How to Build Roof Dormer Vents to Code

Meeting the Code

• 1

  Measure the length and width of your attic. Multiply the two numbers, which gives you the attic's square footage. Remember that the code requires a minimum of 144 square inches of vent opening, also called the net free vent area, for every 150 square feet of attic space. About half of this should come from roof vents, such as dormer vents. Soffit vents typically make up the other half.

• 2

  Plan to install enough dormers to comply with code. Remember that dormer vents come in standard sizes, such as 100 square inches of net free vent area. Factor this into your calculations.

• 3

  Determine how many dormer vents you need to supply half of the IRC's requirements for your attic and start installing your vents.

Build Vent

• 4

  Select a location for your first vent between two rafters. Make it about 18 inches from the ridge peak. Place the dormers on the back slope of the roof.

• 5

  Measure the length and width of your dormer vent. Mark the dimensions on the attic sheathing. Drive nails at the corners through the plywood and shingles.

• 6

  Go up to your roof and draw an outline of the opening with a felt tip pen. Make it 1/2 inch larger than the actual dimensions of the dormer.

• 7

  Cut off the shingles from the outlined area with a utility knife. Cut out the plywood sheathing with a reciprocating saw.

• 8

  Remove any shingle nails near the opening with a pry bar. Place roofing cement under the shingles at the top of the opening. Apply it over the shingles at the sides and bottom of the opening.

• 9

  Slide the metal flange at the top of the dormer vent under the shingles. Let the metal flanges at the sides and at the bottom remain on top of the shingles.

• 10

  Nail the dormer to the roof with two roofing nails at each corner or as indicated by the manufacturer. Apply roofing cement along the edges where the shingles meet the new vent.

• 11

  Install any other dormer vents your roof needs to comply with the IRC as you did with the first one.