Architectural Flashing Supply
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Expansion joints and thermal movement · Lesson 1 of 4

Expansion joints and thermal movement: principles and terminology

Why sheet metal changes length with temperature, which temperature to use, where a run is fixed and where it floats, what stops it moving, and the roof safety rules that apply while you work on joints.

You will be able to: Define thermal movement, fixed point, floating end, restraint and the two kinds of expansion joint, and name the fall protection and protective equipment rules that apply on the roof.

9 min · 11 slides · 7 questions

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Slide 1 of 11

Why sheet metal moves

Metal gets longer as it warms and shorter as it cools. AFS's thermal method gives the change as dL = alpha x L x dT: the metal's coefficient of expansion, times the run length, times the temperature change.

The formula has three inputs and no thickness term.

Sources

Glossary

Thermal movement
The change in length of a metal run as its temperature changes, worked as dL = alpha x L x dT.
Coefficient of expansion
The length change per unit length per degree for a metal; AFS's register gives galvanized or Galvalume steel as 6.5 x 10^-6 per degF.
Fixed point
The place where a run is anchored so it cannot slide; the run moves away from it and back.
Floating end
The end of a run that is free to move; with a one-end anchor it takes all of the movement.
Installation temperature
The metal temperature on the day a joint is set; expansion is worked up from it and contraction down from it.
Restraint
Anything that stops an edge or flashing component from expanding and contracting freely, such as a tight crimp or an attached appliance.
Splice plate
A plate under a joint gap that keeps water out while the lengths move.