Detail of the Week: Expand your horizons...and your roof.

Buildings are big, some of them really big. When a building is big enough, it needs expansion joints to allow the building to expand when it gets really warm and contract when it gets really cold. It sounds like not much of a deal, perhaps, but when you have two 35-foot long steel beams each trying to expand one inch into each other, finishes can crack or warp or worse, the structure can deflect and cause structural failure. I found some photos here recently of an expansion joint system in the roof of a one-story building with a concrete structure and precast concrete exterior.

Here's a photo of the expansion joint coming up to the exterior wall where two precast concrete panels come together.

The expansion joint itself is a product that the architect specifies and the contractor buys and installs. They come in a pretty wide range of shapes and sizes, depending on where you want to put it and how wide the expansion joint is (the more movement that's expected, the wider the joint--this joint is about 2 inches wide). This joint is in the open on a roof, so it looks like a thick rubber half-of-a-tube with a piece on each side that the contractor attaches to wood blocking on top of metal studs. The wood blocking/metal studs are the box underneath the expansion joint bump. The contractor then puts a rubber-polymer roofing material over it (well call it an EPDM sheet in da biz), seals it down with an adhesive, and covers all edges and gaps with a sealant. An expansion joint has to run from one exterior wall to another exterior wall. This one goes to a space between two exterior panels and all the way down to the ground on the other side. You can see the tan-orange sealant on top of the panel gap. On the other side, there's a piece of spongy material adhered into the gap between the panels going down the face of the building.

When a building has a couple of wings or big chunks coming together, the expansion joints have to intersect. This is not always perty. However, this contractor did a really nice job of bringing the three wings of this building together.

This expansion joint also runs to a clearstory, which is the name for a part of a roof that pops up above the rest to get some natural light into a building. You see it hit right behind the column on the end of the clearstory. See the gap in the yellow sheathing on the column and the sheathing on the soffit (the horizontal parts of the roof)? The expansion joint is wrapping up around the column and running along the high side of this angled roof.

How can this be? Doesn't the roof structure have to be connected to some beams or something? Well, sure. The roof joists are attached firmly to the beams on the low side of the clearstory. However, because the expansion joint runs along the high side of this clearstory, the joists sit on Teflon pads. One pad is attached to the joist, and the other is attached to the beam holding it up.

The Teflon pads are the thin light-colored wafers between the rusty-looking steel plates. The upside-down U-shaped piece of steel on top allows the joists to sit at an angle.

Here's the thing about expansion joints: pretty much finish needs an expansion joint. The brick face on a building needs an expansion joint at least every 200 feet (if I recall correctly), and even drywall needs very small, pretty expansion joints, called control joints, in wide expanses of walls.

The little lines in that drywall are control joints. Here, they're being used for aesthetics, to separate different colors in this decorative soffit. The control joint is a little piece of V-shaped plastic about 1/4-of-an-inch wide with little flanges that the drywall contractor muds into the drywall on each side of the joint. It allows the drywall to expand and contract so it doesn't crack and look skanky.

Have I expanded your horizons with my expansion joints? Good. Now go have a good weekend.