Stucco Cracking on Wood-Frame Structures

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Technical Note (National Research Council of Canada. Division of Building Research), 1954-10-01

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Stucco Cracking on Wood-Frame Structures

Tibbetts, D. C.

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DIVISION OF BUILDING RESEARCH

No.

NATIONAL RESEARCH COUNCIL OF CANADA

188

NOTlE

'fE

C

1HIN ][ CAlL

I

PREPARED FOR General Distribution NOT FOR PUBLICATION

PREPARED BY DoC. Tibbetts CHECKED BY C.R.C.,W.H.Bo

FOR INTERNAL USE

APPRQVED QY

ｾ

ｾ

DATE October, 1954

SUBJECT Stucco cracking on wood-frame structures.

Frequently one comes across the unqualified'statement in trade arid technical periodicals that "diagonally applied wood sheathing if5 unsuitable as a backup material for stucco".

These statements associate cracking of stucco with diagonal '

sheathing and refer

to

horizontally applied sheathing as being

superior' for this particular use. The only explanation given in

the literature is that cracking is due to ｡｣ｲｯｳｳｾｴｨ･ grain

shrinkage but this,'while being

a.

possible 'contributor to"

failure, does not remove the fact that this shrinkage occurs in wood regardless of its arrangement in respect to the wall.

Shrinkage ih the length of the piece is negligible and may be discounted for all practical purposes.' This has been confirmed by discussion with Forest Products Laboratories.- It may be that the fault is due more to hailing of the sheathing and that little or no consideration is given to the fact that the distances

between nailing at supports is increased when sheathing is

diagonally applied. It should also be remembered that no

shrinkage occurs at the centre of the sheathing in that the change iIi dimension begins from the edges and'becomes

proportionately smaller as the centre is approached0

Originally the search for literature was carried but in an attempt to determine more concrete reasons for failure'of stucco on wood sheathing; however, while the emphasis in this note is on frame construction it was thought that material gathered on stucco applications over the commonly used backups should be included for future study and to eliminate the need for a further review of the literatureo

,Some Early Tests

1911 - Tests were begun by the United States Bureau of Standards in co-operation with the Associated Metal Lath Manufacturers in an effort to determine what factors caused corrosion and the consequent failure of metal lath embedded in stucco and plaster.

- 2 =

The conclusions reached at that time were that oversanded mortar or the imperfect embedment of the metal lath had -more to do with th$ development of rust than the nature of the cementitious

materials0

As there had been some criticism of the 1911 ｷｯｲｫｾ it was decided to outline a more comprehensive series of tests on a larger scaleo Because there were failures of stucco

on

bases other than metal lath in which corrosion could have. played no part₉ the general opinion was that all types of stucco

construction should be studied0

1.2J2 -

A building200 by 26 feet by 25 feet high was erected

consisting of 56 exterior panels of typical combinations0 ". The

panels were large enough to contain 'ilrihdow openings 2" feet

-6-

inches by

3

feet 6 ｩｮ｣ｨ･ｳｾ These were included since, in practice, cracks usually occurred at the corners of openings in ,,,aIls0

As a result of these tests'the following order of preference as to stucco bases was given:

10 Monolithic concrete not coated with a bituminous compound,

20 Metal lath applied directly to the studs and back-plastered;

30

Brick and tile;

4

0 Metal lath over wood sheathing;

50

\-lJood lath;

60

Gypsum block and plaster boardo

All of the panels over diagonal sheathing developed large and prominent cracks;: These cracks invariably appeared first off the corners of windows and ran across' the-direction of the sheathing, It was believed that they were brought into prominence if not actually produced by the shrinkage of the sheathing,

1916 - It was decided to pay more attention to the method of application in the next series of panels and to investigate further the 'Use of the leaner mixtures0 TlIlenty-tv,TO new frame construction panels forming the walls of a penthouse

or

monitor were erected on the roof of the original test structure,

One side of the monitor was fully sheathed with 6-inch sheathing placed diagonally, the direction of the sheathing

alternating on'adjacent panels o ' These panels included metal lath

and wood lath plastered with a variety of stucco ｭｩｸｴｵｲ･ｓｾ The

deductions from the 1916 tests may be summarized as follows:

, .

