Roofs are usually made up of the following: roof sheathing, underlayment, roofing material, roof intersections, flashing details and ventilation. Proper installation is important to ensure water is kept on the outside. Maintenance is also required to stop any of these individual components from failing.



Roofs rely on gravity, pitch of roof and friction to direct water down and off your roof. These systems rely on overlapping elements?roofing felts, shingles, tiles and flashing details'to redirect the rainfall. The pitch of the roof provides the gravity and the detailing provides the redirection.

Gravity does not play such a role in low-slope roof applications, where gravity only allows water to accumulate in every low spot on the roof. This is why low-slope roofing applications have to have a perfect waterproofing system, both on the roof and at every penetration.

Roof sheathing is attached to the roof framing, trusses or rafters, and provides the nail base for the other components of the roof system.

Roofing underlayment, often made of building paper or ?felt,? is the first weatherproofing layer of a pitched roof. Underlayment should be installed from the bottom of the pitched roof to the top, such that each upper layer overlaps the lower layer. This method of installation channels the water out and down, away from the wood-panel sheathing below. Ice shield is usually applied along bottom rows of shingles as an extra barrier against ice damming and other winter problems.

The roofing material provides the primary waterproof barrier for the structure. For pitched roofs, almost all roofing materials rely on some form of shingling to provide the weatherproof barrier. Like the underlayment, these roofs are installed from the bottom-up, with successive layers overlapping both vertically and horizontally to shed away water. Low-slope roofs can use many different proprietary and non-proprietary systems to form their waterproof barrier, from single to multiple-ply; adhered, mechanically anchored or ballusted; hot mopped or cold applied (solvent, urethane or epoxy-based).

Flashing works in conjunction with all of these elements to reinforce the waterproofing at roof intersections or penetrations.

The purpose of flashing is to direct the flow of water into the intersection down and away from the interior of the structure to the topside of the roofing material. In every case, the top edge of the flashing passes underneath the underlayment, the upper pieces of flashing pass over the lower pieces, and the lower edge of the flashing always passes over the top of the roofing material. This method ensures that water will never be in contact with the sheathing.

Valley flashing protects the valleys where two roof planes meet. This material is available with a V- or W-shaped profile and is placed over the top of the building felt before the roof's finishing material is installed. Some roofers install a "covered valley", this is where the shingles are continued over the flashing which provides a more pleasing view and extra protection.

Step flashing protects the joints between the roof deck and chimneys or dormers. Step flashing fits to each course of shingles and appears to ?step? up the wall of the chimney. Again, each step sits over top of the preceding lower step, keeping water away from walls and sheathing.

Vent pipe flashing fits over flues or pipes. The shape of vent flashing is typically a cylinder with a wide flange at the base, which is lapped into the shingles as the roofing is installed. These are usually rubber based and rely on friction fit around pipe.

Drip edges are strips of flashing material that run along roof eaves and rakes to prevent water from seeping under the finished roof along its edges. They also are manufactured to allow your fascia to slip into a pre-made groove providing a great seal and finish.

The majority of roof leaks occur in locations where the plane of the roof is interrupted by a ridge, another roof intersecting at an angle, a wall or penetration. Even the simplest of rooflines has dozens of potential leaks sites due to chimneys, ridges, valleys, etc.

Also, remember that both temperature and humidity can cause roofing materials (shingles, wood sheathing, flashing) to expand and contract with seasonal changes. The flashing materials will continue to be leak-proof if they can withstand this movement of the roofing materials. Well engineered and properly installed two-part flashing can handle this movement with no problem. Two-part flashing systems consist of a base flashing?often step-flashing?that is laced into the finished roof material. The base is then covered by another metal flashing piece lapped over it. The second piece should not be fastened to the base, so the two pieces can move against each other independently when the roofing materials shift with seasonal change.

Some tips for preventing roof leaks are; always check the roof flashing and the condition of the shingles whenever you clean your gutters. Look for loose nails and any damage to the seals at the edges of the flashing. Roofing cement can dry out and crumble away, exposing joints to water. Fasten loose nails and cover exposed nail heads with roofing cement. Renew flashing seals by chipping out old caulking and mortar along the edges of the flashing. Recaulk the joints between the roof and the flashing. It's much easier to reseal the flashing than it is to patch a water-stained wall or ceiling.

Roger Frost has many other great articles for homeowners on his web site Do It Yourself Articles located at http://diy.napoleon.cc