Glaze Coating – Some Misconceptions
27 – June 1981
Good roofing practice requires that only as much membrane should be laid as can be covered with the final bitumen coating and aggregate surfacing during a normal daily working period.
Short Term Protection
Glaze coating is considered an emergency measure to provide short term protection against moisture on small uncompleted sections of the roof. It is not meant to provide long term protection and herein lies a common misconception which can cause roofing failures.
Short term protection is overnight protection of the exposed felts against dew, sudden rain or snow. Its correct use is on that relatively small area of felts left exposed between the completed pour coat and gravel and the outer edge of the work which includes the temporary water cut-off. When work resumes the temporary water cut-off is removed and the glazed membrane dried and cleaned.
A proper glaze coat consists of a light application of bitumen spread uniformly over the felts at nor more than 1 kg/m² (20#/sq.).
Longer Term Protection
That this is required is not good roofing practice. It can frequently be avoided by more careful scheduling. It is required for uncompleted areas of membrane where it is known work will not resume for some time. Such areas occur around the perimeter where parapet walls are not completed, around openings, around an uncompleted penthouse and where roofing work has been temporarily discontinued.
When such protection must be used a heavier mopped coating is required, approximately 1.2 kg.m² (25#/sq.) if there is good run off and slightly greater if there will be known ponding. All water cut-offs associated with these areas should be reinforced.
The potential disadvantages in both these coatings are:
SLIPPAGE: If the protective coating is over the finished membrane, use extra precautions in applying the pour coat and gravel. These extra precautions will be influenced by the roof slope, type or grade of asphalt, its quantity and area of protective coating in any one place.
If the protective coating is covered by an interply mopping, the quantity of bitumen at that layer is excessive. Thus 2 and 2 construction with a glaze coat or more over the first two plies is encouraging slippage if there is any slope at all. As much as possible of the roofing operation must be carried out in one operation.
BLISTERING: Should small amounts of moisture penetrate the glaze coat at “skips” or thin spots or edges of the felt, it may become trapped in the felt by the next application of hot bitumen and felt to cause a blister. As the area cools a percentage of the moisture vapour is replaced by air which cannot escape through the following mopped plies. This air and water vapour gradually enlarge the blister under subsequent heatings.
In constructing a good roofing membrane, therefore, any temporary coating should be used sparingly so that those areas will be only a small portion of the whole. Consequently getting the building “in the dry” by using part of what will be the finished roofing membrane is poor roofing practice.
To obtain a good roof the correct procedure is either specify and use more temporary roofs to be removed before laying the roof proper or hold off the start of any permanent roofing until a proper roofing sequence can be followed.