Flashing & coil guide · 9 min read
Flashing joints, locks, and laps
Compare common sheet-metal joints, understand what each connection is meant to do, and map its material allowances into the calculator.
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.
Never reverse a weather lap. Water must flow over the joint rather than into its exposed edge.
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
For the current UC example, 36 inches of stock, a 2-inch forming allowance, and 1-inch engagement produce 34 inches of first coverage and 33 inches thereafter. Make a test lock and measure it; those values are not universal.
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.
Do not invent an expansion joint from a generic lap dimension. Location, spacing, fixity, cover geometry, clips, sealants, and movement capacity belong to the specified system or a qualified designer.
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.