Rain penetration and masonry wall systems

(1)

Publisher’s version / Version de l'éditeur: Building Practice Note, 1979-05

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.

https://nrc-publications.canada.ca/eng/copyright

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à [email protected].

Questions? Contact the NRC Publications Archive team at

[email protected]. If you wish to email the authors directly, please see the first page of the publication for their contact information.

NRC Publications Archive

Archives des publications du CNRC

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.4224/20338210

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Rain penetration and masonry wall systems

Davison, J. I.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

NRC Publications Record / Notice d'Archives des publications de CNRC: https://nrc-publications.canada.ca/eng/view/object/?id=f1ace393-a381-4020-86f8-ab0494974cc9 https://publications-cnrc.canada.ca/fra/voir/objet/?id=f1ace393-a381-4020-86f8-ab0494974cc9

(2)

ISSN

0701-5216

BUILDING

PRACTICE

NOTE

by J

.I.

Davison

Division of Building

ekea arch

National Research Council of Canada

(3)

RAIN

PENETRATION AND MASONRY

WALL SYSTFNS

by

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 most of t h e contributing causes have been i d e n r i f i e d a n d

a r e well documented, A l i s t of s e l e c t e d 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 of Rain Penetration

There are three essential requirements for t h e rain penetration of masonry walls:

(1) A f i l m of water on the wall

(2) An opening in the wall to permit the _{entry of the}_water

(3) 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 l t to v i s u a l i z e t h e _{absence of any of}_the_factors_{during rain}_storms.

T h e m 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 t h e t h i ~ s t ; of the units is s a t i s f i e d , there will be water 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 v i b r a t i o n . Openings can also occur in the caulked j o i n t s between t h e wall and door and window u n i t s

-

(4)

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 slope downward 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 b a s i c wall system that will be considered are the s o l i d wall 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 ( F i g . 1). Thc masonry units can b e s t o n e , brick {clay, concrete, sand-lime) or concrete block. The walls can 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 S i n g s sma l 1 ex. The wall s received protection from roof overhangs and archlxectural detail t h a t directed water away from doors 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 t h e b r i c k before the next storm. And despite all t h e s e factors favorable to minimizing r a i n p e n e t ~ a t i o n , there

were

problems w i t h these buildings.

Modern buildings have much thinner s o l i d walls because designers utilize the increased strength of the masonry u n i t s to b u i l d much larger buildings with load-bearing masonry walls. These buildings have more waIl area exposed to tho weather and there is less protection w i t h the disappearance of architectural d e t a i l , such as heavy cornices, in an era of simplified d e s i g n . The use of high strength mortars is required for load-bearing walls, h u t they have poor workability and t h e i r use

i n c r e a s e s t h e possibility of voids at the brick-mortar interface. In a d d i t i o n , walls are more prone t a cracking because of a combination of

increased d i f f e r e n t i a l movement between materials

and

the vibrations from the multiplicity of mechanical equipment that

is

an integral component of modern buildings. Finally, t h e current ''high strength1' masonry units have 1m absorption values and hence l e s s capacity for absorbing water. Modern buildings t h u s have larger w a l l areas, w i t h

(5)

m i n i m u m protection from the onslaught

of

rain, are more susceptible to cracking, and have minimal capacity

f ~ r

absorbing water penetrating the surface.

These

factors place added

emphasis

on the _importance_of_correct detailing and high quality workmanship i n controlling rain penetraticm.

Single-wythe

solid

walls are now extensively used both as aon-load-bearing infill far buildings with structural frames and as load-bearing walls for small and medium sized buildings. T.T.W. (through-the-wall] clay bricks

and

concrete b l o c k s are used in these walls. The T.T.W. clay b r i c k s are

wider

than normal clap

bricks

and

in some cases the single-wythe walls of brick or black serve as interior finish. As mentimed previously, there

is

a higher incidence af unhanded

areas between the units and t h e high strength mortars, and there are problems in detailing and executing effective flashing i n s t a l l a t i o n s . There have r e c e n t l y been ~ e p o r t s of serious leakage problems with these walls,

On

the _{p o s i t i v e}

s i d e ,

_same_designers_have_{developed solid wall} systems that perform satisfactorily and others have incorporated

impexmeable membranes

in

_{s o l i d}_{wall systems}_{with some} _success, _{S t u d i e s} at the D i v i s i o n on the T.T.W. single-wythe wall indicate t h a t parging

[back p l a s t e r i n g ) t h e i n t e r i o r wall surface minimizes t h e rain pelrctration prob l em.

