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Problems in Flat Roofs: A Review of Research
i'rr'::r .il!i"i Itfi-l r:'"1 .*i,. .l-ii3 i: " l.:: Fi;-'iil
PROBLEIdS
IN
A R E V I E WFLAT ROOFS
OF RESEARCHNatlorual ResgRncx
Counclt-D I V I S I O N O F B U I L Counclt-D I N G R E S E A R C H
A l . i A ! - Y ; 1 ' - r
BY G. O. HANDEGORD , T E C H N I C A L P A P E R N O ' I A 2 O F T H E D I V I S I O N O F B U I L D I N G R E S E A R C HOTTAWA
J U N E 1 9 6 4
C A N A D A F R I C E 2 5 C E N T SThis publication is one of a series of reports produced by the
Division of Building Research, National Research Council of
Canada. Noabrldgernent of this reportrnay be published
with-out the written authority of t"Ile Division. Extracts rnay be pub
-Iiehed for purpoeee df review only.
Qopies of this and other publications of the Division rnay be ob-tained by rnailing the appropriate rernittance ( a Bank, Express r o r P o s t O f f i c e M o n e y O r d e r , o r a c h e q u e , r n a d e p a y a b l e . t o t h e Receiver General of Canada, credit NRC) to the Publicatione
Section, Division of Building Research, National Research
Council of Canada, Ottawa. KIA 0R6. Starnps are not accept-able.
A list of the publications of DBR/NRC ie avdilable, on request,
frorn the Publicatione Section of the Division.
NATIONAL RESEARCH COUNCIL CANADA
DTVISION OF. BUILDING RESEARCH
PROBLEMS IN FLAT ROOFS . A Review of Reeearch b y G . O , H a n d e g o r d
A N A ! . Y Z E D
T e e h n l c a l P a p e r N o . l 8 Z of theDivlslon of Butlding Reaearch
OTTAWA June 1964
PROBI,EMS IN FLAT ROOFS . A R e v i e w o f R e s e a r c h
b y
G . O . H a n d e g o r d *
I n t e r e e t l n t h e p e r f o r m a n c e o f f l a t r o o f syeteme has grown during r e c e n t y e a r 8 , b o t h w l t h l n t h e i n d u s t r y and among research organizatione c o n c e r n e d w i t h b u i l d t n g . T h i e h a s b e e n d u e b o t h t o a n i n c r e a s e i n t h e n u m b e r o f r e p o r t e d r o o f i n g f a i l u r e s a n d t o a r e n e w e d c o n c e r n wlth roofing epeclfi-cations and design. The populartty of dead flat roofs and the reeultant l n c r e a e e i n n u m b e r a n d s i z e o f s u c h r o o f s constructed during the laet
twenty years hae no doubt had eome bearing on the number of roofing failuree gaining attentlon, not only because of multlplication, but aleo because roof f a i l u r b s a r e r n o r e p r o m p t l y d e t e c t e d and results more diaaetrous with dead l e v e l r o o f s . G h a n g e e i n r e c e n t y e a r s l n t h e m e t h o d s o f e p e c l f y l n g roofs a n d t h e a d v e n t o f n e w r e g u l a t i o n s a n d g u a r a n t e e s , ae well as new materiale, h a v e c o n c e n t r a t e d t h e a t t e n t i o n o f d e s i g n e r e , c o n t r a c t o r a , ae well as
m a t e r i a l s u p p l i e r s , o n t h e s e p r o b l e m e . T h u s e v e n t h o u g h t h e n u m b e r o f r e p o r t e d r o o f l n g f a i l u r e 6 r e p r e s e n t s a small percentage of the total area of f l a t r o o f s i n s t a l l e d , t h e e e f a i l u r e s c o n s t i t u t e a challenging and frequently c o n t r o v e r s i a l t o p i c a m o n g t h o e e c o n c e r n e d .
