Plaster Cracking Associated with Roof Trusses

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Technical Note (National Research Council of Canada. Division of Building Research), 1965-08-01

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Plaster Cracking Associated with Roof Trusses

Beach, R. K.

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DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

TEClHINlICAlL

No.

450

NOTlE

PREPARED BY _{R. K. Beach} CHECKED BY _{CR C} APPROVED BY _NBH

DATE

- August 1965

PREPARED FOR

SUBJECT

Limited Distribution

PLASTER CRACKING ASSOCIATED WITH ROOF TRUSSES An unusual case of a large crack occurring in the plastered ceiling of a house that had been built with roof trusses was brought to the attention of the Division by the designers of the roof trusses in the hope that an explanation

for the failure could be found. The problem was described as a longitudinal

crack that opened and closed daily in the ceiling of a 15 by 30 -ft living room. Two visits were made to the site to examine the building and obtain background

information. The fir st visit by the author and a technician was made on 26

and 27 May 1965, when measurements of the movements of the crack that

occurred during a daily cycle were recorded. At the same time information

was obtained from the contractor and several of the workmen. The second

visit was made by the author on 15 June, at which time discussions were held with the contractor and representatives of the firms designing and supplying the trusses.

The house, a bungalow approximately 40 by 72 it _{in plan, has the}

front entrance recessed 5 ft and the exterior wall of the living room recessed

3 ft 8 in. A sketch of a portion of the house is shown in Figur e 1. Both the

exterior wall of the living room and the centre bearing partition have steel beams and steel columns down to the foundation which rests on rock.

The roof trusses wer e installed in late October with the partition

in place. Plastering was finished in mid-January, and about two weeks later

longitudinal cracks appeared in the living room, dining room and master

bedroom, as indicated in Figure 1. Cracks also appeared in other areas such

as in the kitchen and main entrance. The latter cracks were patched, but cracks

in the dining room and master bedroom closed up and now cannot be detected

by the naked eye. The crack in the living room ceiling, which has a stippled

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-at each end, though -at the west end the crack in the ceiling did not appear to

be continuous. This crack was both reported and observed to close up

partially on warm sunny days and open up as the outside temperature dropped

at night. The design of the truss and the locations of the bearing points and

the crack in the living room are shown in Figure 2. The crack in the plaster

was coincident with a joint in the gypsum lath both on the ceiling and walls. During the first visit, measurements were taken of the movement

that was occurring in the ceiling. Two methods wer e used; one with dial

gauges and the other with suspended scales on which readings were taken by

means of a precise level. One gauge previously installed by the contractor

measured the vertical movement of the ceiling. Two additional gauges were

in stalled to measure the relative vertical movement between the top and bottom chords of the truss and the horizontal movement of the crack itself. The suspended scales were used to measure vertical movement in the bottom

chord of the truss at different locations. The locations of the gauges and

scales are shown in Figure 1. The weather during the period of observations

changed from sunny and hot to cloudy and cool. This was reflected by the

movement of the roof trusses, but no relationship between movement and

outside conditions could be established. Measurements taken during the

period from 8; 45 p. m., 26 May to 3:15 p. m., 27 May showed that the

maxi-mum vertical change in deflection amounted to 0.02 in., while the width of

the crack changed O. 002 in. The width of the crack was estimated to be

O. 02 in.

Additional background information was obtained from the contractor,

truss fabricator and workers on the job. Details as to the extent of the

plaster cracking and other relevant factors varied somewhat as the people

involved were relying on memory only. Lack of accurate drawings of the

building and the trusses also added to the difficulties of trying to determine the significant cause or causes of the cracking.

After the first visit was completed, a verbal report was made to the inter ested parties. At this time ther e was insufficient evidence to indicate that any single factor was the prime cause of the trouble, but several factors

were considered to be contributing to the failure. It was anticipated that all

the cracks would show up next winter due to thermal contraction of the top

chord and the effect of snow on the roof. Corrective action was therefore

required to patch the crack in the living room and to prevent all the cracks from

showing up again next winter. .

The successful repair of cracked plaster is always difficult, partic-ulary if daily movement of the crack, even though slight, is occurring.

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-The use of a non-shrink plaster patching material and

reinforce-ment of the joint in the gypsum lath were discussed. Loading of the lower

truss members during patching to increase the crack width was also mentioned. Releasing the load after the repair was made would subject the patch to a

slight compressive load sufficient to offset the shrinkage of the plaster. It

was suggested that stiffening the trusses could probably be achieved by

fasten-ing 2 x 8's to the side of the bottom chords of the trusses such that they would

bear on the walls. They would then be expected to help carry live loads and

would reduce the tendency of the bottom chord to deflect as the top chord underwent thermal mov ement.

The corrective action taken by the contractor consisted of bolting or screwing 2 x 10's to the bottom chord, cutting back and patching the crack

and then re-stippling the ceiling. A few days after this work was finished the

crack reappeared. The contractor then proceeded to take down the plastered

ceiling and reinstall the 2 x 10's free of the trusses and dropped an inch so that they would support the plastered ceiling but be independent of the trusses.

When the second visit was made, about half the ceiling had been

re-lathed so that it was impossible to examine the original repair work. It

was noted, however, that the extension of the crack into the two end walls could

be seen and apparently had not been repaired. As the cracked patch could not

be inspected it was impossible to determine the cause of the re-cracking. although the patching technique was considered to be the most likely factor responsible.

