Workmanship and rain penetration of masonry walls

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Workmanship and rain penetration of masonry walls

Davison, J. I.

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Ser

TH1

BUILDING

PRACTICE

NOTE

WQRKMAP6SHIP AND

RAIN

OF MASONRY WALLS

Atlantic Regional Station, Division of Building Research National Research Council o f Canada

Y E O R W S H I P AND RAIN PENETRATION

OF MASONRY

J.I. Davison

Poor workmanship has been cited as the major cause of leakage problems in masonry buildings. The wall system and design detail are a l s o factors, but the ultimate performance of the wall will reflect the calibre of workmanship during the construction period.

It is d i f f i c u l t to define workmanship accurately, The Oxford Dictionary defines it as "the relative finish or execution seen in the

manufactured article." Primarily workmanship, as it relates to masonry, r e f l e c t s the skills and a b i l i t i e s of the mason. But many other factors can influence the end result which, as defined above, is f i n a l l y judged in "relativett terms. Thus workmanship c o ~ l s i d e r e d good in one part of the

country, might be considered mediocre in another. The nebulous character of workmanship makes it t h e obvious culprit, sometimes erroneously, when

leakage problems occur.

Modern masonry buildings are much bigger than those of yesteryear and have larger wall areas exposed to t h e weather. In addition they have less protection, are thinner and

mom

Approximately 28 per cent of a brkk wall c o n s i s t s of mortar j o i n t s , and it has been established that rain penetration usually occurs at the unit-mortar interface. Thus it follows that 20 per cent of t h e wall area is vulnerable to rain penetration. Because this is obviously more of a _{problem than it}

once was, t h e quality of workmanship is a key f a c t o r in controlling it. The fast pace of modern construction coupled with the practice of awarding contracts to the lowest bidder are not conducive to excellence in workmanship. The mason is often faced with the dilemma of choosing between expediting the job to meet a time schedule or a price limit, or taking extra time t o achieve t h e calibre of workmanship that will ensure a wall capable of maximum resistance 20 rain penetration.

The calibre o f workmanship will be influenced by the quality of the masonry materials, their conditign at the time of use, the compatibility of the mortar and the units, and w a a t h e ~ conditions during construction. Some

of these factors vary considerably fropi day t o day, and although most masons do a remarkable job i n compensating for these variables, every effort should be made to assist him by controlling these factors whenever possible.

The compatibility af rha mastar-unit combination requires a knowledge of the properties of the two materials, The rate of water absorption of the masonry unit is probably the most important of the external factors affecting workmanship. If i t i s very high it should be reduced by soaking t h e u n i t prior to use so that the mortar does not dry out too f a s t . Care should be taken to select s mortar that w i l l be compatible with t h e brick. For example, a clay brick with a high absorption should be combined with a mortar having a high water retentivity to ensure maximtlln extent of bond and minimum rain penetration. In certain circumstances the designer may place a p r i o r i t y on high compressive strength rather than protection against ~ a l n penetration, and must accamplish t h i s

by using

if

philosophy underlying the design.

Tho proper e m a g e of masonry materials on the construction s i t e and the preparation of the martar are discussed in Building Practice Note Ma. 5, "Quality Control in Preparing Masonry Mortar1'. The Note contains

a

on-site, and thus contribute to better workmanship in the end product. Daily variations in the weather during t h e constructim p e r i o d ,

particularly in air temperature and relative humidity, w i l l affect mortar propertics such as consistency and tine of s e t . This in turn can result in

' f v a ~ i a b l e " workmanship. Fortunately the mason c m minimize the effect of weather by rerapering the mortar to s u i t the condition.

Specific instances of poor workmanship that have recently been reparted from t h e f i c l d include empty head and collar j o i n t s , broken bond b~twecn bricks and mortar, cavity spaces plugged with martar droppings, weep holes that w e r e too small, and ripped and reverse sloped flashings. Rather than d e a l with these points specifically ground m l e s far goad workmanship, designed to prevent these and orher problems, will be reviewed.

1. Mortar. me mortar should be soft and plastic when placed in the wall. The mason obtains the desired degree of workability by tempering the mortar wirh the addition of water and

remixing

used

is

unit and the retempered mortar will more than compensate far the loss of some compressive strength caused by the addition of water.

2 . Joining of Work. Where new work joins masonry that is partially or t o t a l l y set, the exposed surface of the s e t masonry should be cleaned and lightly wetted so as to obtain the best possible bond with t h e new work. All louse b r i c k and mortar should be removed. Providing a clean surface and wetting the existing masonry w i l l ensure that it will not e x t r a c t water f r o m

the fresh mortar and thus reduce its a b i l i t y to form a maxirum extent of bond with the s e t masonry. If it becomes necessary to "'stop D ~ P '

a

horizontal run of masonry it should be done by racking back one-half brick length in each course.

