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Technical Note (National Research Council of Canada. Division of Building Research), 1962-09-01
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Flat Roofs in Canada
Baker, M. C.
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https://nrc-publications.canada.ca/eng/view/object/?id=280bdf90-5d23-4def-a164-5116a39c6843 https://publications-cnrc.canada.ca/fra/voir/objet/?id=280bdf90-5d23-4def-a164-5116a39c6843
DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
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APPROVED BY R. F. L. DATE
- September 1962
PREPARED FOR Record of Report to CIB Commission W13, Flat Roofs.
SUBJECT FLAT ROOFS IN CANADA
Most roofs applied in Canada to-day are flat except for house roofs and the roofs of certain exotic-type buildings. This change to flat roofs has developed since World War I I, concurrent with the trend to large one-storey buildings. The annual volume of built-up roofing in Canada at the present time appears to be something less than two million squares. (One
square represents 100 square feet or 9.29 square metres.) In the U. S. A. , the annual volume is reported to be in excess of 30 million squares. Although the volume in Canada is small compared to that in the U.S.A., it nevertheless represents quite a large increase in the use of built-up roofing on flat roofs in recent years. Estimated volume in 1940 was approximately 500,000 squares and in 1930 approximately 100,000 squares.
The usual specification requires four plies of IS-pound bitumen-saturated rag felt applied with either coal tar pitch or asphalt, and with a protective dressing of crushed stone or gravel. In 1955 more than twice as much coal tar pitch was being used as asphalt, but by 1961 the situation had completely reversed in favour of oil refinery asphalt, obtained from the steam distillation of petroleum. Rag felts are made from wood fibre pulp, to which is added scrap paper and a small percentage of rag. Because of problems with synthetic rags, most manufacturer s now dispense with the rag entirely, but the name rag felt has been retained. The fibre felt weighs approximately 6 lb (2.72 kg.) per 100 square feet (9.29 sq metres), and is saturated with 9 lb (4.08 kg.) per 100 square feet (9.29 sq metres) of asphalt or coal tar pitch saturant. The saturated felt weighs approximately 15 lb (6.8 kg.) per 100 square feet (9.29 sq metres). Asbestos felts have been in use to some extent for a
number of years, and more recently glass fibre felts have come into use. A rigid form of insulation is usually placed on top of the structural roof deck, and the roofing membrane applied over the insulation.
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-Many of the roofing manufacturers producing roofing materials in Canada are subsidiaries of companies in the U. S. A., and consequently the tendency is to follow U. S. practices. With the large increase in the U. S. roofing market after World War I I there followed a corresponding development of new roof-deck materials, and this was true also in Canada to a lesser extent. Prior to this, two main types of roof decking predominated, cast-in-place concrete and wood plank or wood sheathing. At present there is in use an ever-increasing variety of multiple unit-type roof decks of lightweight aggregate concrete, steel, and asbestos-cement, to mention a few. Some of these materials have insulating properties, but generally additional insulation is specified by most architects.
The inclusion of a vapour barrier to prevent penetration of water vapour into the insulation from inside the building is usual for most building types. Following recommendations of the U. S. roofing industry, Canadian manufacturers and roofers are recommending the inclusion of vapour barriers on all roofs. The vapour barrier usually consists of two layers of rag felt mopped to the top of the deck or, in the case of steel deck, a vinyl-type film applied with a special adhesive.
The system of bonding roofs was introduced into Canada by the Barrett Company in 1916 at the same time that it was introduced in the U. S. A. Roofing applied according to a standard application specification by a roofer,
approved by the bonding agency, is bonded against repair and maintenance expense for periods up to 20 years. The system was abandoned in Canada in 1960. It is believed that inadequate supervision of workmanship and a lack of concern for
design detail by architects and for protective maintenance by owners were encouraged by this system.
The roofing industry in Canada is at present faced with a variety of roofing defects, and the proportion of defective roofing appear s to be increasing.
It is reasonable to assume that many of the more recent roofing failures are
related to other changes that have recently taken place in building practice. There has been an increasing use of dead level roof decks, and consequently poorly
drained roof surfaces. Precast concrete and other types of multiple unit roof decks are now widely accepted, and many of these are subject to creep deflections, often in exces s of load deflections. This leads to permanent deflections resulting in ponding of water on the roof. There is the use of greater thicknesses of lighter insulations to achieve better thermal resistance, giving a soft underbedding for the roof membrane and subjecting it to greater variations in temper ature. The increasing use of vapour barriers, while restricting the entry of moisture into the construction, introduces problems from moisture trapped in the construction. Air-conditioning and humidification of many building types are common, thus aggravating the problems of moisture transfer into the building components.