Diagonal sheathing of improperly seasoned wood or wood

that has absorbed ｣ｯｲｴｾｩ､･ｲ｡｢ｬ･ moisture is apparently

unsatisfactory for stucco backingo With only such wood

available, horizontal sheathing appears to be the better construction, provided the wood frame is sufficiently braced o

1922 - Through the co-operation of the National Steel Fabric Company; ten of the sheathed pahels in the monitor were replaced with hew panels designed to give a comparison of the effectiveness

of three types of metal backing; that is, expanded metal lath "

weighing

3

04 Ibo per square yard, 20 grolge galvanized woven' wire

lath with V-rib stiffeners, and three weights of two by two-inch mesh-steel fabrico

The study of the conditions of these 1922 panels indicates that crazing and map cracking may be eliminated by"

controlling the mixing watero The reinforcing value of the lath

was'not shown conclusively because although the penthouse was nearly'six years old when the tests were carried out there was

considerable shrinkage ahd expansion of the ｳｨ･｡ｴｨｩｮｧｾ This

had

a

greater destructive effect on the stucco slabs than any

restraining effect of the reinforcing fabrico It was seen

that for the best results in frame and stucco construction sheathing should be placed horizontally and at right angles to

the studso The results also suggested that if sheathing is to

be used some means of providing a flexible tie of the stucco slab should be developed o

ＱＹＹｾ

-

There remained certain unanswered questions on the

existing practice for cement stucco and it was decided to erect

one more series of test panels0 This involved replacement of

32 ｰ｡ｮ･ｬｳｾ

twenty backed with horizontal sheathing;

twelve back-plastered or paper-backed on open studso These tests were carried out with the co-operation of the

National Lumber Manufacturers Association, the Portland Cement Association, the Associated Metal Lath Manufacturers, and other individual manufacturerso

4

-After one year the conclusions were as follows:

le In so far as the stucco itself is concerned

back-plastered and possibly paper-backed construction seems· preferable to sheathed construction, and it appears that when sheathing is used it should be placed horizontally;

20 If the wire laths and the steel fabric are

considered of similar construction, being made

of wires at right angles to each ｯｴｨ･ｲｾ the .

results indicate that the heavier members have

a greater reinforcing value. The relation of

proper spacing and size of member, ｨｯｷ･ｶ･ｲｾ

requires further study;

30

There is no positive indication of advantage of

one type of furring over another in attaching

metal lath to sheathingo While there is some

slight indication in favour of the loose tie, ｴｨｾ

number of panels involved was so limited that a more extensive study would be necessary before any recommendation for a change in present practice could be madeo

While most of the literature reviewed advocated the use of horizontally· applied sheathing where frame structures are

desired there are a few proponents of the use of diagonally applied sheathing; this serves to further confuse the picture,

10 Dwelling House Construction, by Albert GoHo Dietz,

ScDo, Massachusetts Institute of Technologyo

nIT

walls must be frame construction they should be

well braced, diagonally ｳｨ･｡ｴｨ･､ｾ and so framed as to

avoid shrinkage and expansion in the frame and sheathing which would cause cracks in the stuccoo Foundations must be firm and unyielding, with ample and carefully proportioned footings to avoid ..

differential settlement which would be reflected in cracked stucco0 "

20 Structural Clay Products Institute Technical Note,

VOlo

5,

Noo

6,

June

1954,

"Recommended Backing for Architectural Terra Cotta and Ceramic Veneer - for exterior or load-bearing wood stud construction, the studs should be spaced not more than 16 inches on centre over which diagonal wood sheathing, building paper, and galvanized mesh

is placed to receive the cement scratch coat."

