Flashing joints, locks, and laps

Choose the joint for its exposure and function

A simple overlap can be easy to form and fit where the specified drainage detail permits it, but leaves a visible step. A receiver or interlocking seam can conceal an inserted edge and help organize a long run, at the cost of more developed material, accurate folds and fitting work.

A backed butt joint can separate the visible face joint from the support or weathering detail behind it. A movement joint has a different purpose again: it must let the specified movement occur without defeating the weather barrier. A tight-looking seam is not proof of watertightness, and an ordinary lock is not automatically an expansion detail. Choose from the actual roof or wall system requirements before calculating allowances.

Simple overlap

One piece passes over another in the direction that sheds water. It is easy to fabricate, but the joint can remain visible and its performance depends on orientation, overlap, fastening, sealant requirements, slope, and exposure.

• Use engagement as the installed overlap that reduces repeating coverage.
• Use lock allowance as zero when no receiver is formed.
• Account for any starting return as the first-piece end allowance.

S-lock or receiver joint

One edge is folded back twice to create a shallow S-shaped receiver. The raw edge of the following piece slides between those folds, producing a clean interlocking joint while concealing the inserted edge.

First coverage = stock width − lock allowance − first-end allowance
Repeat coverage = stock width − lock allowance − engagement

Flat-lock seam

Both adjoining edges are folded so they hook together and can be closed flatter than a raised seam. The developed allowance depends on the folds, material, tools, and whether the seam is soldered, sealed, or otherwise specified.

• Enter the complete formed-edge consumption as lock allowance only after developing the real profile.
• Enter any lost installed width from mating the seam as engagement.
• Do not assume a flat lock is watertight merely because the edges interlock.

Butt joint with backing splice

The visible faces stop near one another while a separate backing piece supports or weathers the joint. This can preserve a clean face and lets the splice be detailed independently.

• Main-piece coverage is based on the intended face module and joint gap.
• Calculate splice-plate material separately from the main flashing blanks.
• Confirm fastening, sealant, drainage, and movement requirements for the actual detail.

Expansion or movement joint

Long metal runs expand and contract. A movement joint separates sections and uses a purpose-designed cover, receiver, or splice arrangement so the metal can move without opening the weather barrier or buckling.

Map the joint into the calculator

Keep every input editable and label the joint used on the printed fabrication sheet. The calculator handles arithmetic; the selected construction detail determines the right dimensions.

• Stock width: raw width available for one piece.
• Lock allowance: width consumed forming the receiver, hook, hem, or lock that no longer advances along the wall.
• Engagement: width of each following piece inserted into or overlapped with the previous piece.
• First-piece end allowance: separate loss at the starting termination.
• Profile height: coil consumed down the roll for each straight or tapered blank.