Plaster Cracking in a Ceiling Related to Roof Framing

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Technical Note (National Research Council of Canada. Division of Building Research), 1968-01-01

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Plaster Cracking in a Ceiling Related to Roof Framing

Ball, W. H.; Blackall, T. N.

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DIVISION OF BUILDING RESEARCH

No.

NATIONAL RESEARCH COUNCIL OF CANADA

503

NOTlE

'f

EClHIN ][CAIL

PREPARED FOR Inquiry and Record Purposes

SUBJECT _{PLASTER CRACKING IN A CEILING RELA TED TO ROOF FRAMING}

This Note describes plaster cracking in a ceiling of a house. The possible relationship of the cracking to the framing details of the roof and ceiling are discussed. The owner reported that the house was completed in the fall of 1963 and was occupied in early 1964. Figure 4 is a view of the house from the street. Figure 1 is a plan of the second storey of the house with the cracked ceiling. The four lines of cracks are identified in this figure as Crack A-A, Crack B-B, Crack C-C and Crack D-D.

The framing details of the roof and ceiling are shown in Figures 2 and 3. Attention is drawn to the method of end support for the roof rafters that were intended to provide a no-thrust system. In actuality, however, the rafter s are butted tightly at the peak and are tied together with a scab or gusset. It is probable that shrinkage of the ridge beam and in the supporting structure allowed the beam to lose contact with the rafters so that it could not carry roof loads without developing an outward thrust.

The ridge beam had a measured depth of approximately 11 in. on March 22, 1967; many rafters were not bearing on the beam (Figure 5) although in places a measure of bearing is achieved in-directly through the gusset. The detail at the lower end of the roof rafters is normal but the nailing is probably inadequate to resist much outward thrust. The stub joists (2 by 4' s) which are attached

2

-to the roof rafters, and provide support for the ceiling between the wall and the last ceiling joist (2 by 6), would accept thrust from the rafters and could in turn stress the plaster.

Another possibility is that the thrust in the rafters caused a slight outward movement of the wall (see Figure 6). Again, the stub joists might have followed the wall movement resulting in the plaster being overstressed and cracking. It is thought that Crack A-A shown in Figures 8 and 9 resulted from one or both of the loading conditions described. The owner reported that this crack was in evidence when he first inspected the house. The crack was filled to his satisfaction before the

purchase was made, but it reappeared after the house was occupied. An attempt was made to determine whether the wall was bowed outward but the results were inconclusive. There appear, however, to be clear indications of outward movement of the rafters relative to the wall in the form of narrow unpainted strips on the underside of blocking and rafters (Figure 7). An obvious criticism of the framing arrangement is the lack of adequate bearing area for the rafters on the ridge-beam. The rafters should have been notched to form a flat bearing area on the ridge-beam.

The crack B-B parallels the ct;iling beam shown in Figure 1•.. This beam supports the kitchen and dining room ceiling joists on metal joist hangers (Figure 10). A view of the crack taken from just above floor level is shown in Figure 11. This crack shows

signs of periodic movement. This method of support might encourage periodic movement due to expansion and contraction in the wood.

Seasonal change in moisture content of wood in the attic may vary from 20 per cent in winter to 14 per cent and less in summer.

The crack C-C runs transversely to the line of the ceiling joists (Figure 12). It appears to be a tension crack which is now stable. The cause of this crack is not obvious. There is a possibility of differential drying of the ceiling joists (i. e. drying and thus shrinking above the line of the top surface of the insulation

3

-in advance of the lower portion) caus-ing a general, but temporary, downward deflection of the ceiling. Another possibility is that

some temporary loading of the ceiling joists might have overstressed the plaster. Crack D-D is associated with the corner of the brick-work of the fireplace (Figure 13) and could have resulted from dif-ferential movement between the wood-framing of the ceiling and the fireplace.

GENERAL COMMENTS

It is unusual to find cracking as extensive as in this case. Of the four cracks identified, the first two are probably of greater interest because of the structural implications involved. It is apparent that for good performance to result, structural systems -even those used in simple house structures - must be based on recog-nized principles, and constructed to accept anticipated loads without excessive displacements or deflections.

REMEDIAL MEASURES

The problem facing the owner is to repair the damage with reasonable assurance that the cracks will not recur. Cracks C-C and D-D should be filled and will probably not re-open. Crack B-B may continue to be a problem unless it is possible to minimize movement of the beam or the beam-joist connections. One approach would be to attempt to stabilize the beam environment by a re-arrange-ment of the vapour barrier and insulation (Figure 14).

The permanent repair of Crack A-A will be very difficult as long as there is a possibility of an outward thrust of the rafter s occurring. This might be remedied by restoring positive bearing between the roof rafters and the ridge-beam using ledger

strips (Figure 15). Care should be taken not to raise the joists, but to just bring them into firm bearing on the ledger. Another approach would be to devise a system of cross-ties to restrain outward move-ment of the lower ends of the roof rafters. This approach would be costly, however, and difficult to install.

.

LIVING ROOM

-.J

BEDROOM

--

-0

r

-

_-

oU'!,--c

U

BATH

I

l

\

_I

II

-

_D

,

..

\

DINING

II

-

b--,

KITCHEN

llA

!

!

FIGURE I

KEY TO MAJOR CEILING CRACKS, SECOND FLOOR

,

L

:::!

_l'

:-1i10IRECTlON OF

ROOF RAFTERS

ROOF RIOGE BEAM)

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===-?

r

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"

1

"

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1E

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CEILING BEAM/

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\. STUB JOISTS'

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FIGURE 2

APPROXIMATE LAYOUT OF BEAMS, JOISTS AND RAFTERS

OVER PROBLEM AREAS

....

'

FIGURE 3

. ,-"

",

.1---METAL JOIST HANGERS

/ '

2"x6" JOISTS

FIGURE 10

\.:

,.

ADD INSULATION LEVEL

WITH TOP OF JOISTS

POLYETHYLENE

FIGURE 14

PROTECT THE BEAM WITH POLYETHYLENE AND AT LEAST

2" OF INSULATION

INSTALL 2''x

a"

LEDGER STRIPS

WITH TAPERED TOPS.

SHORT LENGTHS

MAY BE REOUIREO

TO ATTAIN UNIFORM

BEARING.

FIGURE 15

SUGGESTED METHOD OF RESTORING RAFTER BEARING TO

RIDGE BEAM

Figure 4 Front Elevation of House

Figure 5 In Many Locations Rafters Do Not Rest On Ridge Beam

Figure 6 Narrow Exposure at Blocking Indicates Where Wall May Have Moved Outward

Figure 7 Paint Line Indicates Movement of Rafters Over Top Plate

Figure 8 Crack A-A In Kitchen

Figure 9 Crack A-A In Dining Room.

Figure 11 View of Crack B-B Taken From. Just Above The Floor

Figure 13 Crack D-D. Note Loose Brick At