Ｍ Ｍ Ｍ Ｍ Ｍ Ｍ ｾ Ｍ Ｍ Ｍ Ｍ

-•

Literature reviewed in which the use of horizontally applied sheathing i5 recommended:

10 Stucco Investigations at the Bureau of Standards with Recommendations for Portland Cement Stucco Construction - Circlllar of the Bureau of Standards Noo 311, December 13, 1926. Covers the series of tests conducted between 1911 and 1926.

2 0 Handbook of Building Construction, Vol. II, p0982, by Hool and ｊｯｨｮｾｊｮＬ 1929.

30 TIBH - 20, National Bureau of Standards, DoSo,· July 14,

ＱＹＳＶｾﾷﾷ "Recommendations for Portland Cement Stucco

Construction" •

4 0 Proposed Standard Specifications· for Portland Cement Stucco and Portland Cement Plaster, American Standards Association, May 11, 19380

50 Standard Specifications for Portland Cement Stucco and Portland Cement Plastering, American Standards Association, A4204-1946, A4203-l9460

60 PlastererVs Manual9 Portland Cement Association,

19520

70 Portland Cement Association9_{s Specification for}

Stuccoo

8 0 Gypsum Lime Alabastine Plastering Specification "C", Exterior Stucco on Wood Frame Construction.

Technical Bulletin ｎｯｾ ｬｾ "Reinforced Portland Cement Stucco",

ｰｾ･ｰ｡ｲ･､ by tbe Metal Lath Manufacturers Association, March,

1952, does not mention the method of application of wood sheathing.

Generally there are four fundamental factors to be considered for best results where stucco is used:

10 The building must be designed and properly constructed for stucco;

20 The material must be applied to a suitable base;

30 The mortar must be properly proportioned and carefully mixed from good materials;

40 It must be applied by workmen having a knowledge of the material and skilled in its application, because workmanship cannot be written into specificationso

- 6 ｾＬ

The following is a summary of the literature reviewed -with respect to the application of stucco on wood-framed "buildihgs o

It remains to be shown that the practice of applying stucco over diagonally applied sheathing should be discontinuedo

Framed construction to be finished with stucco should have special bracing to prevent cracking and be set on well-built

foundations to avoid the possibility of settlemento Small

move-ments due to these Causes would have no effect when the structure is finished with wood siding, but when stucco is used they may

cause unsightly crackinpo Diagonal braces of 1- by 6-inch

boards let into the studs on their inner faces at the corners of each wall and bridging betweeh the studs at least once in each

story height add much to the rigidity of the frame0 If sheathing

is used this bracing may not be necessary, tut it is an added factor for the stability of the structureo

The ｳｴｬｾ､ｳ are usually 2 by

4

inches, placed 16 inches

on"centre when sheathing is used and 12 inches on centre in

back-plastered or open-frame ｣ｯｮｳｴｲｵ｣ｴｩｯｮｾ The studs should be

continuous and not interrupted by horizontal members to avoid

any lack of uniformity or point of weakness in _{the wall}

construc-tiona

Framed structures may be sheathed or not? but the

most successful application in so far as the stucco is concerned

i s _{to omit the sheathing and use either the back-plastered or the}

open-frame constructiono If sheathing is used it should be placed

horizontally across the wall studs and not diagonally? although

in many s ectiohs of the country diagonal sheathing is used to

furnish greater rigidity to the frame0 Good rigidity may be

obtained by using the system of bracing described ｡｢ｯｶ･ｾ and by

placing the sheathing horizontally a considerable saving will be effected in both labour and materialso

With regard to the structural stability of a frame

ｾｴｲｵ｣ｴｵｲ･ it is highly probable that a building to which diagonal

sheathing has been applied will be more rigid than a

corner-braced frame to which the sheathing has been applied horizontallyo Much work has been done in the fields of building codes and

structural design to introduce the structural advantages of

diagonal ｾｨ･｡ｴｨｩｮｧＢ If such applications are to be restricted

by limiting the types of cladding that may be applied over them then it is important that steps be taken to prove or disprove that the wood backup is the only contributing factor to the reported stucco failureso

Information on various lathing practices for stucco application is contained in Technical Note Noo 187 of the Division of Building Researcho