How to Build a Vapor Barrier for a Metal Roof


A vapor barrier is used to block moisture diffusion through concrete or ceilings. Vapor barriers are usually foil or plastic sheets laid out over a structure as a protective lining. The vapor barrier on a roof structure prevents water vapor from rising up from the house and rotting the roof structure. All types of roofs need a vapor barrier, even a metal roof. A vapor barrier serves as a type of insulation against harsh weather extremes and can preserve the life of your metal roof.

Things You'll Need

  • Fiberglass insulation with a vinyl face Construction scissors Tape measure Slap stapler 3/8-inch staples 2-by-4 boards 16-penny nails 1/2-inch roofing plywood 30-pound roofing felt
  • Measure and cut 3-inch fiberglass insulation with a vinyl face. You can buy this product at most hardware and roofing stores. The rolls come 6 feet wide.

  • Overlap the seams when placing the sheets on the roofing plywood, and staple them down with 3/8-inch staples and a slap stapler.

  • Nail down 2-by-4 boards from the ridge beam down to the eaves, placed every 16 inches. These are the spacers needed for proper ventilation between the roofing plywood layers. Use 16-penny nails and a hammer. Hammer a nail every 12 inches down each board.

  • Apply another layer of 1/2-inch roofing plywood over the spacer boards. Nail them to the spacers using 16-penny nails. Cover the entire roof.

  • Cover the second layer of ply with a 30-pound roofing felt. Use a slap stapler and 3/8-inch staples to apply this vapor barrier over the entire roof. You are now ready to lay your metal roof.

Tips & Warnings

  • When securing your insulation and vapor barrier to the roof structure, make sure to apply staples in enough areas scattered around the surfaces to keep them down in case of high winds.
  • The insulation is vitally important if the roof rafters are exposed on the inside of the house, such as with cathedral ceilings and gambrel roofs. Vapor barriers must be installed to preserve the life of the metal roof.

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