With the plaster ceiling removed, deformation in the bottom chord

of the truss was readily seen. Measurements were made and it was determined

that the deflection at the heel of the truss amounted to as much as 3/8 in" while

at about midspan it was as much as 5/8 in. below the limit of the bearing walL

Several cracks in the ceiling on the opposite side of the building were

now quite easy to see though they had not been previously. About half the

cracks were parallel to the trusses and the others perpendicular. They were

rnJstly associated with the corners of partitions and appeared to be cOl'!?:pression

failures of previously patched cracks. It is possible that these were related

to movement in the trusses that resulted from the removal of the ceiling in the living room.

DISCUSSION

It has long been known that trusses cannot perform as designed when interior partitions are present to interfere with the deflection of the bottom

chord. It is also known that moving the point of support inwards on a truss to

4

-of deflection. In spite of this, plaster problems associated with roof trusses

do not seem to be any more serious than with other systems. This case has

been somewhat different in that the crack in the living room has closed

con-siderably from a reported width of 1/8 in. Although no cracks could be seen

in the dining room and master bedroom, there were many reports that these ceilings were cracked and that the cracks had disappeared without being

patched. It is, of course, impossible for a plaster crack to reheal itself,

and although they cannot be seen the cracks are still there. There are

there-fore two questions to be answered: what caused the cracks, and why did

some close up?

Dealing with the plaster fir st, it is well known that cracks can occur as the result of shrinkage of the plaster, shrinkage of the wood frame and

the use of poor plaster. There was really no evidence to suggest that the

plaster in this particular case was not satisfactory. Plaster with a high

moisture content has only about half its normal tensile strength, however, and this was most likely the situation when the cracking occurred as it was

reported to have appeared only two weeks after plastering was completed. One

may suspect that the occurrence of heavy snowfalls at that time of year might

have produced sufficient stress in the plaster to cause it to fail. This

possi-bility is countered by the fact that the trusses were installed at 2/3 of their design spacing, and the contractor reports that very little snow was ever on

the roof. Although a strong wind off the lake could be anticipated, the shape

of the building is such that the wind would not be expected to apply any

signi-ficant load to the roof. Workmen working in the attic could easily deflect the

bottom chord and cause a local failure, which would then act as the focal line for other stresses such as those caused by plaster shrinkage.

There are several factors that must be included when considering

the trusses. Creep in wooden members and nailed joints could contribute to

a permanent set or deflection in the bottom chord. In many cases, compression

members were not in contact with the top and bottom chords so the compression

load was being transferred by the nailing plates. It was evident that the gaps

were built in and were not the result of shrinkage of the wood. Previous tests

have indicated that although nailing plates are not intended to carry such a load

they usually will do so satisfactorily. Such a situation may give rise to high

local stresses and these will be relieved by some relative movement at the joint. Shrinkage of the wooden member s, particularly the 2 x 12 wedge at

the overhang, is probably the most significant factor. Considerable shrinkage

occurs in the lumber used in house construction and a 2 x 12 could be expected

5

-and the height of the truss at the support, the end of the 2 x 4 lower chord would be deflected downward an equal amount since it is much more flexible than the

2 x 8 upper chord. Similarly, this would cause an increase in the deflection

of the lower chord between supports, which, due to the location of the truss members, may actually be larger than the amount of shrinkage in the wedge.

Shrinkage in the wooden structural frame may also have an effect

on the performance of the trusses. It is known that the interior wooden members

have a lower average moisture content than those located in the exterior portion

of a building. As a result, the interior walls will shrink more than the outer

walls as the members, mainly the joists and plates, dry out. This will allow

the trusses to deflect slightly and so begin to approach the deflection that they

would have if simply supported by the outside walls. Since the tensile stress in

the plaster varies with the deflection in the truss, the difference in shrinkage of the bearing walls may determine whether or not plaster cracking will occur

if the plaster is close to its failure stress.

In this case, however, both walls of the living room are designed as loadbearing members and have steel columns and beams while the other

end of the trusses rests on a normal frame wall. This eliminates the

bene-ficial differential shrinkage am, in fact, produces a detrimental differential shrinkage between the centre bearing wall and the exterior wood frame wall.

The effect that shrinkage of wooden members will have on the plastered ceiling depends on the percentage of the ultimate shrinkage that has

occurred at the time of plastering. The major factors here are the moisture

content of the wood when built into the house, the outside weather conditions, ventilation of the attic and the actual construction schedule. It is not

unreasonable to assume that the moisture content of the wood was in the order of 25 per cent and it is known that the attic ventilation was poor as no vents

were originally provided at the ridge. These and the other factors indicate

that the majority of the shrinkage in the wood used in the house would not occur until after plastering was finished.

Along with an increase in the drying rate, as the weather warmed up.

the attic temperature and roof temperature would also increase. The

tempera-ture of the ceiling and bottom chord of the truss would remain relatively steady

due to the presence of the ceiling insulation. The top chord would increase

bow upwards and so lift the bottom chord. This would tend to close the cracks in the plastered ceilings and produce a yearly cycle of movement that is similar to the daily cycle.

The trusses are therefore being deflected downward by one set of

forces and upward by another. In the case of the living room, this downward

deflection exceeds the upward deflection and the crack remains, while in the dining room and master bedroom the reverse apparently is true and the cracks

ar e hidden. Whether or not the cracks in the dining room and master bedroom

show up again next winter will depend on the relative effect of the various forces. It is impossible to pr edict this, but since the possibility of the cracks reappear-ing is quite high, suitable action should be taken to offset such a possibility.

In summing up, the causes of the major plaster cracks in the ceilings of the living room, dining room, master bedroom and other areas are the

inability of the roof trusses to function properly due to the presence of interior load and non-Ioadbearing partitions and the shrinkage of the wood used to

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PART PLAN SHOWING LOCATION OF CRACKS AND INSTRUMENTS

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SECTION OF TRUSS SHOWING BEARING POINTS AND LOCATION OF CRACK

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