Many roofing problems are due to faults in construction, p a r t i c u l a r l y a t f l a s h i n g d e t a i l s , a n d s u c h f a i l u r e s h a v e o b v i o u s c a u s e a a n d l o g i c a l r e m e d i e s . s t i l l o t h e r s a r e d u e t o i m p r o p e r a p p l i c a t i o n
t e c h n i q u e s a n d s u b s t a n d a r d m a t e r i a l s which can be countered by adequate i n s p e c t i o n . T h e p r o b l e m s o f m o s t c o n c e r n t o s o m e m e m b e r s o f the
l n d u s t r y , a n d o f m o g t l n t e r e s t t o r e s e a r e h w o r k e r s , a r e t h o s e r e s u l t i n g f r o m m o r e o b s c u r e c a u s e s and which result in blisters, wrinkles, and c r a c k s i n t h e r o o f i n g m e m b r a n e .
t r Officer -in-charge, R e s e a r c h , N a t i o n a l
Prairie Regional Station, Division of Building R e s e a r c h C o u n c i l , S a s k a t b o n , S a s k a t c h e w a n :
O n e o f t h e r n o s t n o t a b l e e a r l y s t u d i e s o n t h e s e t ) T ) e 5 o f p r o b l e m s w a s t h ; r t c a r r i e d o u t s o m e t w e l v e y e a r s a g o a t t h e U n i v e r s i t y o f M i n n e s o t a , u n d e r t h e d i r e c t i o n o f P r o f e s s o r C . E . L u n d ( 1 ) . A t t h a t t i m e , t w o t y p e s o f b l i s t e r i n g p r o b l e m s w e r e m e n t i o n e d , w e a t h e r b l i s t e r s a n d s t r u c t u r a l b l i s t e r s . W e a t h e r : b l i s t e r s w e r e i d e n t i f i e d a s s m a l l s u r f a c e b l i s t e r . s t h a t w e r e c o n f i n e d t o t h e r o o f s u r f a c e c o a t i n g a n d w e r e a t t r i b u t e d t o t h e expansion of volatiles in the bitumen, or to small air bpbbles in the top c o a t . T h e y w e r e t h o u g h t t o b e t h e r e s u l t o f t h e n a t u r a l p r o c e s s o f
weathering and not the cause of failure during the normal life of the roof. S t r u c t u r a l b l i s t e r s w e r e d i s t i n g u i s h e d f r o m w e a t h e r b l i s t e r s a s b e i n g m u c h l a r g e r i n s i z e a n d o c c u r r i n g w i t h i n t h e r o o f i n g p l i e s r a t h e r t h a n i n t h e t o p s u r f a c e c o a t i n g . T h e i r f o r m a t i o n w a s c o n s i d e r e d t o b e d u e t o t h e e x p a n s i o n o f a i r a n d w a t e r v a p o u r . t r a p p e d b e t w e e n p l i e s d u r i n g c o n s t r u c t i o n , u n d e r t h e a c t i o n o f h e a t f r o m t h e s u n . I t w a s s u g g e s t e d t h a t , o n a d e c r e a s e i n t e m p e r a t u r e , t h e s e b l i s t e r s d i d n o t r e t u r n t o t h e i r o r i g i n a l s i z e b e c a u s e o f p e r m a n e n t s t r e t c h i n g o f t h e f e l t . E m p h a s i s was placed on the fact that water vapour in these air pockets could
m a r k e d l y i n c r e a s e t h e p r e s s u r e s t h a t w o u l d d e v e l o p a n d t h e p o s s i b l e w a y s i n w h i c h w a t e r v a p o u r c o u l d b e t r a p p e d o r e n t e r t h e s e p o c k e t s
\ t r l e r e d i s c u s s e d . M o i s t u r e i n t h e r n a t e r i a l s d u e t o p o o r s t o r a g e c o n d i t i l o n s a t t h e s i t e , w a t e r e n t e r i n g t h e u n f i n i s h e d r o o f b y r a i n o r b y d e w f o r m a t i o n during the night, or trroisture entering the roof components from within t h e b u i l d i n g o r f r o m w e t d e c k m a t e r i a l s u n d e r w i n t e r c o n d i t i o n s w e r e c o n s i d e r e d .
