Design Roofing to Optimize Durability and Function
Roof coverings provide a first line of defense against the elements. They also tend to be the most exposed component of a building’s exterior envelope. Therefore, roof coverings should be selected, detailed, and installed to provide durable resistance to water penetration. Wind, urban wild-land fire, and hail resistance are important roof system considerations in some regions (see Chapter 8).
The design considerations in this section are intended to enhance or help fulfill the objectives for a roof installation as found in the 2015 International Residential Code (IRC) which states:
R903.1 General. Roof decks shall be covered with approved roof coverings secured to the building or structure ….Roof assemblies shall be designed and installed in accordance with this code and the approved manufacturer’s installation instructions such that the roof assembly shall serve to protect the building or structure.
Building codes don’t address many of the details required for a complete and proper installation of the many available roofing products. Therefore, the statement regarding “in accordance with … manufacturer’s installation instructions” should not be taken lightly! Roofing industry guidelines, as provided in Section 4.4, are also important resources to ensure a durable and effective roofing installation.
Service Life—There are a variety of roofing materials with a wide range of estimated service life as shown in Table 4–2. Metal, concrete or clay tile, and slate roof coverings tend to provide the greatest durability as measured by estimated service life. But, they also represent the more expensive roof covering choices. Thus, more than three-quarters of all homes use composition roof shingles. The estimated service life varies significantly even within a given roof covering type. Differences in manufacturer warranties may be considered as one means of assessing expected service life. However, warranties and service life estimates must be taken with a grain of salt because of all the uncertainties which may affect actual installed performance.
Roof Pitch—As shown in Table 4–2, roof pitch is an important factor in the selection of a roof covering type. Steep-slope roof systems are designed for installation on slopes greater than 3:12 (14 degrees). Steep-slope roofs are water-shedding, not waterproof. Therefore, roof pitch is limited in accordance with Table 4–2 for various steep slope roofing products. To prevent water leaks, these roof systems rely on fast drainage, adequate overlapping of elements, and use of underlayment as a back-up layer of protection. While lower roof pitches are possible in some cases with doubling underlayment, low roof pitches are more prone to water intrusion and underlayments commonly used are not waterproof due to fastener penetrations or possible damage during roofing installation. Using enhanced roofing underlayment materials and practices (as discussed in Chapter 9) can help prevent water intrusion during extreme wind-driven rain events. They also offer improved protection against wetting of building materials during construction when the building is temporarily “dried-in” with roof underlayment until installation of the final roof covering. Examples of enhanced roof underlayments include mechanically fastened synthetic membranes, adhesively attached membranes, and specialty roof sheathing products with sealed joints.
Low-slope roofing is more common in commercial building construction, but does find occasion for use in residential construction. Typical roofing material selections may include “hot-mopped” built-up roofs (BUR) with gravel ballast or synthetic membranes such as EPDM, PVC, and TPO which may be held in place by ballast, adhesion, or mechanical fastening. Low-slope systems are designed as waterproof roof systems, and use roof covering membranes designed for pitches of as low as ¼:12. While low-slope roofs are commonly known as “flat roofs,” a dead flat roof surface is a serious design mistake since water will accumulate or pond on the roof and not drain. It is very important to use proper installation practices and skilled installers for these types of roofing systems because any defect in the installation, such as a faulty seam or joint, is destined to result in a leak which may go undetected until substantial damage has occurred. In addition, as with steep slope roofing, there are wind hazard-related design considerations associated with use of these roofing methods.
