Sheet Metal Hem Types and When to Use Each

A hem is a folded edge on a sheet metal piece — the return bend that stiffens an edge, eliminates the raw cut, and in many cases creates the locking mechanism that connects two pieces together. Specifying the wrong hem type leads to pieces that can't be field-installed as drawn, or that fail structurally under wind load. This guide covers every hem type commonly used in architectural sheet metal and trim fabrication.

Why Hems Matter

Raw-cut sheet metal edges are sharp, flexible, and structurally weak. A hem solves all three problems: it removes the sharp edge, stiffens the leg by doubling the material at the edge, and often provides the geometry needed to lock into a cleat or mate with another piece. On exterior trim, hems also affect water management — a properly formed drip hem directs water away from the substrate rather than letting it follow the face of the trim by capillary action.

Open Hem (Open Return)

An open hem folds the edge back approximately 180° but leaves a small gap — typically 1/8" to 3/8" — between the hem and the face of the material. It stiffens the edge and removes the sharp cut without fully closing the fold.

• Use when: You need edge stiffness on a visible face and a fully closed hem would telegraph through Kynar-painted material (full closure can crack the coating on tight bends in painted steel).
• Common applications: Face legs on drip edge, coping face legs, rake trim visible faces.
• Standard allowance: 3/4" of material consumed per open hem. Add this to your leg dimension when calculating developed width.

Closed Hem (Flat Hem)

A closed hem folds the edge fully back to 180° with the hem completely flat against the face — no gap. The result is a clean, crisp edge that is structurally stronger than an open hem.

• Use when: The piece is mill-finish or unpainted aluminum, or when the design calls for a tight, finished edge with no visible return gap.
• Avoid for: Pre-painted Kynar or PVDF coatings in thicker gauges (24 ga and heavier). The tight bend can crack the coating on the outside radius.
• Standard allowance: 1/2" to 5/8" of material consumed per closed hem.

Drip Hem (Hemmed Drip Edge)

A drip hem is an open hem specifically positioned to project beyond the substrate face, creating a water-break that causes water to drip free rather than travel up the back of the piece by capillary action. The hem itself points away from the wall and down at a slight angle.

• Use when: Any exterior trim edge that is not protected by an overhang — drip edges, coping face legs, head flashing face legs, rake trim.
• Minimum projection: 3/8" beyond the wall face to ensure water breaks free cleanly. Less than 3/8" and water will track back under the trim on wind-driven rain.

Pittsburgh Lock (Pittsburgh Seam)

The Pittsburgh lock is a mechanical seam that joins two flat sheet metal pieces edge-to-edge. One piece has a pocket hem; the other has a single hem that slides into the pocket and is then hammered or rolled flat to lock. The result is a strong, flat joint with no exposed fasteners.

• Use when: Joining two flat panels (ductwork, box gutters, flat-seam roofing). Not typically used on trim profiles.
• Common on: Box gutter fabrication, rectangular downspouts, HVAC duct.
• Material consumed: The pocket side uses approximately 1" of material; the hook side uses 3/4".

S-Lock (S-Cleat / Drive Cleat)

An S-lock creates a sliding connection between a coping or panel and a continuous cleat. One hem forms an S-shape that captures the cleat edge. When the cap is placed on the cleat, it snaps or slides into engagement, holding the piece down without through-fasteners.

• Use when: Installing coping caps, fascia covers, or any trim where you need a concealed mechanical attachment that allows thermal movement.
• Standard geometry: The S-hem typically engages a 1" x 24 ga continuous cleat. Verify that your coping hem depth matches the cleat height you are ordering.

Standing Seam

Standing seam is a field-formed or factory-formed hem that rises vertically above the roof plane. Two adjacent panels each have an upward-bent leg; the legs overlap and are either hand-seamed (single or double-folded) or mechanically seamed with a power seamer.

• Use when: Low-slope or architectural metal roofing panels where concealed fasteners and thermal movement capacity are required.
• Seam heights: Typically 1" (low profile) to 2" (standard) to 3" (high profile for low-slope applications).
• Not for trim: Standing seam geometry is specific to panel systems. It is not used on brake-formed trim pieces.

Hem Type Summary Table

Hem Allowances and Developed Width

Every hem consumes material. When you calculate the developed width of a trim piece — the total flat material width before bending — you must add the hem allowances for every hemmed edge. A drip edge with an open hem on the face leg, a flat back leg, and no hem on the top requires a hem allowance only for the face. A coping cap with S-lock hems on both legs and a closed hem on the face requires three hem allowances added to the top flat and leg dimensions.

Trimgy's drawing canvas lets you select the hem type for each edge of your trim profile and calculates the correct developed width automatically — so you order exactly the right amount of material. Draw your profile with correct hem allowances →