need to use a de-icer more than once to melt through to the roof and to keep drainage paths open.

Ice removal is not a good do-it-yourself project. But homeowners can shovel heavy snow off the top of the ice, which might keep the weight load under control.

Ice thickness and weight of ice can be reduced with de-icers such as urea or even wood ashes. Both are slow and work only in relatively mild weather. To ensure water run-off, create drainage paths as described above. Ashes must be directly on the ice, with no snow over or under the ashes, so they can trap the sun’s heat.

Chemical De-icers

Many de-icers don’t show their ingredients on the packaging. Others list ingredients without showing the relative importance of each.This is no help in deciding which de-icer is safe for a roof or better at cutting drainage paths or reducing ice weight.

In general, the least expensive, most effective de-icers are highly corrosive and should not be used on a roof. Urea, the least corrosive, is also the least effective. In between are several products that are a bit more expensive, still effective and reasonably low in corrosive action.

In general, larger rock-like products tend to cut through ice quickly. Finer, powder-like products tend to perforate the ice. This creates a honeycomb effect that makes the ice lighter. Liquid products are the most effective for detaching blocks of ice from the surface.

What to Avoid: Salts containing oxidizing agents (these accelerate corrosion and rust and can deteriorate other roofing materials) such as:

  • NaCl (Sodium Chloride)
  • CaCl2 (Calcium Chloride)

Safer materials

  • CMA (calcium magnesium acetate)

The following are normally used as fertilizers:

  • Urea
  • KCl (Potassium chloride)
    (NH4)2 SO4 (Ammonium Sulfate)

Life Safety

Ice is slippery and in emergency conditions medical help may not even be able to get to you. Not only can you slip, but ladders can slip. Removing ice from the edge of a sloped roof can release large fields of ice higher up that can slide down on top of you. During the 1998 ice storm, more than one person died from icicles falling from above when they were simply standing in the driveway below.

Double and triple your safety precautions, or stay away from the roof. Rope off areas and access doors where overhead ice is heavy or slides made occur. Never work alone. Always have someone on the ground to ensure that what you throw off the roof is landing safely.

On a sloped roof, always tie the ladder down and have a safety rope over the top of the roof secured on the other side.The safety rope should be attached to a full safety harness, like mountain climbers use — it is not there just in case you slip — it is there because you will slip and more than once.

Special ice cleats are available in shoe repair and hardware stores for attaching to shoes and boots, making them much like golf shoes. These are good for not slipping, but are not good for shingles. Walking on ice-covered sloped roofs is best left to professionals with professional equipment.

More information available on the CMHC web site Removing Ice from Roofs