Tl^CHlTICAL I!'T-70WJ.TI0F Olt BUILDING MATERIALS

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^OR USE IN THE DESIGN OF LOW-COST HOUSING

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THE NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPART?4E!TT OF COMMERCE

WASHINGTON, D, C.

July lU, l93^»

RECO^OU^NDATIONS

EOR

PORTLAND CE^'H'NT STUCCO CONSTRUCTION

This is a digest in nart of Circnlar No, 3H (December 13, 1926),

"Stucco Investigations at the Bureau of Standards ’with Recommendations

for Portland Cement Stucco Construction",^ by Frank A, Hitchcock,

issued by the Bureau of Standards, The uart selected deals with recormnendations resulting from a studv of test data,

GEI^twral design AND CO’^STRUCTION

Exuerience has shown that uortland cement stucco may develop

certain defects which cannot always be guarded against. But, the

nroduct may, if urouerly auulied, be deuended upon to be structurally

sound, durable and cauable of giving satisfactory service over long

periods with low maintenance cost. Four fundamental factors upon which

good stucco is deuendent must be taken into consideration:

(1) The building must be urouerly designed and constructed,

(2) Stucco must be applied to a suitable base,

(3) The mortar must be urouerly urouortioned and carefully

mixed from good materials,

(U) The stucco must be apulied by skilled worlcmen .

The construction should provide overhanging roofs, gutters,

suitable flashings, driu grooves, cappings, etc,, so as to prevent

Water from getting back of the stucco and the concentration of flow

over the face of the walls. The importance of these requirements cannot

be too strongly emphasized.

^Available from Superintendent of Documents,

Office, Washington, D. C. (Price 15 cents)

Government Printing

072136-C - 1 -

Frarae structures to "be finished irith stucco may or may not be

sheathed. If the stucco is to be back-nlastered, sheathing is omitted.

Othervrise, sheathing should be used, ulaced horizontally and not diagonally. Suecial diagonal bracing should be urovided in either case to -orevent

racking, and well built foundations should be provided to avoid settlement

and cracking of the stucco surfaces. Araole rigidity may be obtained with

corner bracing, and by ulacing the sheathing horizontally savings in both

labor and material may be effected.

The sheathing should be covered with a good grade of wateruroof

naner, a-^ulied from bottom un, lanned 2 inches, and well secured with

large head roofing nails. The metal lath should be nailed or stapled,

not more than S inches arart, and all butted joints securely '"ired together, Tack-ulastere d t:5,pDe of construction requires that the metal

lath be nailed directly to the studs. After brown and scratch coats of

stucco have been aunlied to the exterior, the interior side is backulastered between the studs with the same material so as to embed the

base comoletely and nroduce a thicker wall, ’lalls of this t’v'De lose

the insulating value of sheathing and naper and, to comoensate for this,

a layer of felt or other insulating material is usually auplied between

the studs in such a manner as to form a double air suace between the

inside -olaster and the outside stucco.

hasonrv walls nrovide a rigid base for stucco. Then stucco is

aunlied to masonrw walls, nrecautions should be taken in their construction to avoid settling. Units, such as cinder '*^lock, hollow tile,

etc,, should be reasonably straight and true and should be laid with

rawed joints in cement or cement lime mortar. The surface should be

rou^i or of coarse texture and free from dust, dirt and loose uarticles.

Cld walls wl'ich have been nainted or treated^ with wateroroofing material

should be covered with a metal base before stucco is applied. It is

preferable to confine stucco to vertical surfaces. It should never

extend lower than one foot above grade.

MTERIALS

Cement : Cement should comnly with the Federal Specification for

"Cement; Portland SS-C-191."^ To obtain white or colored stucco, the

latter through the addition of color uigments or yellow sand, white

cement should be used for the final coat.

1

since

The Federal Snecifications mentioned herein have

the nublication of Circular Fo, 3H*

been developed

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Fine Ajg;ig:resate : The fine aggregate of natural sand or stone

screenings to be mixed with the cement should, when dry, pass through

a No. 8 square ouening sieve (0.0937 inches sQuare) and from 20 to 25

percent through a No. 50 sieve (0,0117 inches) and should be free from

loam, vegetable and other injurious matter.

Water : Only water that is suitable for drinking uurposes should

be used,

hydrated Lime : Hydrated lime should meet the requirements of the

Federal Specification for "Lime; Hydrated (For) Structural Purposes,

SS-L-351,"! The use of lumu lime may produce questionable results;

therefore, it is not recommended.

