Wetting of walls by rain

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Wetting of walls by rain

NATIONAL RESEARCH COUNCIL O F CANADA DIVISION O F BUILDING RESEARCH

T H E WETTING O F WALLS BY RAIN b y T . R i t c h i e a n d J. I. D a v i s o n ANALYZED I n t e r n a l R e p o r t No. 367 of t h e Division of Building R e s e a r c h OTTAWA J a n u a r y

1969

P R E F A C E

Water i s by f a r the m o s t t r o u b l e s o m e agent in building. It i s a l m o s t always involved in the degradation of m a t e r i a l s , p a r t i c u l a r l y when they a r e exposed t o the weather. The d e g r e e of s a t u r a t i o n of m a s o n r y i s a n i m p o r t a n t f a c t o r i n determining the damaging effects of freezing. Rain penetration i s a continuing problem. In view of t h e s e and o t h e r considerations in the design of buildings, i t i s i m p o r t a n t to know m o r e about the distribution of r a i n o v e r building s u r f a c e s . M e a s u r e m e n t s m a d e with the aid of rain-collecting cups on two buildings, one in Ottawa and one in Halifax, a r e now reported.

M r . Ritchie in Ottawa and M r . Davison i n Halifax a r e r e s e a r c h o f f i c e r s with the Division. Both of t h e m have been engaged in studies of v a r i o u s a s p e c t s of m a s o n r y o v e r a number of y e a r s .

Ottawa

J a n u a r y 1969

N. B. Hutcheon A s s o c i a t e D i r e c t o r .

THE WETTING O F WALLS BY RAIN

T. Ritchie and J. I. Davison

I m p o r t a n t Canadian building p r o b l e m s

,

including m o i s t u r e penetration, e f f l o r e s c e n c e on m a s o n r y , f r o s t damage and e x c e s s i v e dimensional changes, o c c u r a s a r e s u l t of the wetting of walls by rain. T h e d e g r e e t o which a wall i s wetted m a y be expected t o depend on m a n y f a c t o r s , including the geographical location of the building, the d i r e c t i o n the wall f a c e s and t h e extent t o which the wall is shielded by o t h e r buildings and objects.

The s t a t e of knowledge concerning t h e onslaught of r a i n on buildings was reviewed by L a c y i n 1965 ( I ) , who d e s c r i b e d methods u s e d t o m e a s u r e the intensity of wetting of walls by m e a n s of r a i n gauges and p r e s e n t e d the r e s u l t s of m e a s u r e m e n t s involving s e v e r a l buildings. B e c a u s e of the n a t u r e of the flow of a i r around buildings during r a i n s t o r m s , non-uniform wetting of walls o c c u r s . C a s u a l o b s e r v a t i o n s of w a l l s a f t e r r a i n s frequently show t h i s t o b e the c a s e , c e r t a i n a r e a s , usually those n e a r the top, obviously being much d a m p e r than the r e m a i n d e r of the wall. In addition t o providing a review of information on the intensity of rainfall on walls, L a c y a l s o c o n s i d e r e d compiling m a p s of driving r a i n index t o indicate v a r i a t i o n s with geographical location i n the d e g r e e of s e v e r i t y of t h e e x p o s u r e of walls t o rain.

The purpose of t h i s r e p o r t is t o d e s c r i b e a study m a d e of r a i n falling on the walls of two buildings, one located i n Ottawa, Ontario, t h e o t h e r i n Halifax, Nova Scotia. The buildings a r e occu- pied by the Division of Building R e s e a r c h and the Atlantic Regional Station, respectively, of the National Re s e a r c h Council. The walls of the f o r m e r building a r e constructed of hollow clay t i l e s with a n e x t e r i o r f i n i s h of painted stucco; the Halifax building is f a c e d with sandstone slabs. Only the e a s t wall of e a c h building was studied. Rain-collecting cups w e r e attached at s e v e r a l locations on the wall s u r f a c e and "run-off" gauges w e r e i n s t a l l e d along the b a s e of the wall t o collect r a i n w a t e r running down the wall s u r f a c e . RAIN GAUGES