3 . Bed and Head Joints. Solid masmry units should be laid with full bed and head joints, except where they are used for weep holes or ventilation. [lo1 low masonry u n i t s should have full bed and head j o i n t s in both inner and outcr shells. The webs should be l a i d i n full bed in all courses in p i e r s , columns and pilasters, in the starting course on footings and solid

foundation walls, and where adjacent to cells

or

Full bed joints can be obtained by spreadfng a thick bed of mortar and making a shallow furrow with the t i p of the trowel. FurrorJing a V-shaped trough in the bed t o a depth that impairs full joints is not acceptable hecausc it

can

where

deterioration of the masonry as a result o f the freeze-thaw action that will subsequently occur-

The length of the bed j o i n t being laid out will depend

on

of the u n i t s and t h e air temperature. The limiting factor is t h e ability of the mortar at t h e end of the bed to r e t a i n enough plasticity to form a

c u m l ) l c t e bund wllcrh the last unit is placed. 'Che maximum length should be

1 metre (3 feet) when concrete block and low suction clay bricks are being used. I f b r i c k suction exceeds %I gms. /30 sq. in ./min, and is not m o d i f i e d by wetting, thc lcngah of spread should bc ~ c d u c e d to ensure t h a t t h e b r i c k s are being laid in plastic mortar.

Full head joints are obtained by buttering the end of t h e unit with mortar, then pressing

the

i n t o

vertically when the unit is inserted, Attempts t o fill head and bed joints by slushing dr dashing mortar into

the

are

The National Buildi~lg

(8

Masonry units should

be

Mortar joints that are expos-kd t n the weather arid have become "thumb-

11rinr" hard should hc tpolcd w i t h a round j o i n t e r t o form n coficavo surfacc. Tooling is neither a substitute nor a remedy for incompletely filled mortar j o i n t s , but it is deferice against rain penetration. Tt compacts the surface of the j o i n t making the martar more dense, and it helps seal any f i n e cracks that may have developed between the unit and the mortar. mere are a number of

tooling configurari~as used to achieve certain aesthetic effects, but the concave shape is best far pratectian againsT rain penetration. If naiZs or

line p i n s are used, the holes l e f t when they are removed should be plugged immediately, while the adjacent mortay is green enough to a l l o w adequate

bond

w i t h t h e patch.

4. Cavity Walls. Failure to prevent mortar extruding from the j o i n t s from dropping i n t o rhe cavity will enable water to cross the c a v i t y barrier via

mortar "bridges" and thus penetmte the interior wall. It is fundamental to t h e success of the system that the cavity be kept clean. The conventional method recommended to achieve t h i s is to place wood strips, equipped with w5re or rope pulls, m mmal ties crossing the cavity. As the walls are built up to the next lwel ~equiring t i e s , the strips are withdrawn, cleaned

of

wythe

.

When placing flashing at the base of the cavity care should be taken to ensure that the flashing material is not punctured. This means chat the

surfaces against which the flashing material is laid should be smooth and free of any sharp projections. It is equally important

t h a t

a

positive slope toward the autside in order rhat any water that penetrates the exterior +he and drains t o the bottom, will be d i r e c t e d back outside through t h e

weep

with

some

Weep holes, spaced horizontally at 600 mm C24 in.) intervals, should be located in head joints in the first course above f l a s h i n g s . The most effective method of forming them is to omit a l l or part of t h e mortar in t h e head j o i n t , Some masons like ra leave a free-draining material in the weep hale t o prevent

the entry of insects into the cavity. Others prefer to use one of a number of preformed plastic Or metal weep holes; these must be a minimum

of

C*

in diameter. The important consideration is that the weep holes must be free draining. This means that they must be big enough to carry off the water and t h a t the cavity area behind them

mu-st

enmanee

5.

CTlW u n i t s ) ' . The parging mortar should be the same as that used to lay up the wall and it should be applied to a _{thickness of 6 . 5 to 9,s ntm [# to}

#

mrrar

6 . _{Tuck Pointing. In any pointing operation the existing mortar j o i n t s must}

be raked back to a depth of at l e a s t 12.5 mm

( 3

me

ra

7. Pratecrian af Work. The tap surface of uncompleted masonry walls should be covered a t the end of each day's work to protect them fmm inclement weather. New walls that have not had t i m e to cure properly can be damaged

by r a i n and snow, and by very high or vary low temperatures. Covering with a tarpaulin or some other waterproof material is adequate protection against rain. Better protection may be necessary to prevent frost damage if thc

w i l l l s a r e built without the benefit of an cnclasure during winter weather.

Good workmanship in masonry construction is d i f f i c u l t to define because of the many variables involved. The end result reflects the ability of the mason to correctly make a number of arbitrary decisions on the job, and his

skill in

them.

The

of modern construction

A knowledge of the philosophy behind the design and o f t h e properties of t h e materials selected far the project will help t h e mason make these

decisions. hoper use of this information in conjunction with some

reasonably f l e x i b l e "ground rules" will enable him to achieve a standard of workmanship t h a t w i l l r e s u l t in a masmry wall capable of resisting rain penetration.

1 . Clay Mason~y Manual, Cutler and Mikluchin. The Brick and T i l e Institute of Ontario, 4824 Yonge Street, Wii.lowdale, Ontario

W N

2 . _{Caravaty and Plumier. Principles}

of

Meadow

Road, Skokie, I l l i n o i s 60076.

4. PICMA Tek Note #85, National Concrete Masonry Association, P.O.

b x

5. Technical Notes on Brick and Tifc Construction, Nos. 7R and RE, Brick Institute of America, 1750 Old Meadow Road, Mcliean, Virginia 22102.

6 Building Practice Notes Nos. 5 , 12 and 13. Division of Building Research, National Research Council, Ottawa, Ontario KIA qR6.

7. Ritchie, T. Water Penetration Tests on 'ITIY Walls. Building Research Mote No. 8 6 , Division o f Building Research, National Resea~ch Council, Ottawa, Ontario KIA OR6,