T h e e > c p e r i m e n t s t h a t w e r e c o n d u c t e d d e m o n s t r a t e d t h a t , u n d e r sirnulated winter conditions, moisture could accumulate in the insulation under a roof membrane when no vapour barrier, or only apartial vapour b a r r i e r , w a s e r n p l o y e d , ' T e s t s u n d e r s i m u l a t e d s u r r r m e r c o n d i t i o n s ; s h o w e d t h a t t h e r e w a s a p o s i t i v e t e n d e n c y t o b l i s t e r i n g i n r o o f s y s t e m s . w h e r e .moisture was trapped in the insulation between the roofrng and v a p o u r b a r r i e r , 4 n d t h a t t h i s t e n d e n c y w a s m o r e p r o n o u n c e d a f t e r t h ' e s p e c i m e n h a d f i r s t b e e n e x p o s e d t o w i n t e r c o n d i t i o n s .
, 3
-T h e r e s u , l t s o f t h e M i n n e s o t a s t u d y w e r e u s e d i n l a r g e m e a s u r e , a s a b a s i s f o r t h e a n a l y s i s o f r o o f i n g p r o b l e m s i n s u b s e q u e n t y e a r s , a n d a s a g u i d e t o g o o d r o o f i n g p r a c t i c e s . L a t e r p u b l i c a t i o n s s u g g e s t e d t h e use of edge venting or ssnall pipe vents through the roofing membrane as a means for relieving pressure and for drying out wet or damp insulation. I n 1 9 5 9 , a v e r y d e t a i l e d a n d c o m p r e h e n s i v e r e p o r t o n s t u d i e s o f f l a t roofs in Norway was publislred by the Norwegian Building Research
Institute (2). This report dealt mainly with ventilation methods in use and proposed for the roofing systems common in Norway, and the majority of these were not tlryical of designs used in North America. Little ernphasis w a s p l a c e d o n t h e u s e o f v e n t s t o r e l i e v e p r e s s u r e , b u t r a t h e r a s a
moisture removal technique. Th.e migration of residual moisture from c a s t - i n - p l a c e c o n c r e t e d e c k s o r f r o m l e a n c o n c r e t e s c r e e d i n g w a s a l s o considered a significant factor in the wetting of insulation. In one of the roofs investigated, where corkboard insulation was used in a manner
s i m i l a r ' t o C a n a d i a n p r a c t i c e , g r o o v . e d c o r k s l a b s w e r e r e c o m m e n d e d w i t h a d a m p - p r o o f b a r r i e r o v e r t h e c o n c r e t e s l a b .
Based on the general conclusions of this study, ventilation of insulations applied over concrete slabs or other impermeable membranes was strongly advocated for the purpose of drying out the roofing assembly.
In the main, both the Minnesota work and the approach in Norway placed heavy ernphasis on the role of moisture as the cause of roofing
failures, recognizing both the possibility of moisture acculrlulating in the insulation by condensation or because of unsuitable weather during
c o n s t r u c t i o n . I n 1 9 6 0 , W a r d e n , i n t h e U n i t e d S t a t e s , s u g g e s t e d t h a t t h e role of rnoieture may have been over -emphasized and that blister
formation wa6 more liltely due to the pumping action of the roof
I t s a n d w i c h r r under cyclical temperature chg.nges (3). faking into account t h e g a s e o u s e x p a n s i o n a n d c o n t r a c t i o n f o r c e s i n v o l v e d , i t w a s c o n t e n d e d that the amount of air within the roof would increase with time. It was r e a s o n e d t h a t a i r c o u l d e n t e r t h e s y s t e m t h r o u g h t h e r o o f i n g m e m b r a n e i t s e l f b e c a q s e b i t u m i n o u s m a t e r i a l s w e r e s e m i - p e r m e a b l e t o a i r .
- 4 - .
