7.4 Sheet Metal Fabrication, Expansion Joints & Area Dividers

7.4 Sheet Metal Fabrication, Expansion Joints & Area Dividers

Sections 7.1 through 7.3 covered what perimeter metal, flashings and drainage components must do. This section covers how the metal is actually made and how the roof is allowed to move. Roofing Components is a twenty-percent, ten-question block on the CR-42 exam, and shop fabrication, soldering and movement accommodation are the parts of it that separate a roofer from a sheet metal mechanic. Every failure discussed here has the same root cause: metal that was restrained when it needed to move.

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1. Metal Selection: Thickness, Gauge and Weight

Sheet metal is specified three different ways depending on the material, and confusing the systems produces the wrong product on the truck.

Two practical rules follow. First, a smaller gauge number always means thicker metal, but 24-gauge stainless is thinner than 24-gauge galvanized — always confirm which table the specification is using. Second, the ANSI/SPRI/FM 4435/ES-1 edge-metal standard discussed in Section 7.1 sets minimum thicknesses for the fascia and cleat by design wind pressure, and those minimums override any general rule of thumb.

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2. Fabrication: Brakes, Hems, Cleats and Seams

Field-formed and shop-formed metal share the same vocabulary:

• Hem — the edge is folded back on itself. An open hem stiffens the edge and removes the cut-hazard; a closed hem does the same and creates a locking edge for a cleat. Every exposed metal edge on a roof should be hemmed. A drip hem at a fascia bottom edge forms the capillary break described in Section 7.1.
• Continuous cleat — a strip of metal, usually one gauge heavier than the metal it holds, fastened to the substrate along the entire length and engaged by the hemmed edge of the fascia or coping. The cleat holds the metal against uplift while allowing it to slide longitudinally. This is the single most important detail in expansion control.
• Drive cleat (drive slip) — a U-shaped connector driven over two hemmed edges to join sections without exposed fasteners.
• S-lock (S-cleat) — an S-shaped connector for transverse joints in gutters, copings and gravel stops; it locks the sections while permitting slight movement.
• Flat lock seam — interlocking folded edges, often soldered, used for flat-seam roofing and complex flashing shapes.
• Standing seam and batten seam — vertical seams raised above the water plane, discussed for panel roofing in Section 4.3, also used for large flashing panels.

[!IMPORTANT] Never face-fasten metal that must move. A screw or nail driven through the face of a coping, gravel stop or gutter creates a fixed point. On a long run, thermal cycling elongates the hole, breaks the sealant, and the fastener becomes the leak. Use a continuous cleat and hemmed engagement; where a fastener is unavoidable, fix one end and let the rest float.

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3. Soldering: What Can Be Soldered and What Cannot

Soldering creates a genuinely watertight, structurally continuous joint — the reason copper gutters and stainless pans outlive sealed joints by decades.

Technique that matters: mechanically fasten the joint first with rivets or a lock seam so the solder is sealing, not carrying load; overlap the joint at least one inch and fill the entire lap; pre-tin both surfaces; heat the metal, not the solder, so the solder is drawn into the lap by capillary action; then neutralize and rinse acid flux residue, which will corrode the metal if left. Traditional 50/50 tin-lead solder remains common in roofing; lead-free alloys are used where runoff may reach potable or edible-garden collection.

Because aluminum and Galvalume cannot be soldered, their joints must be sealed and mechanically locked — butyl tape or non-hardening sealant inside a lapped, riveted joint, or a fully engineered slip joint with a cover plate.

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4. Thermal Movement: The Governing Calculation

$\Delta L = \alpha \times L \times \Delta T$

Where α is the coefficient of linear thermal expansion, L is the length, and ΔT is the temperature change the metal will actually experience.

Worked example — 40-foot aluminum coping in Phoenix:

A dark-finished coping on a west parapet reaches roughly 170 °F on a July afternoon and drops to about 40 °F on a January night, a ΔT of 130 °F.

