"Understanding Your Home" by Building Inspector Mark Visser
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About half of the roof was covered in moss growth due to poor drainage. There were also three large blisters on this roof.

Aggregate protects the bitumen from erosion and deterioration by the sun's ultraviolet rays.

Built-Up or Tar-and-Gravel Roofs
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Built-up roofs. Tar and gravel roofs.

Cant strips
. These triangular shaped pieces of wood or pre-shaped metal, are installed on flat roofs along the outer edge and where roofs meet chimneys, adjacent masonry walls or parapet walls (a short section of masonry wall above the roof line). The cant strip is needed to avoid a right angle bend in the roofing. The roofing should be extended over the cant strip and not less than 6" (150 mm) up the wall and capped with counter flashing.

Built-up roofs are also known as tar-and-gravel roofs. Several layers of felt are bonded together with fluid bitumen. The last layer, an extra heavy one, is called a float coat.
Aggregate, (gravel) is spread in the wet float coat. It protects the bitumen from erosion and deterioration by the sun's ultraviolet rays.
Life expectancy is approximately 5 years per layer of felt, with a minimum of 3 layers (15 years) to a maximum of 5 layers (25 years). It is not possible to tell how many layers of felt have been used after the installation is completed.

The roof's life expectancy will be reduced by poor ventilation, standing water, bare spots and by walking on it.
Felt should be stored of the ground and be covered. It will be permanently damaged if it absorbs moisture.
Blisters. Water absorbed felt will eventually turn into a vapour. This is accompanied by an expansion of 1000-1500 times the liquid volume, creating blistering and delamination of the bitumen and felt. In the case of an actual leak into the sub-surface, the water will be constantly replaced and larger areas will blister.
Ponding. Standing water will reduce the life span of the roof. Water traps debris, silt and wind born seeds which will deteriorate the roof at a faster rate.
Drains should be cleared (or replaced) to drain away any standing water. Bitumen and gravel can be added to low areas to improve drainage.
Bare spots are no longer protected from the sun's ultraviolet rays and should be repaired by applying a re-coat with bitumen and proper sized gravel.
Alligator cracks. Adding more bitumen is only a short term remedy. Where possible, remove the alligatored material and re-coat.
Tar or asphalt? Tar and asphalt are not compatible and before attempting any repairs, you have to find out what kind of material has been used. You can test this by breaking off a piece of roofing and dropping it in a glass of varsol.
- Asphalt will dissolve fairly quickly.
- Tar will not dissolve at all.
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