P r o f e s s o r F r a n k J o y h a s f u r t h e r e x p a n d e d t h i s t h e o r y i n a
recent publication of the Pennsylvania State College Experiment Station (4). He suggests that, under the influence of daily temp€rature changes in a roof, air and water vapour may be drawn into the roofing insulation through unintentional openings in the vapour barrier or at the roof perimeter. This inflow of air is presumed to take place at a slow rate, since long and
relativel.y resistant paths are involved. Under a subsequent rapid increase in roof temperature on a bright sunny morning, this rnoist air tends to expand,, creating pressures that must be relieved through the same paths, unless the roofing membrane gives way. At this tirne of rapid temperature rise, the roofing membrane is hot and pliable, and, if a point of bond weakaess exists, there may be little rnore than the weight of the roofing membrane and gravel to hold it down. The calculated pressure exerted within the roof could approach 700 pounds per square foot at 140'F, which is considerably greater than the weight of one square foot of roofing. As a r e s u l t , r o o f b l i s t e r s b e g i n t o f o r m .
As a blister gf,ows in size, the pressure required to continue its growth rnay actually diminish, since the total lift is proportional to the area and the anchorage at the rim is proportional to the diameter. Thus, for a small blister, the lift per inch at its rim doubles as the diameter is doubled.
Eventually, reduction in temperature rise, or lateral or down-ward air leakage from the blister, will stop its growth. The roofing
membrane will have cooled by this tirne and the bitumen regaiaed sorne of its rigidity, As the ternperature continues to drop, the blister becornes rnore rigid and rnore air is drawn into the enlarged space. Rapid heatirrg the ne><t day results in a repetition of the sarne phenomenon and the
blister continues to grow until some more di.rect passage for pressure relief occurs.
In keeping with this theory, Prof,essor "Ioy suggests that the tendency for large blisters to form in a non-tight roofing systern is
5
-increased by the presence of liquid water directly under the roofing. This ' water can reeult in a much larger increase in pressure, and is also
detrimental to the bond between roofing felts and insulation.
This effect of moisture on the bond between layere of the roofing system wae algo suggested by Lund in the early Minnesota work. More recently, Brotherson has shown that moisture itself is the primary cauee of another type of failure, cori:monl.y referred to as wrinkling or ridging (5). Hie examl.nation of roofs and experiments in the laboratory clearly indicate that the expansion of some roofing felts on an increase in moisture content could produce ridges in the roofing membrane. He points out that the joints in insulation boards provide a drain for the hot bitumen which may leave the bottom face of the felt uncoated at this point. Since these points also constitute a location for the concentration of moisture under winter conditions, the peculiar location of these wrinkles can be readily er<plained. He suggested as a possible remedy the use of a coated felt base sheet used as the first ply. It is of interest to note that one of the incidental
flndings of the Norwegian study previouely mentioned was that a base sheet consisting of alrrminum foil appeared to be a possible factor in the satis-factory performance of certain roofs.
The problem of cracking of roof membranes has not recelved the sarne degree of attention in research as has been devoted to blistering or ridging, Bdotherson has suggested that cracking may eventually result from the degradation of the bitumen and felts at a wrinkle and the materials may easily lose the protection of the gravel surface and suffer accelerated weathering. He suggested that additional information is needed on the physical properties of roofing membranes and on the stresses induced by e4pansion and contraction under changes in temperature and moisture
content. It has also been suggested that the freezing of ponded water could s t r e s s t h e r o o f i n g m e m b r a n e t o a s e r i o u s d e g r e e , b u t t h e r e h a s b e e n n o evidence presented to support this suggestion.
6
-One can easily appreciate that in practice the roofing membrane is subjected to an extreme range of ternperature and humidity conditions, not only on a yearly, but on a daily basis. On a bright summer day, surface t e m p e r a t u r e s o f b l a c k r o o f s m a y r i s e t o v a l u e s 4 0 t o t 5 d e g r e e s o r m o r e above air temperature. Indeed, it has been estirnated that the surface temperature of a black flat roof on a low buildtng adjacent to higher
buildings may reach 230" F.on a sunny day in summer. At night, under a clear sky, the surface temperature of the same roof may drop below the minimum air temperatsre by 5 or 6 degrees.