The Cavity Wall

The cavity wall (Fig. 2) offers

a

_&orepositive approach to the problem of rain penetration. In t h i s system there are t w o walls [wythes) s e p a r a t 4 . b ~ an a i r space.

The

concept accepts the premise that same

rain

will penetrate the exterior wythe, and provides a positive barrier in the form o f an a i r space to s t a p the advance of water into the wall

sys tern.

The masonry u n i t s for c a v i t y walls can also be stone, clay, or

concrete or combinations of t h e s e materials. Again the papular composite wall consists of an exterior wythe of clay brick and an i n t e r i o r wythe of

concrete b l o c k . The w y t h e s are tied tagether with masonry units or

corrosion resistant metal ties. The w i d t h of t h e c a v i t y can vary between 50 and 100 mm depending

on

_{t h e}_type_of_wall_{t i e . Metal}_ties_should_have a "dripmv feature so t h a t water travelling along the tie w i l l fall E T Q ~ t h e "dript+ t o The base

of

t h e cavity. The base

sf

t h e cavity

is

flashed

t o the o u t s i d e and weep h o l e s are provided by leaving t h e mortar out of v e r t i c a l j o i n t s at intervals of 400 mm in the-bottom course of the wall.

Thus water penetrating the exterior wythe runs down its inner

surface to the base of the cavity where it _{is intercepted by}_{t h e flashing}

and directed back t o the exterior through t h e weep h a l e s .

The success of the system depends on good design d e t a i l and on t h e correct interpretation and application of the design during construction.

(6)

The cavity must

be

kept free

of mrtaf

droppings that c a n bridge the a i r baprier and thus provide a path for water to enter the interior wythe. The flashing material must be of high quality and must

be

installed so that it slopes

toward

the &&side. Failure t a respect t h e s e essential requirements can

resulr

in serious leakage

p r ~ b l e m s

t h a t are d i f f i c u l t and costly to correct.

Unfortunately

t h e

c a v i t y space is _often

_used

_{to adjust dimensional} inaccuracies b u i l t into the building frame.

Xf

the frame is a b i t too long,

the mistake

is corrected by reducing the cavity w i d t h , making it more d i f f i c u l t to keep it free sf _mrtardroppings, T h i s practice of arbitrarily adjusting the cavity

width

to

compensate

for

errors made b y other trades should be d i s c o u r a g d because it _'inevitab

_Ly

_jeopardizes the _{a b i l i t y}

_of

_{t h e wall}_to_resist_{r a i n penetration.}

In summary, t h e excellent performance record of properly detailed and well constructed conventional cavity w a l l s indicates that the sysrem is an effective method

of

eonrrolling

~ a i n

penetration.

Concluding statement

The a b i l i t y of any masonry wall system to resist r a i n penetration ultimately reflects the competence of the design d e t a i l and the q u a l i t y of workmanship during cunstmctian. These t o p i c s will be discussed in future notes _in

_{t h i s}

_Series.

The following publications relating to rain penetration and masonyy walls are available from t h e _{D i v i s i o n of}_Building_Research.

T.

Ritchie. Rain Penetration of Walls of Unit Masonry.

CBD

6

T . R i t c h i e . Cavity Walls. CBD 21

T . R i t c h i e . Water _Penetration_Tests

_{of T.T.W.}

_{Brick Walls.} _BRN ₈₆ T. R i t e h i e .

T.T.W.

B r i c k Walls. NRCC 13865

(7)

B R I C K C O N C R E T E B L O C K BRICK HEADER T I E F O U N D A T I O N F I G U R E 1 S O L I D W A L L OUTER W A L L INNER W A L L A I R S P A C E ( 5 0 m m ) M E T A L T I E B R I C K F L A S H I N G TO FORM C A V I T Y G U T T E R W E E P H O L E (MORTAR OM I T T E D ) F O U N D A T I O N F I G U R E 2 C A V l T Y W A L L