$\Delta L = 0.0000129 \times (40\text{ ft} \times 12\text{ in/ft}) \times 130\text{ °F} = 0.0000129 \times 480 \times 130 = \mathbf{0.80\text{ inches}}$

Four-fifths of an inch of movement in a single 40-foot run. If both ends are pinned, that movement has nowhere to go: the coping buckles, the joints tear open, and the sealant fails within a season. Note also that aluminum moves nearly twice as much as galvanized steel for the same run and temperature swing, which is why aluminum details need closer joint spacing than steel details.

Design Rules That Follow

1. Fix one point, float the rest. Every continuous metal run gets a designed fixed point, usually at its center or at a corner, with sliding engagement everywhere else.
2. Space the joints. Follow the SMACNA Architectural Sheet Metal Manual tables for the metal in question. In practice, aluminum and zinc need markedly closer joint spacing than copper and steel because their coefficients are higher, and manufacturers of edge systems publish their own maximum lengths.
3. Build the joint with a gap. A slip or butt joint needs an actual air gap — commonly about 1/2 inch — with a backing plate, a hemmed cover plate, and a sealant bead over a backer rod so the sealant works in extension rather than being pinched.
4. Every change of direction is a movement point. Corners restrain movement in two axes; provide a joint near each corner rather than running a single length around it.
5. Continuous gutters need joints too. A 200-foot gutter without expansion joints will tear at its hangers or split at its outlets.

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5. Roof Expansion Joints

A roof expansion joint accommodates structural movement. Its location is dictated by the building, not by the roofer, and it must be placed:

• Wherever a structural expansion joint occurs in the building frame — the roof joint must align with it
• Where steel framing, structural steel or deck direction changes
• At the junctions of L-, U- and T-shaped building wings
• Where the deck material changes — steel deck meeting concrete, for example
• Where an addition joins an existing structure
• Where interior conditioning differs sharply on either side of a demising wall
• Where differential movement between a wall and the roof deck is expected

Construction is a raised double curb, with the top of the curb a minimum of about 8 inches above the finished roof surface — the same height rule as base flashing. Membrane and base flashing run up each curb and terminate; a prefabricated joint cover with a flexible elastomeric bellows spans between them, fastened on one side and allowed to float on the other. Cant strips are used at the curb-to-deck angle for bituminous systems.

Two placement errors are chronic and both are testable:

• Never route an expansion joint into a drain sump or across a drain. The joint becomes a dam and the drain becomes unreachable.
• Never let the joint block drainage. The curb runs uphill to the perimeter, or drainage is provided on both sides with independent drains or scuppers. A joint laid across the flow path creates permanent ponding on the upslope side.

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6. Area Dividers

An area divider looks like an expansion joint but does a different job. It is a nonstructural raised double-curb assembly — typically two wood members on a wood plate mounted to the deck, flashed on both sides — installed to break a large roof field into smaller sections. Area dividers:

• Relieve accumulated thermal stress in adhered membranes over long runs
• Provide a clean termination for phased work and for tie-ins between roofing campaigns
• Separate roof areas with different assemblies, warranties or ownership

They are commonly used on large adhered low-slope roofs at intervals on the order of 150 to 200 feet, following the membrane manufacturer's requirement, and they are placed on the high side of the drainage pattern or elevated on blocking so water can pass beneath. They are not a substitute for a true expansion joint: an area divider cannot accommodate structural movement, and installing one where a structural joint is required will tear the membrane at the curb.

Fabrication and Movement Checklist

• Correct gauge table used for the specified metal (galvanized, stainless, copper by weight, aluminum by decimal)
• ES-1 minimum thicknesses verified for edge metal against design wind pressure
• All exposed edges hemmed; continuous cleat one gauge heavier than the metal it holds
• No face fasteners through metal that must move
• Solder used only on solderable metals; aluminum and Galvalume joints locked, riveted and sealed
• Acid flux residue neutralized and rinsed after soldering
• ΔL computed for each continuous run; fixed point designated and joints spaced accordingly
• Slip joints built with a real gap, backing plate, hemmed cover and sealant over backer rod
• Roof expansion joints aligned with structural joints and never crossing a drainage path or sump
• Area dividers placed per membrane manufacturer spacing and never used in place of a structural joint