Installing a bathroom vent helps to prevent mildew, peeling paint and sweaty water pipes by reducing the humidity levels in your home. Although installing the bathroom fan itself is simple, many DIY homeowners shy away from the prospect of messing with the roof. Although you may require professional help if you wish to install a roof cap on a tiled roof, installing a roof vent on a shingle roof is not a difficult process, provided you're comfortable with heights.
Climb up into the attic after you've installed the bathroom fan in the ceiling. Examine the inside of roof directly above the bathroom fan vent outlet. Drive a long nail right through the plywood sheathing with a hammer, midway between two rafters or roof trusses at the closest point possible above the extractor fan outlet to minimise the length of flexible duct tubing you'll need to install later.
Place a sturdy ladder against the house. Climb on the roof to ensure that the nail penetrating the shingles is at least 2 feet from any roof fittings such as vents, chimneys or valley ridges. Remove the nail and adjust the position if necessary. If you do so, patch the nail hole with a blob of bitumen roofing cement and sprinkle some roofing gravel gathered from the gutter over the patch.
Measure the diameter of the tubular roof cap vent tube with a steel tape measure. Tie some string around a piece of chalk and loop the other end around the nail. Draw a circle on the shingles slightly larger than the diameter of the roof cap vent.
Cut this circle out right through the shingles and plywood sheathing with a reciprocating power saw. Grasp the nail and pull the plug out.
Insert the roof cap into the hole with its outlet pointing down toward the eaves. Mark the outline of the upper half of the roof cap flashing on the shingles with the piece of chalk, and remove the roof cap.
Lift the shingle directly above the roof cap to reveal the roofing nail attaching the lower shingle to the roof. Pull this nail carefully out by first prying it loose from under the shingle with a pry bar. Remove any nails from under the next row of shingles that are inside the marked flashing square on the right and left of the hole.
Measure and mark the outline of the square roof cap on the two shingles on either side of the roof vent above the centre line with a tape measure, straight edge and chalk. Cut this square out of the shingles with a utility knife.
Ease the upper edge of the roof cap flashing under the shingles on either side of the hole above the centre line. Slide the flashing upward under the shingles while you insert the roof vent tube through the hole in the roof. This will allow the upper two rows of shingles to overlap the flashing and fit snugly around the square roof cap, while the bottom half of the flashing overlaps the lower shingles to form a watertight transition.
Lift the upper row of shingles. Secure the roof cap to the roof by hammering three roofing nails through the flashing and into the plywood sheathing an inch from the top edge of the flashing. Repeat by hammering in three more evenly spaced nails through the lower edge of the flashing.
Run a bead of outdoor-grade silicone caulking compound around the exposed edges of the roof cap flashing. Seal the heads of the exposed nails with a dime-size dab of compound. Dip your finger in water and smooth down the edges of the compound to ensure that you have a watertight seal.
Return to the attic above the bathroom fan. Connect the roof cap vent tube to the bathroom vent tube with a suitable length of flexible duct tubing. Secure the tubing to the roof cap and bathroom fan by wrapping overlapping layers of duct tape around both joints to compete the installation.