The rain-collecting cups, r e c t a n g u l a r m e t a l boxes with a n

1

open f a c e m e a s u r i n g 8 in. by 25 in., w e r e attached t o the wall with the longer dimension in the v e r t i c a l direction. The depth of the box was such t h a t the open f a c e was 2 in. f r o m the wall s u r f a c e . Rain e n t e r i n g the box f e l l into a trough at the b a s e f r o m which it d r a i n e d through a p l a s t i c tube t o a collecting bottle. The run-off gauges,

1

thin rectangular m e t a l boxes 1 2 in. long, a

,

in. wide and 4 in. deep with an open top, w e r e attached and sealed t o the wall with the long dimension horizontal. Water flowing down the wall s u r f a c e entered the open top of the gauge, and was drained through a plastic tube f r o m the bottom of the gauge into a collecting bottle. A r a i n cup and a run-off gauge a r e shown in F i g u r e 1.

Six r a i n cups w e r e mounted on the wall of the Ottawa building. T h i s wall, 9 2 f t long and 14 f t high, i s the e a s t wall of a penthouse forming the t h i r d s t o r e y of the building, the r e m a i n d e r of which i s two s t o r e y s high. The f l o o r a r e a of the penthouse is s m a l l e r than that of the r e m a i n d e r of the building, and the penthouse walls adjoin the roof of the lower p a r t of the building on the e a s t side and e l s e - where ( F i g u r e 2). The top of the e a s t wall of the penthouse is 45 f t above grade.

The r a i n cups w e r e attached t o the wall n e a r i t s top and bottom, a cup being located 1 ft f r o m the top of the wall and another d i r e c t l y beneath it, but 5 ft above the b a s e of the wall. Two such p a i r s of cups w e r e located

6

in. f r o m e a c h end of the wall and another p a i r was located 18 f t f r o m the n o r t h end. A free-standing cup, placed

10 f t i n f r o n t of and a t the s a m e height a s the bottom cup of the middle p a i r , was a l s o included in the study. A run-off gauge was attached t o the wall beneath the lower r a i n cup of e a c h p a i r , but located 1 f t l a t e r a l l y f r o m it.

A s i m i l a r a r r a n g e m e n t was made on the e a s t wall of the Halifax building which is 183 ft long and 30 f t high ( F i g u r e 2). A penthouse on the building has walls which r i s e 5 f t above the height of the m a i n walls. F o u r r a i n cups w e r e attached t o the e a s t wall, located in p a i r s a t each end of the building, 1 f t f r o m the c o r n e r s . The upper cup was 1 f t f r o m the top of the wall, the bottom cup

6

f t f r o m the b a s e of the wall. A run-off gauge was attached t o the wall beneath e a c h p a i r of cups but displaced l a t e r a l l y 1 f t .

A fifth r a i n cup, placed on a r a c k on the penthouse roof, was 40 f t above the b a s e of the wall.

RESULTS

The amount of r a i n collected in a p a r t i c u l a r cup naturally varied g r e a t l y f r o m rainfall t o r a i n f a l l depending on its intensity.

111 g e n e r a l , rainfall a t Halifax is m o r e intense than that a t Ottawa

and the amount of r a i n collected in the cups a t the f o r m e r place was t h e r e f o r e g r e a t e r than that obtained a t Ottawa.

At both Halifax and Ottawa t h e r e w a s a significant difference between the amount of r a i n collected i n the t o p cup and in the

corresponding cup n e a r the b a s e of the wall, the f o r m e r frequently catching s e v e r a l t i m e s the amount caught by the l a t t e r . The r e -

lative amounts a r e depicted in F i g u r e s 3 and 4. On s e v e r a l o c c a s i o n s , however, p a r t i c u l a r l y a t the Halifax building, m o r e r a i n was caught i n the l o w e r cup than in the higher one. The variation i n the amount of r a i n caught in the upper and l o w e r cups probably depended on the wind speed and i t s angle with the wall, a s well a s the g e o m e t r y of the building, but the m a n n e r in which they influenced the r e s u l t s is not known. Another f a c t o r which affected the r e s u l t s was t h a t on a few occasions the t u b e s joining the cups t o the collecting b o t t l e s b e c a m e plugged up.

