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ISSN 0701-5216


J .I. Davison

Division of Building

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National Research Council of Canada

Ot'tawa, May 1979

J . I . Davison

The problem of rain panetrat ion h a s been a major concern of t h e masonry industry f o r many years. As a result of much time-consuming a n d c o s t l y research mostof t h e contributing causes have been i d e n r i f i e d and a r e well documented, A list of selected publications on the subject, available from the Division of Building Research, will be found a t the end of this Note, t h e purpose of which is to review some of t h e basic f a c t s r e l a t i n g t o rain p e n e t r a t i o n and the masonry wall systems currently in u s e .
Causes ofRain Penetration

There are three essential requirements for t h e rain penetration of masonry walls:
(2) A f i l m of water on the wall

An opening in the wall to permit the entry of the water
A force to drive t h e water through


the opening.

These factors work in combination - the absence of any one of them will eliminate the problem but unfortunately it is d i f f i c u lt to visualize the absence of any of the factors during rain storms.


Them will always be water on soma part of t h e walls of b u i l d i n g s when i t rains. Roof overhangs and architectural detail provide some protection for walls of small buildings, and will direct water away from protected wall areas. Initially the masonry units may absorb some water. But as soon as the t h i ~ s t ;the units is s a t i s f i e d , there will be water of on unprotected wall areas.

It is a l s o virtually impossible to f i n d a masonry wall without openings. I n e v i t a b l y there will be small unbmded areas between u n i t s and mortar, or cracks resulting from d i f f e r e n t i a l movement and vibration. Openings can also occur in the caulked j o i n t s between t h e wall anddoor and window u n i t s -

But even though water may bridge tho openings, it will not enter without same force to push it thmugh. These forces include kinetic energy, gravity, capillary suction and t h e pressure created by wind blowing against the wall. Kinetic energy and capillary suction are significant under certain conditions, and gravity is even mare important if the openings slopedownward and to the i n s i d e of the wall. But the most important force is the pressure gradient created by wind blowing against the exterior surface of the wall. In most rain storms the driving force is a combination of the above forces. There are very few rain storms where one or more of t h e s e forces are not present.

Wall Systems
The two basic wall system that will be considered are the solidwall and t h e cavity w a l l .
The Solid Wall
A s o l i d wall consists of masonry u n i t s l a i d close t o g e t h e r w i t h the j o i n t s filled w i t h mortar. The wall can vary i n thickness w i t h d i f f e r e n t wythes b e i n g tied together by masonry units or metal ties (Fig. 1). Thc masonry units can be stone, brick {clay, concrete, sand-lime) or concrete block. The wallscan h e all of one unit, or a combination of d i f f e r e n t u n i t s . A currently popular composite wall combines an exterior wythe of c l a y b r i c k with a back-up wythe of concrete block.

P r i o r to the 20th century, s o l i d walls were m c h thicker, and bui 1 Sings sma l 1 ex. The wall s received protection from roof overhangs and archlxectural detail that directed water away fromdoors and windows, and the clay b r i c k s were much more absorbent than those in today's market-place. Water striking t h e walls was absorbed by the b r i c k s , which acted l i k e a sponge holding the water until it evaporated during drying weather t h u s restoring the absorptive capacity of the b r i c k before the next storm. And despite all t h e s e factors favorable to minimizing...
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