In winter, the minimum surface temperature of the roof membrane may fall well below miaimum air temperature (as much as 14 degrees in $aekatoon), with the maximum surface temperature rising to values Z0 to 30 degrees above air temperature under bright sunshine. At the same !ime, the moisture conditions to which the membrane is e:rposed may vary widely, depending on previous weather conditions and on the moisture conditions within the roofing system'.
To compound the cornplexity of the situation, not only the extrerne exposure limits must be considered, but the rate of change and the frequency of wetting and drying, heating and cooling cycles may have
an important bearing on the behaviour of the materials involved Joy has shown this to be important to the forrnation of blisters. He also refers to the dramatic effect of repeated wetting arrd drying of unsaturated rag felts, presented by Martin in Australia, where a sample, exposed to weathering for five years, suffered a 60 per cent reduction in area. Recent work by the Division of Building Research in Ottawa, and by
other research workers, has indicated that biturninous membranes have potentially large coefficients of erpansion which are not constant with t e m p e r a t u r e ( 6 ) .
Much more information is needed on the prdperties of the various components of the roofing system and the effect on these properties of the environment to which they are elq)osed in practice.
- ' l
T h i s i s c o m p l i c a t e d b y t h e f a c t t h a t e a c h c o m p o n e n t i n f l u e n c e s t h e c o n d i t i o n s t o w h i c h t h e o t h e r s a r e s u b j e c t . L o w d e n s i t y , e f f i c i e n t insulations force the roofing rnembrane to undergo a rnore extreme cycle of exposure than if no insulation was used. At the sarne tirne, the r o o f d e c k i s k e p t a t m o r e c o n s t a n t c o n d i t i o n s . A n a s t u t e s t r u c t u r a l
d e s i g n e r m a y w e l l r e c o g n i z e t h e s i t u a t i o n a s f a r a e t h e d e c k i g c o n c e r n e d and reduce the number of expansion joints accordingly. The situation e > q p e r i e n c e d b y t h e r o o f i n g m e m b r a n e i s t h u s m a d e e v e n m o r e a c u t e - a n i n c r e a s e i n d i m e n s i o n a l c h a n g e w i t h f e w e r j o i n t s d e s i . g n e d t o t a k e c a r e o f t h i s i n c r e a s e d m o v e m e n t . ' W h e n o n e a t t e m p t s t o c o n s i d e r h o w t h e s e i n d u c e d
stresses are distributed throughout the roof, or epeculate on the coincidental i n f l u e n c e o f m o i s t u r e , t h e p r o b l e m b e c o m e s c o m p l i c a t e d t o t h e e x t r e m e ,
With all these fa.ctors considered, it is truly remarkable that such a limited number of roofing failures has occurred. Whether this is a tribute to the roofing industry or to the special properties of the roofing materials is difficult to answer. One can say, however, that the
complexity of the situation makes it extrernely difficult to predict just what conditions can be e:rpected in roofs other than in a very general and
semi-quantitative way. This situation is common to many technical problems in building. All that the designer can do is to use the available i n f o r m a t i o n t o h e l p h i m i n m a k i n g d e s i g n d e c i s i o n s . T h e r e s e a r c h w o r k e r can use the available inforrnation to examine principles and ideas for f u r t h e r r e s e a r c h a n d d e v e l o p m e n t .
One rnight conclude from the studies and assessments that have been.carried out that all possibitity of blistering and wrinkling could be avoided if a roofing system could be built that would have no air or moisture in it. This is obviously impractical, and still does not eliminate the possibili.ty of cracking or other problems due to therrnal expansion and contraction of the roof system components. What we must look for is a compromise solu.tion based on consideration of the relative importance of the various factors involved.
