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 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 defense 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.
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.
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 (Figure 2).