bowed trusses

Old 09-26-02, 07:23 PM
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bowed trusses

I'm buying a home which inspection has determined has
bowed (about 20) trusses although not severely. I'm trying to figure out the cause of the problem and what a potential fix might be if the situation calls for it. One possibility is that the builders let the trusses lay out on the ground and they were bowed at installation

another might be that the attic has no soffits and heat might
have played a role.

However these are uneducated uesses. Anyone have any more experienced thoughts?


Old 10-08-02, 09:00 PM
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Wood Truss Uplift

Wood Truss Uplift

An increasingly common effect of wood shrinkage is the upward bowing of wood trusses in winter This causes cracks between the partitions and the ceiling of up to 20 mm in severe cases. Wood truss uplift is primarily caused by the differential longitudinal movement of the upper and lower chord members.

Air in a well-ventilated attic space contains approximately the same amount of moisture as the outside air In winter the relative humidity of the outside air is fairly high; consequently, the top chords and web members will absorb moisture until equilibrium is reached with the surrounding air. The higher moisture content causes the top chords to lengthen.

The lower chords, however experience a different phenomenon. Since in modern houses they are often covered with up to 300 mm of insulation, their average temperature in winter is closer to the indoor temperature. This causes the air spaces in the insulation adjacent to the wood to have a much lower relative humidity than the air adjacent to the top chords. As a result, the air spaces adjacent to the bottom chords absorb moisture from the wood until an equilibrium moisture level is reached. The moisture content in the lower chords may decrease to less than 10% during the coldest winter months, and cause the chords to shorten. 2 As the lower chords shrink and the top chords expand, the peaks of the trusses are forced upward. This forces web members attached near the peaks to pull the lower chords upward, which, in turn, causes cracks between the ceiling and the partitions. If the chord members contain compression or juvenile wood, the amount of movement can be significantly increased. 3 Using unseasoned lumber may be a significant factor in truss uplift problems, particularly when the ceiling is installed before the moisture level of the trusses has been reduced to a reasonable level. Tests on roof trusses containing unseasoned juvenile wood have demonstrated that either upward or downward movement can occur as the wood dries, depending on whether the juvenile wood is located in the upper or lower chords.

A number of factors can influence the degree of uplift. Roof slope is one; the lower the slope, the greater the amount of arching for the same difference in moisture content between the upper and lower chords. The amount of insulation is another factor; the more insulation, the greater the difference in moisture content between the upper and lower chords. Differential shrinkage resulting from the lower moisture content of the partition framing in winter compared with the exterior wall framing, can also contribute to the separation of the ceiling membrane from the partition.

Thermal contraction, in the winter months, of the top chord relative to the bottom chord is insufficient to counteract arching caused by moisture changes. The weight of the roof assembly and the snow load, in most cases, only partially counteract truss uplift.

Even if seasoned lumber is used, roof truss uplift may not be avoidable without changing the present system of construction so that the top and bottom chords are exposed to the same environmental conditions. Because of the costs and adjustments this would entail, it seems more practical to modify the current system to allow the wood trusses to bow upwards without causing damage to the interior finish.

This can be achieved by eliminating ceiling fasteners within 300 mm of the partitions, and by coupling the ceiling to the partitions at their juncture so that the trusses can move upwards without breaking the joint between the partition and ceiling.4 The ceiling membrane can be coupled to the partition by special clips or corner beads nailed to the tops of the partitions so that the ceiling membrane is forced to flex, rather than tear away from the partition, as the truss moves upward

Alternatively, 19 x 140 mm boards can be nailed to the tops of the partitions. The boards must be fitted between the trusses where the partitions are at right angles to the trusses (unless the ceiling is supported by furring strips).

If such "floating corners" have not been provided, damage at the partition can be masked by installing cove moldings fastened to the ceiling supports only. This permits the molding to slide up and down the wall with the seasonal movement of the trusses. Suspended ceilings can also be used. It may be necessary to seal cracks with adhesive tape before installing the molding or the suspended ceiling, to prevent air leakage into the attic if the vapour barrier has been damaged by the arching effect.

Concluding Remarks

Wood shrinkage can cause many problems, from nail popping to truss uplift. using lumber whose moisture content does not exceed 19% should significantly reduce the incidence of most of these problems. It is possible to allow for truss uplift by using floating corners that will permit the ceiling to flex without tearing away from partitions. If floating corners are not used, corrective action is normally limited to concealing the damage by means of moldings or suspended ceilings.

CBD-244. Effects of Wood Shrinkage in Buildings
A.T. Hansen
Canadian Building Digest
Retrieved 08 October 2002

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