8
-Perhaps the closest approach to the ideal of eliminating air and moisture would be a conctruction consisting of impermeable insulation in which all joints remained filled with impetmeable material and over which w3.s placed an impermeable roofing rnembrane, containing no air pocketo. Assurance would have to be given that the joints in the insulation remained tight arid that the roofing membrane could withstand the stresses introduced by changing temperature conditions or ice build-up on the surface.
Using another approach, it could be reasoned that a ventilated space be provided between the insulation and the roofing membrane in order that pressure relief and moisture removal could be accomplished. In such a design, the vapour barrier could be less than perfect but must stiil exhibit relativelyhigh resistance to the flow of air and vapour. The requirernents of the roofing membrane, as far as temperature stresses are concerned, need not be as stringent since its supporting deck would be subject to similar conditions. Some means of preventing the entry of moisture into the felts frorn below migtrt be necessary to avoid moisture expansion, particularly at joints in the deck. The basic system coreld rePresent a return to past practice rrhere this arrangement was used for the purpose of creating a sloped roof.
Cost, fire, and other considerations might make such a system unsuitable for many applications. As an alternative, grooved channels connected to perimeter vents might be used in a roofing rf sandwichrt with semi-permeable insulations to provif,e pressure relief. Similarly,
highly Porous irlsulations might be used without channels to provide pressure venting. It is unlikely, however, that such venting arrange-ments could provide an effective means for moisture removal unless forced circulation of air was provided. With only natural ventilation, a tight vaPour barrier would be essential if no moisture accumulation could be tolerated in the insulation. Even with a perfect vapour barrier, some initial moisture might be present in the insulation and this would have to be prevented from entering the lower plies of felt to avoid wrinkling problerns.
9
-It is often not fully realized that the small amounts of moisture in a normally dry material may migrate under a temperature gradient and result in a concentration of moisture at the cold face. For instance, a f -in. thickness of fibreboard insulation, having a moisture content of l0 per cent, contains about one-sixth pound of water per square foot, but if all this water migrates into one square foot of l5-lb felt, the felt will have a moisture content exceeding 100 per cent. It is entirely conceivable that the ineulation may be at an even higher moisture content than this if weather conditions during construction are not ideal. It is therefore lmportant that some means be provided to prevent the entry of such moisture into the bottom of the roof, membrane in addition to the normal vapour barrier.
The possibility of moisture being trapped in the insulation under certain conditions points up the fact that the vapour barrier under the insulation can be a bane as well as a blessing. One may even argue that, in certain situations, elimination of the vapour barrier rnay provide the b e e t d e s i g n c o m p r o m i s e .
It is difficult to dispute the fact that during winter, the usual interior and exterior conditions of a building are such that water vapour will diffuse into a porous roof insulation and condense bene4.th the rgof membrane if no vapour barrier is provided. It may also be said, however, that in sur-nmer the flow will be reversed, the insulation tending to dry out. If the moieture that can be removed in sunrmer is equal to or greater than the moisture accumulation in winter, no progressive increase in moisture content will be experienced from year to year That this is possible can be demonstrated for most normal internal environments and for the weather conditions in most regions of Canada. There are, of course, a good many other factors that must be taken into consideration.
The roofing system muet first of all be capable of holding the winter moisture without dripping or without permanent damage to the
l 0
-various components involved. Secondly, the possible increase in heat loge through the wetted insulation rnust be recognized and allowed for in design.
Not enough information ie available at present to assess fully the effects of accumulated motsture or frost on the durability or stability of the roof. The actual physical effects on the materials or the bonds between materials are not clearly defined. There have, however, been some
investigations of the theimal effects of moisture in roof insulations which are particularly intere sting.