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-The roofing defects are generally characterized by a breakdown of the roofing membrane and a wetting of the insulation. Evidence of this breakdown is indicated by leaking inside the building in late winter, usually after a thaw and particularly in February and March. Frequently this occurs in the early life of the roofing, from one to six years after construction. It has not always been possible in roofing field studies to determine whether the moisture initially entered the insul-ation from the inside or from the outside of the building. The penetration of moisture into the insulation from the interior by diffusion is not considered to be capable of providing the degree of wetting often encountered, although there is some evidence to indicate the possibility of wetting by mass air flow around roof penetrations such as at ducts and pipes, or at joints or cracks in the roof deck.
Of greater significance may be the problem of moisture trapped in the roofing during construction. There has been in Canada in recent years a large in,,:, crease in the an ount of winter construction, leading to the application of roofing under conditions where it may be difficult to meet the requirements nece ssary to obtain good roofing. Protective cover is widely used for other phases of construction, but has not yet been employed by the roofing industry. Apart from the hazards of snow and ice on roof decks, and the possibility of wet material from improper storage, there is great danger of inadequate adhesion of felts due to application of materials to cold surfaces, or due to overheating materials to counteract the cold. The overheating may also destroy the desirable properties of the bitumen. For some buildings large quantities of moisture are released into the closed-in building from wet finishing operations, such as concreting and plastering. Under certain conditions, such moisture can penetrate into the roofing system.
Two of the mos t common characteristics to be found in roofs that have failed are: (a) wrinkling of the membrane over insulation joints, and (b) splitting of the membrane across the complete width of a roof. Even though Canada has a wide range of climatic conditions, early surveys seem to indicate these problems exist in all parts of the country.
Failure due to the formation of ridges in the membrane is most prevalent along insulation joints where insula tion is applied over the structural deck. Subsequent erosion and mechanical damage leads to breaks in the membrane, wetting of the in-sulation, numerous leaks inside the building, and eventual complete deterioration of the roofing membrane. In some cases, the vapour barrier retards the penetration of water into the building. This failure appears to be usually confined to flat and low sloped roofs, has occurred on roofs with and without vapour barriers, over various types of insulation, and is believed to occur only with rag felts.
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4
-Splitting of the membrane is characterized by a complete separation, frequently
i
to ｾ in. wide, for lengths up to 100 ft, usually along the continuous unstaggered joints of insulation, accompanied by a corresponding separation of the insulation. Splits are usually parallel to the direction of laying of the felts. This is not confined to anyone type of felt, and with rag felts in some cases splitting and wrinkling has occurred on the same roof.CURRENT RESEARCH ON FLAT ROOFS
The Division of Building Research is interested in a broad range of problems concerned with roofs. Many of the problems are brought to the attention of the Division through inquiries from architects, builders, and owners and deal with roof terraces, roof deck construction, insulation, vapour control, and the more recently developed roof coatings. The Division endeavour s to keep the construction industry informed on these matters, and conducts investigations of roof construction practices and roofing failures. Current laboratory activity is principally confined to work on built-up roofing, as indicated by studies listed below.
1. Durability of asphalts - The evaluation of the durability of asphalts has been studied for about two years and this work continues. Asphalts from various sources have been studied during natural and accelerated degradation. Physical and chemical changes are determined in an effort to relate initial properties to subsequent perfor-mance. During this study it was noticed that the low temperatures of winter caused cracking of the materials without any accompanying chemical change.
2. Low temperature properties - This study, principally concerned with brittleness, uses a much improved and modified form of the Fraaus brittle point apparatus.
Development work is still in progress but results indicate that this may be used to demonstrate and evaluate self-healing properties, degradation changes and effects of overheating.
3. Roof temperatures - A continuous record has been made of the surface tempera-tures for several forms of built-up roofing test panels on outdoor exposure, and this study is continuing.
4. Engineering properties of built-up roofing membranes. - A start has been made on a study to assess temperature effects on strength and deformatiop. in relation to
shrinkage and embrittlement, in view of the high incidence of roof membrane splitting.
5
-5. Combined heat and moisture flow - A study is under way to determine the rate of moisture gain and the moisture content distribution of various insulating materials exposed to a temperature gradient, with the ｣ｾ､ surface sealed and the warm sur-face open to controlled air conditions. Observations are being made for a number of controlled air conditions, and a number of cold surface temperatures.
