Roof Structure

Before you can start any work on a roof, you need to understand how the roof’s elements combine to create a waterproof layer. This varies according to design, age, type of covering, and climate. Some roofs have all the elements shown below; others have only some, but the principles tend to remain the same. Roofs are supported by outside walls, ceiling joists, and interior bearing walls. They are sheathed in layers of plywood, waterproofing, and shingles. The roof’s job is to shed water away from the structure.


The features described here are standard on most residential roofs. Further information on roofing felt, vapor barriers, shingles, and other roof coverings. In order to shed water properly, make sure you overlap these materials in the correct direction. Typically material is installed from the bottom of the roof toward the ridge, so that the lower layer is overlapped by the next layer of the same material.



Most roofs have some kind of shingle or tile covering to provide the waterproofing. Ways of overlapping roofing materials are shown below and here.


These provide attachment points for tiles, and hold down felt. They are evenly spaced to give the correct overlap between rows of tiles. The spacing (gauge) depends on roof pitch, tile type and fastening method, and climate. Roofs to be covered with asphalt shingles do not need battens.

Roof structure A typical roof is constructed from a roof truss, covered with roof sheathing, underlayment, and a roof covering on top. The layers are carefully overlapped to ensure the roof is fully waterproof.

Roofing under-layment

Felt, or another kind of under-layment, is laid below shingles to create a waterproof barrier. It is now required by building regulations, but it may not be present on older buildings. It is laid over the sheathing, which may be made of plywood or another material (see below), in overlapping horizontal strips.


Boarding laid on top of the rafters is known as sheathing. It is required by local codes. Sheathing adds rigidity to the roof frame and provides a nailing surface for fasteners. OSB, particleboard, and plywood are common sheathing materials. Center each sheet of sheathing on a rafter below. If you are planning to install wood shakes or shingles, you may want to space sheathing to provide more ventilation, as it helps to dry out wood roofing materials after wet weather. When installing roof sheathing, remember to only walk where the rafters run underneath to help prevent any problematic bowing from occurring. You may want to make chalk lines along the rafters.


Check your local regulations regarding roofs. There may be requirements for almost every detail, including materials, ventilation, and fasteners. The regulations are much stricter with regard to historic buildings, for which certain styles have to be maintained, and attic conversions, for which stringent safety rules must be followed.


Roof designs can be complicated. In order to shape a roof, hips and ridges combine with the trusses to create the spaces inside the home. The shape of the roof must also shed water effectively. When detailing a roof hip or ridge, it's important to make the connection secure, and to properly install the sheathing, underlayment, and flashing materials so that water does not enter the structure. Shown below are a few common details used to create most rooflines. See this page for more information about detailing roof flashing.


A valley is used if the external walls of the home turn an internal corner. It is designed to direct water down towards its intersection point, and then the water runs down the valley and off the roof.

Deep eaves

Typically, deep eaves should have 12 in (300 mm) or more continuously vented to ensure that sufficient air intake is available. This is required to properly vent the attic.

Outlook overhang

Outlook overhang is shown here on the gable end of the home. It is supported by look-outs, which extend from the adjacent rafter. Although it requires more material. This is a sturdy method.


A hip design can sometimes be found at a corner of the external walls of the home, just like a mountain, a hip allows rainwater and snow to fall off either side of the hip and then slide off of the roof.

Shallow eaves

Typically, shallow eaves have less than 12 in (300 mm) of continuous venting. This is popular among more contemporary designs. You must also vent to ensure proper air circulation in the attic.

Ridgehung overhang

Ridgehung overhang is less material intensive than outlook overhang. In this design the overhang is hung from the ridge at the apex of the roof. The boards are installed flat.

Tuesday, November 4, 2008 13:21 PST