Flat-Lock Metal Roofs

What used to be called a “tin roof” is actually made from terne-coated steel. These days, this material is used mostly on restoration projects. These roofs have a series of rectangular metal panels, or pans, folded together along the edges. The seams must be beaten flat with a mallet, and then hand-soldered.


Slope

Flat-lock roofing is called for on any roof with a pitch less than 3:12. On steeper roofs, standing-seam metal should be used.


Flat-Lock Metals

Terneplate. This is the most common and inexpensive material used for flat-lock metal roofs. Originally, terne metal was a copper-bearing steel coated on both sides with an alloy of 80% lead and 20% tin. Since lead has been outlawed, a second-generation coated steel, using a 50% zinc, 50% tin alloy coating, is typically used instead but may be very difficult to source these days.

Unfinished terneplate must be painted on the underside before installation, and on the top side after installation. Use a rapid-dry water-based acrylic formulated specifically for zinc-coated steel. Typically, roofs should be repainted every 10 or 12 years.

Prefinished terneplate cannot be soldered; it is used for field-formed standing seam, not for flat-lock roofs.

TCS (terne-coated stainless steel) is a traditional material that doesn’t need to be painted to prevent the base metal from corroding. However, it has the same silver color as unfinished terneplate and may be painted for appearance.

Copper is expensive but looks great. Be forewarned that the runoff stains the house. Copper also expands and contracts more than steel, so it should be used only on short roof runs. Lead-coated copper used to be a popular choice but is no longer widely available; existing roofs are often painted for appearance with a slow-drying linseed-oil-based paint.


Flat-Lock Installation Checklist


Prepaint the underside of flat-lock metal panels, including all cleats, flashing, and drip-edge stock using a rapid-dry acrylic paint. Never use an aluminized paint, which will promote galvanic corrosion.

If using coated-steel, repaint the underside of the panels, including all cleats, flashing, and drip-edge stock using a rapid-dry acrylic paint. Never use an aluminized paint, which will promote galvanic corrosion.

  • Lay roofing panels over a wood deck, preferably plywood. Do not use treated wood; the salts will attack the metal coating.
  • Install metal panels over a rosin-paper slip sheet. Do not use asphalt-impregnated felt; the asphalt will attack the metal coating.
  • Form pans from 20x28-in. sheets, using a metal brake (see pattern below).
Figure: Flat-Lock Panel Pattern
Cut flat-lock metal roof panels from 20x28-in. panels (for terneplate) or 18x24-in. (for copper), and then bend as shown.
Cut flat-lock metal roof panels from 20x28-in. panels (for terneplate) or 18x24-in. (for copper), and then bend as shown.

  • Panel edges along rakes and eaves lock over a drip-edge flashing (follow as shown in figure, Standing-Seam Edge Detail, below). Lock panels to L-flashing bent from flat-stock metal at vertical walls.
  • Stagger panels, and secure to roof deck, as shown in the figure below.
Figure: Flat-Lock Panel Layout
A typical layout shows how the staggered pans cover the roof. Uphill pans should always overlap the edges of lower pans.
A typical layout shows how the staggered pans cover the roof. Uphill pans should always overlap the edges of lower pans.

  • Use 2x4-in. prepainted cleats and two 1-in.-long roofing nails to secure panels. Fold over flap on cleat to cover fastener heads.
  • Fold seams closed, crimp tightly, and beat down with a mallet and wood block. Seams should be single locked a minimum of 1/2 inch.
  • Solder seams with a 50/50 solder and rosin flux, using a 3-pound minimum soldering iron. Be sure to wipe off all excess flux.
  • Paint soldered roof with a minimum two coats of the same rapid-dry acrylic paint used to prepaint the underside of the panels.
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