Expansion Joints and Curbs

04/15/2005 |

Buildings.com - Roofing Newsletter

When inspecting most low-slope membrane roofing systems, you will encounter raised curbs that dissect the roof area. Because most are raised above the floodplain of the roof, there are generally not troublesome, except perhaps at crossovers and terminations.

Are Expansion Joints Really Needed?

To be accurate, what you are looking at is not really the expansion joint. A joint is a gap between structural elements. That gap runs from the foundation, upwards through the structural beams supported by double columns, the roof deck, and roof system. The roofing industry provides expansion joint covers. Beams are supported independently on each side of the joint, and the deck is discontinuous as well.

The purpose of the structural expansion joint is indicated by its name: It allows for expansion, contraction, or even seismic relief so that the structures on each side of the joint can move independently.

Large roof structures extending over acres are a fairly new development. Massive poured concrete buildings are rare today because of the associated costs of foundations to support the weight, dead weight of the deck and supports, and labor involved. If a concrete deck can be poured as a monolith, there is no need for joints (just in the roof system). However, for the lightweight steel, precast concrete, and OSB structures used today, the buildings are far more dynamic and structural allowance is needed between adjacent sections.

Justin Heshell, FAIA, a highly regarded roof consultant, has indicated that cumulative movement expected in large, unbroken rectangular roofs has never been confirmed as a cause of roof splitting. Rules for placing roof joints every 200 feet or so regardless of the structure below are not validated by fact. Differential movement, on the other hand (for example where cambered precast “T” sections settle to different degrees), has been indited, but that suggests that relief joints probably are needed only where the deck's anticipated movement will occur, and that is related to the size and shape of the deck units, not roof size.

Since the oil embargo of the 1970s, roof systems have been using quite a bit of above-deck thermal insulation. This, in turn, reduces the thermal loads on structural elements. Other than movements that might occur when a building first turns on its heat, seasonal and diurnal movements are negligible.

With wood, plywood, and OSB, seasonal moisture changes result in movements of the decking. Gaps of 1/8 inch between deck units are intentionally installed so that the decking will not buckle at maximum moisture content. However, structural joints every few hundred feet are not indicated here either. Instead, above-deck components are attached intermittently, so that stresses are not concentrated directly over these wood joints. At various times, the base sheet of a membrane roof system has been attached by spot-mopping, strip-mopping, sprinkle-mopping, nailing, and stapling. Only when mopping comes all the way to the ends of adjacent boards does tearing stress build up to the point where the membrane might split.

APA (the Engineered Wood Association) provides guidelines for the conditioning of decking before installing shingles – for example, because these minor ridges in shingle underlayments might telegraph through the conformable shingles and be unsightly. In membrane roofing, we are more likely to have above-deck thermal insulation so these small deck movements do not telegraph through or affect the membrane. A more serious problem is when the insulation boards themselves cup or warp, either from moisture or thermal or manufacturing stresses. These ridges are unsightly in unballasted roofs, and may cause fishmouths at laps and seams of roof membranes.

So, Are Those Expansion Joints Really Needed?

Fortunately, that decision is not the building owner’s problem. That is a design issue that hopefully has been addressed by the structural engineer and architect. As building owners and managers, our concern has more to do with the joints we have (not some hypothetical design criteria) and figuring out whether we have a real expansion joint -- something called an “area-divider,” or a “control joint.” Also, while most joints are well-designed and constructed, we have the typical problems at terminations and crossovers. If the building has been around awhile, odds are good there are all sorts of patches and goop that have been unsuccessfully applied.

Types of Joints

A cornerstone publication on the various types of joints and how they work is the Canadian Building Digest #202, by R. G. Turenne (you can find his entire article at the website shown below). Turenne points out that there are three types of joints that have been used in membrane roofing.

One of these is the structural expansion joint cover already mentioned; the second one is where the deck is continuous, called an area divider; and the third is a patch for areas that are splitting, termed a control joint.

Since Turenne addresses the control joint first, let's start with that. "The control joints are frequently placed after the roofing membrane has been placed. Allowance for movement is made by cutting the membrane and insulation along a predetermined line … unfortunately, the use of membrane control joints is based on a number of invalid assumptions, one being that bituminous materials behave elastically and that all thermally induced movements are completely reversible," Turenne explains. Fig. 1 illustrates such a control joint.

If such splits are occurring in your roof system, the real culprit is non-uniform attachment of the insulation or membrane to the deck. Stress accumulates along the shear line until the concentrated forces exceed the membrane's strength, usually over an insulation joint. The solution is to reattach the roofing system to the deck. There will no longer be movement in the split area, hence no need for a flexible control joint that is supposed to open and close as the membrane moves back and forth.

The other two joints are useful. The area divider, as its name implies, divides a big roof area into smaller, usually rectangular units. In this case, it is installed over a continuous deck as its purpose is not to accommodate movement. The divider can effectively cut off passage of leaking water from section to section, and serves as a permanent separator of chemically different materials. For example, one side might be an old, still performing coal-tar-pitch, gravel-surfaced built-up roof system while the other is a reflective PVC single-ply system. Each side of the area divider is flashed with materials compatible with its roof membrane, and the divider is capped with some sort of cover, usually a metal cap. See Fig. 2.The third type, the true expansion joint cover, is illustrated in Fig. 3. Note that the joint goes entirely through the deck and supporting structural elements. This differs from the area divider, in that two vertical curbs are needed, and the joint cover is either a flexible material to accommodate movement by flexure or a metal system installed in such a way that the cover allows slippage while deflecting water.

Next month, we will look at how to maintain these expansion joint covers and how to redesign crossovers and transitions – the usual causes of leakage.

Helpful Websites:
Canadian Building Digest #202
National Roofing Contractors Association
Sheet Metal and Air Conditioning Contractors’ National Association

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