Height Restrictions for Brick Veneer on Wood Frame Backing

Abstract

The most documented reason for the height restriction on wood backing appears to be the vertical differential movement caused by expansion of the brick veneer and shrinkage of the wood framing. Thermal expansion and shrinkage can occur in steel stud backing, but to a much smaller degree. In the current US masonry standard, height restrictions still exist for brick veneers supported by steel stud backing, but they are larger than those for wood. Concrete and masonry backing will experience similar moisture and thermal expansion as the brick veneer, and there is no height restriction for these backing systems. A solution to this problem would be to use engineered lumber. As the calculations will show, the expected shrinkage when using engineered lumber is much smaller than that of sawn lumber. The expected differential movement when using sawn lumber is 1.70 in. (4.32 cm) for a 30 ft. (9.1 m) exterior wall. If the floor joists are switched to LVL spruce-pine I-joists, similar differential movement (1.59 in., 4.04 cm) can be obtained by going up to a 50 ft. (15.2 m) wall. By switching the entire framing system to engineered lumber, the wall height can go up to 120 ft. (36.6 m) before getting similar differential movement (1.58 in., 4.01 cm). Masonry standards should include an allowance for a height increase for brick veneer supported by engineered wood construction. Another possible reason for these restrictions has to do with seismic concerns. It could be related to the horizontal differential movement between the brick veneer and the wood framing. If this is a concern, the wood framing could be stiffened with additional sheathing in order to reduce the amount of movement during a seismic event and increase the wall height. In this case, the standard should include an allowable horizontal movement due to seismic loading for wood framing supporting brick veneer. Finally, to get a small increase in allowable height, a standard mean roof height could be maintained. The code currently restricts the height at the plate to 30 ft. (9.1 m) and at the gable to 38 ft. (11.6 m), giving a mean roof height of 34 ft. (10.4 m). If this mean roof height is maintained, the height of the wall could be increased slightly to allow for larger story heights.