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Do you think of the buildings that you design and construct as dynamic entities? Believe it or not, they are. Over their lifetimes, buildings experience movement that must be considered in the earliest stages of their creation—from conceptualization through construction.

Wood Shrinkage

With the increasing costs of materials, multi-story buildings framed with timber are becoming more popular choices. One often forgotten characteristic of timber framing is shrinkage. As wood dries, it shrinks.

For a typical story, changing the moisture content from 19% to 10% can result in a ¼ inch of shrinkage. If the weather is very wet during the time before the building is enclosed, the wood may have a moisture content above 19% which will cause it to expand, magnifying the eventual shrinkage due to drying. In the absence of good air flow after a building is enclosed, the drying may take a year or two.

This amount of shrinkage by itself may not present a problem if the finishes have the flexibility to absorb this amount of movement. However, if there are rigid finishes like stone or brick, wood shrinkage may become an issue as the wood shrinks and the finish does not, causing misalignment of windows and doors. For example, if a window is built into a wood-framed wall, and the wood shrinks, the window sill may move down. If there is a brick veneer which is built up to the window sill and does not allow for movement at the sill, the brick may block the movement of the sill causing the window to bind.

The solutions lie in anticipating and calibrating the amount of shrinkage, which can be estimated during design based on the moisture content specified. Careful detailing can compensate for this shrinkage, and the movement it causes.

Brick Expansion

Regarded as one of the longest lasting and strongest building materials, brick has been used as a building material for thousands of years. (Mud bricks have been discovered in ruins dating back to 7500 B. C.!) During most of its history brick was used by itself to build walls. It may have been combined with wood or metals for floors and roofs, but, traditionally, the walls were constructed of solid brick. In more recent times, however, brick has been widely used as the weather envelope for the walls only, and has been combined with wood, concrete, concrete block and other materials to create walls.

With wetting and drying due to weather, a brick will slowly grow. This is a permanent, irreversible expansion. When a building was constructed of solid brick (as buildings were in the past), the entire wall would expand and not cause problems. However, with the modern practice of using brick as a veneer over a backup structure, the brick will expand, but the underlying structure may not.

As this expansion continues over a period of 20 to 30 years, I have seen cases of brick veneer literally trying to tear apart the structure behind.

I have also seen instances of brick overhanging a different material below it, and even causing the cracking of materials that confine it. If the brick is broken by expansion joints, the expansion has somewhere to go. But in the absence of expansion joints, a 100-foot length of brick could expand ½ inch over a 20 year period.

Unfortunately, the rate of expansion has been poorly documented, and is dependent on the type of clay used and its exposure in the building. Over the life of a building, even expansion joints may close, requiring new joints to be created.

A similar expansion can occur over time in structural clay tile. Again, detailing expansion joints into the walls can give the expansion somewhere to go and prevent future problems.

If the building combines a wood structure (which may shrink) with a brick veneer, the effects of each will be magnified.

Good detailing can reduce problems associated with this movement, such as allowing space for movement at window sills and other wall penetrations. Proper communication between the designer and the Structural Engineer is essential, both of whom should take into consideration the opposing characteristics of these two building materials when collaborating on the architectural details.

Let’s talk about your project’s potential pitfalls for expansion and shrinkage, and how to plan for the inevitable characteristic changes of these universal—and indispensable—building materials. Give me a call at 717-627-6873 or send an email to Tom@zugassociates.com