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In 1980, the CHBA put out a bulletin on the problems of drywall seam cracking at the ceiling joint of interior partition walls - caused by something that was identified as truss uplift. It was a strange phenomenon that only happened sometimes in some parts of some houses - a real construction phantom. The best guess scientific explanation appeared almost as strange as the uplift itself.

The top and bottom cords of trusses in insulated attics sit in very different temperature and humidity regimes and, if the cell structure of these pieces of wood themselves are critically different (wood grown under compression on the downhill side of a tree, wood grown under tension on the uphill side of a tree or juvenile wood), they will expand and contract in such a way as to cause the truss to arc upwards. The upward movement has been recorded as anywhere between 1/4 to 2 inches rise, with 1/2 inch being the most common.

We are now almost 20 years wiser. Many 15 year old houses still have seasonally rising and falling ceilings, there seem to be fewer occurrences in new housing but it still does happen, and the scientists' first best guess still appears to be the best explanation. It appears that we have not stopped the trusses from moving but rather we have learned to accommodate our building practices to live with it. The important thing today is to assure that newcomers to the industry understand why ceiling joints on partition walls need to be floated. As the Ontario New Home Warranty program has stated: "Servicing and repairing ceiling/wall separation complaints can cost 10 to 20 times the value of controlling the problem during construction.

Unfortunately, no one has studied or listed if some types of trusses have more or less "up-lift potential" than others, except for a general feeling that steeper pitched roofs have less problem than lower slopes. As well, the drywall companies seem to have dismissed the problem as not theirs, but a problem of underlying structure. The research that has gone on recommends that the last ceiling screw be 18 inches away from any partition wall. Some drywall companies still have a 12-inch recommendation, apparently worried about sagging but 18 inches gives a much better hiding and flexibility. A 1 x 6 top plate attached to the partition walls but not to the trusses seems to give the strongest joint, with drywall clips a good second choice.

Whether your trusses will experience seasonal lift or not is a hard question to answer. Floating partition wall/ceiling corners is an insurance policy that allows you to not worry about the question.

Copyright 2005 Prest-on Drywall Fasteners