Hair or ~^iber : When used in scratch coat work, esuecially when

it is to be back-ulastered, hair or fiber should be of first quality,

long, free from foreign matter, or long fiber well combed out. It

should be carefully incoruorated in the mortar to insure even distribution and freedom from clots.

Coloring Materials : For the coloring of stucco, the use of

mineral pigments incoruorated in the mix is advisable. Available color

materials are briefly listed, below, many tones or shades of which may

be obtained by various combinations:

Natural iron oxides - red, brown, chocolate

Iron hydroxide - yellow

Manganese dioxide - black (grayish)

Carbon gas black - black

Ultramarine - blue

Chrome oxide - green

Metal Bases : Exuanded metal lath should be galvanized or painted and weigh not less than 3*^ pounds uer square yard and should

be not less than 24 gage in thickness.^

Woven wire lath may be either galvanized or uainted and have

2 1/2 meshes to the inch. The wire used should be not lighter than

19 gage.

MIXING AND APPLICATION

Mixing : Through mixing of stucco mortar is of first im.portance,

Dry miixing should, be continued until the mix is uniform, in color and

wet mixing until it is of a uniform, consistency. Each batch should

^The Federal Suecifications mentioned herein have been developed

since the publication of Circular No, 3H*

p

“^The uresent specification recognizes three weightd and three

thicknesses less than the number given here.

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"be carefully measured and nrouortioned exactly alike in order that

consistency and color he maintained throughout the work. Machine

mixing should he run for at least five minutes and hand mixing from

10 to 15 minutes after the vrator has been added,

Pronertions ; A recommended uronortion is 1 sack of cement to

3 cubic feet of sand or stone screenings and 10 uounds of lime.

Quantities of Materials : The following table gives auuroximatc

quantities of materials neccssarv to cover 100 square feet of wall

surface.

Thickness of Coat

Proportions 1:3 with one-tenth

hydrated lime

Cement

Sacks

Lime

Pounds

Sand

Cubic feet

l/U inch 0.73 7.0 2.20

^/S inch 1.10 10.3 3.30

1/2 inch I.U7 13. s 4 . 4o

3 /U inch 2.22 20.

g

6.60

1 inch 2.94 27.6 S.go

Consistency : Only enough water should be used in mixing to

uroduce a mortar that may be readily worked and applied. The exact

amount for the materials at hand is determined by trial and when once determined should be closely adhered to in all the mixes,

Retenpering: Mortar should not bo retemucred by the addition

of water,

Waterproofing Materials; In rirouerly proportioned and applied

stucco, waterproofing materials do not appear neccssarj/ nor have

tests shown that any benefit is derived by waterproofing studs in

back-plastered, construction,

Mumber and Thicloiess of Coats ; "^irst class work requires three

coats of stucco on frame structures and. two coats on masonry. However, when the masonry wall is not true, three coats may be desirable.

The first or scratch coat should thoroughlj?" cover the base and

be bonded firmly to it. It should be at least one-fourth inch thick

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over the face of the hase and have fairly deep and rough scoring. l!he

second or hroTO coat shonld he aurilied the following day and should have

an average thickness of from three-eighths to one-half inch. After, this

coat has hecn straightened and has oartially stiffened, it should he

gone over with a wooden float to compact it and then lightly scored to

receive the third or finish coat, which should he about . one-fourth inch

thick.

We tting of Surfaces : Immediately preceding the auulication of the

scratch coat, the surface of the wall (uarticularly masonry walls)

should he wetted down, hut not saturated, so that there will he free

water on the surface. Too dry a. surface ahsorhs water from the mortar

before it has set. On the other hand, a wall co’Troletely saturated loses

its ahsorntive nower or "suction". Only -a slight amount of suction is

necessary for urouer bonding, which suction tends to draw fine cement

narticles into the nores of the wall, Uuon this action, the bond of

the stucco d.euends. The wetting of each coat is necessary before annlying the succeed.ing coat. The same urecautions auply to wetting of each

coat before auulying . the next as amply to ivetting a wall.

Ammlication of Coats : Coats should he ammlied continuously in one general direction without allowing the stucco to dry at the edges. If

immossihle to work the full width of the- wall at one time, the Jointing

should he made to come at some natural division of the sur^^e, such as

the side of a door or window omening.

Cur ing a.nd Protecting: To obtain best results, stucco snould. he

thoroughly wetted nights a.nd mornings for several davs after the final

coat is apmlied and after it has snif ficiently hardened to allow smraying with W8.ter, Preslily ammlied; work s’lould. he mrotected from sun,

rain and freezing weather, ho exterior mlastering s-iould he attempted

when the tem.merature is below Uo°P unless some protection is given the

stucco for at least ^8 hours.

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