Studies carried out by Powell and Robinson at the United States Bureau of Standards have involved the measurement of heat flow and moisture content in a variety of roofing systems under simulated summer and winter conditions (7). The teets showed that a vapour barrier under an insulation tended to keep it dry and maintain its insulation value if the insulation was initially dry. If the insulation contained appreciable
moisture, a vapour barrier or even a concrete deck opposed the escape of moisture from the insulation to the room during surnmer exposure and kept r the insulation moist. Moisture present initially in a green concrete deck, however, tended to migrate into the ineulation under winter conditions if no intermediate vapour barrier was preeent.
It was found that highly permeable insulations, without a deck or vapour barrier, tended to accumulate rnoisture rapidly in winter with a marked increase in apparent heat flow. Their drying rate in sumrner, however, was egually rapidr as was their regain in insulating value.
Moderately perneeable insulations under the same ionditions accumulated moisture slowly in winter, with dnly a rnoderate reduction in insulating value, and dried quickly in surnmer, to approach the insulating value of the dry material. It is suggested by Powell and Robinson that, if such a roof should develop a leak, it would probably be quickly detected because no vapour barrier was used. Once the roof is repaired, the insulation would tend to recover its original insulating value due to self -drying, and
expensive replacement of large areas of insulation and roofing would be eliminated.
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-None of this work was undertaken at below -freezLng
t e m p e r a t u r e s , b u t r e s u l t s of some preliminary work carried out at the DBR Prairie Regional laboratory in Saskatoon have suggested that, with certain insulations, the increase in heat flow during winter may not be e x c e s s i v e . I t s e e m s possible ttrat, under more gever€ temperature conditions, the accumulated frost may be confined to a smaller thick-ness of insulation and have'less effect on heat conduction.
This type of roof system is not entirely new, eince it is b a s i c a l l y t h e s a m e as that used with considerable success,in the ingulation of rnetal buildings with sprayed-on asbestos insulating
materials. It is also exemplified by roofing systems which incorporate a thick wood deck exposed to the building interior. Both of these
systems aPPear to perform satisfactorily with relh.tively low interior humidities, although little information is available on the change in their thermal properties during the. year. It is possible, however, that, with a more complete knowledge of a1l the factors involved,.the same principles might be applied to the design of more sophisticated constructions for use under more severe conditions.
In summary, it can be eaid that, although a good deal of con-structive and enlightening research into roofing problems has been undertaken in the past, there is still a need for more information on the physical properties of roofing componentg and on the nature of the
con-ditions to which they are subject. Much can be accomplished to improve our present roofing practice and design through an appreciation of the basic principles involved. At the same time, application of'these principles can perhaps bring about alternative and m.ore efficient new designs and more satisfactory applications for new and traditional materials.
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-R e f e r e n c e s
1 . L u n d , C . B . a n d R . M . G r a n u m . P r i n c i p l e s Affecting Insulated Built-up R o o f s . U n i v . o f M i n n e s o t a E n g . Exp. Stn. Bulletin No. 34,
May 1952.
z, Ho1mgren, J. and r. Isaksen. ventilated and unventilated Flat, c o m p a c t R o o f s . N o r g e s ' Byggforskningsinstitutt Report No. 27, O e l o , 1 9 5 9 .
w a r d e n , w a r r e n B . A Theory of the Mechanism of Blistering. In BRI Monograph No. I A Study to Improve Bituminous Built -up R o o f s , 1 9 6 0 , p . l 4 - 2 6 .
Joy, Frank A. Premature Failure of Built-up Roofing. The Pennsylvania State University, College of Engineering. Better Building Report No. 5, Sept. 1963.
Brotherson, Donald E. An Investigation into the causes of Built-up
Roofing Failures. University of Illinois Small Homes Council
B u i l d i n g R e s e a r c h C o u n c i l Research Report 6l-2, Oct. 1961. cullen, william c. solar Heating, Radiative cooling and rhermal
Movement - Their Effects on Built-up Roofing. NBS Technical N o t e Z 3 I , L 9 6 3 .
Powell, F. J. The Effect of Moisture on Heat Transfer through
Ingulated Flat-Roof constructions. Proceedings of the
Inter-national Institute of Refrigeration Meeting, Aug. zo-24, 1962.
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