Differential Movement: Tips & Definition for Roofers

Differential Movement

Differential movement is a term that you have probably heard if you have spent any time as a roofer. It can cause alarm in some cases, or it can be a regular part of the job. In any case, as a roofer, you should be able to identify and define differential movement as it pertains to the roofing system of a building. 

Differential movement in a roof can cause severe damage. Or it can be a regular part of the roof’s “settling” process. The following post will look at the definition of differential movement, how it can affect the structure, and some tips on how to handle it. 

What is Differential Movement?

Differential movement occurs at points in the roofing system where independent parts are adjoined. Installed parts settle at different rates causing unsettled structural formation. Specifically, other parts of the roof adjust, settle, or move (this phenomenon itself is natural) at different speeds. 

When Does Differential Movement Occur?

Differential movement often occurs when there has been an addition of some kind to a commercial or residential building. For instance, adding a bedroom to a house is often the culprit for residential differential movement. Because new materials are being adjoined to old materials, the settling rate will not match. As you can imagine, signs of differential movement typically appear where old materials interact with new materials.

The flashing of a roof will also be a tell-tale sign of differential movement. Therefore, if you inspect for differential movement, one of the first places you should look at is the roof’s flashing.

What Can Differential Movement do to a Roof?

First of all, you should be aware that other things can cause differential movement. For example, shifts in the soil under a house could have visible effects on the foundation and the roof. Tree roots can also cause differential movement by shifting the foundation of the structure. If one side of a system is sunken lower than the other, this will cause a faster settlement rate on the side that is sunken. 

As you can imagine, differential movement of any kind can cause some severe problems. However, the manifestations of differential movement may also be somewhat benign. Not all differential movement occurs at such an opposing rate as to cause severe damage. For example, slight differential movement can occur over decades without causing any serious problems. Still, the phenomenon can impact a structure in many ways, including but not limited to:

  • Foundation Cracks: This is one of the more severe side-effects of differential movement and can cause a structure to become unsound. 
  • Interior Wall Cracks: Inexplicable cracks in interior walls may be another sign of differential movement. 
  • Ill-fitting Doors: If you have ever had to shave down a new door because it doesn’t fit in the jamb, the problem may have been caused by differential movement in the structure. 
  • Tilting Chimney: Differential movement in the roof or the foundation may cause chimneys to tilt or sink to one side or another. 
  • Distorted Walls: Severe differential movement may cause the structure’s walls to bulge in sections under extreme stress. 

Differential movement can cause both cosmetic and structural damage. For example, cracks in the wall of a house may be a mild sign of differential movement, but inspect the whole structure o make sure that no adjustments are necessary.

Roofers, in particular, should inspect the flashing first. If you see a gap of 2-6 inches in the flashing between a vertical wall and the roof deck, this is most likely a sign of differential movement that has to be corrected. Sunken areas of the roof near flashing could also signify something that needs to be done sooner than later. 

Tips for Differential Movement

How you deal with differential movement will depend on a lot of factors. First, how severe is the movement? Then, what type of structure is the roof system? And, of course, where the movement is occurring? In general, though, here are some tips for dealing with differential movement as a roofer:

  • Look for Wrinkles: One of the fastest ways to identify differential movement on a roof is by looking at the flashing. If there is a pattern of diagonal wrinkling in the flashing, you are likely dealing with differential movement. 
  • Roof to Wall Expansion Joint: If the roof decking is independent of a vertical wall, a roof-to-wall expansion joint should be installed to correct and prevent differential settlement. The flashing should only be anchored to the roof deck, and there should be a woodblock at the base. 
  • Dealing with Roof Openings: Differential movement often occurs at roof openings (vents, skylights, etc.). To prevent differential movement from happening at these vulnerable points, roofers must form a curb. The curve isolates base flashing and counter-flashing, so that differential movement becomes a non-issue. The curb should be made from metal and should sit at least 8 inches above the finish roofing material.

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