At both Ottawa and Halifax the amount of r a i n collected differed a t the v a r i o u s locations along the wall. M o r e r a i n , f o r example, was g e n e r a l l y caught i n the cups n e a r the n o r t h end of the wall a t Halifax than n e a r t h e south end, while a t Ottawa the r e v e r s e w a s g e n e r a l l y the c a s e .

A r a i n cup a t Halifax mounted on a r a c k on top of the penthouse usually caught about the s a m e amount a s the top cups on the wall. S i m i l a r l y , the amount caught by the f r e e - s t a n d i n g cup at Ottawa,

10 ft f r o m the wall s u r f a c e , c o r r e s p o n d e d g e n e r a l l y t o t h a t caught by the cup mounted on the wall behind it a t the s a m e height.

T h e d i f f e r e n c e s in t h e amount of r a i n caught i n cups along the wall w a s r e f l e c t e d in the amount of w a t e r collected by the r u n - off gauges. At Ottawa the gauge n e a r the south end of the wall g e n e r a l l y collected appreciably m o r e w a t e r than the o t h e r s ; a t Halifax the r e v e r s e was g e n e r a l l y the c a s e , the gauge a t the n o r t h end collecting m o r e than t h a t a t the south end. T h i s situation m a y r e f l e c t not only differences in the amounts of r a i n hitting the wall d i r e c t l y above the gauges but a l s o possible l a t e r a l movement of the f i l m of w a t e r flowing down the wall s u r f a c e , the f i l m being deflected by the wind's p r e s s u r e . B e c a u s e of the w a t e r - a b s o r p t i v e capacity of the walling m a t e r i a l , a c e r t a i n amount of t h e r a i n falling on the wall would be a b s o r b e d by it, thus reducing the amount

flowing into the gauge. The a b s o r p t i v e capacity of t h e wall i s not constant. It i s a t a m a x i m u m value when t h e wall h a s had an opportunity t o d r y completely before again being wetted, and a t a m i n i m u m value when it i s n e a r l y s a t u r a t e d f r o m a previous rain.

CONCLUSIONS

M e a s u r e m e n t s with r a i n cups attached t o the w a l l s of two buildings indicated that when t h e s e walls w e r e wetted by r a i n the d e g r e e of wetting was highly i r r e g u l a r . A p a r t i c u l a r l y m a r k e d d i f f e r e n c e w a s noted in the amount of wetting received by the upper p a r t of t h e w a l l s and t h a t r e c e i v e d by the lower portion, the upper p a r t g e n e r a l l y being subject t o m u c h m o r e intense wetting. The

r e s u l t s of t h i s study m a y t h e r e f o r e account f o r the frequent o c c u r r e n c e of m a s o n r y m a t e r i a l s being damaged n e a r the top of w a l l s while

apparently sound a t lower l e v e l s , since the r a t e of d e c a y of a m a s o n r y m a t e r i a l f r o m f r o s t action, c h e m i c a l interaction, and c r y s t a l growth depends i n l a r g e m e a s u r e on the amount of m o i s t u r e i n the m a t e r i a l , e s p e c i a l l y when f r e e z i n g is the agency of decay.

M e a s u r e m e n t s of "run-off" f r o m wetted w a l l s r e f l e c t e d t h e d i f f e r e n c e s i n wetting which o c c u r r e d a c r o s s t h e w a l l s and a l s o indicated that a considerable portion of the r a i n w a s a b s o r b e d by t h e wall b e f o r e it could flow down the s u r f a c e and e n t e r the gauge.

REFERENCE

(1 ) L a c y , R. E. Driving - r a i n m a p s and the onslaught of r a i n on buildings. M i n i s t r y of Technology, Building R e s e a r c h Station, P a p e r 54, Re s e a r c h S e r i e s , G r e a t B r i t a i n , 1965.

F i g u r e 1. Rain cup (top) and run-off gauge on the wall of t h e building i n Ottawa.