High & Dry: What Homeowners Need to Know About Roofs, Part 1

By
Home Inspector with Inspect It 1st

Along with the foundation, the roof is the most important component of the home. The roof protects the top surface of the home from the effects of the weather, provides structural support to framing members, and helps maintain the thermal blanket (temperature control) of the home.

Roofs provide security and psychological well-being to occupants in addition to physical comfort. Along with food and clothing, roofs embody one of the essential components of our modern, indoor lifestyle.

Although there are many different forms of roofing materials, all roofs have five basic components:

  1. Sheathing - The sheet material such as plywood, wood boards or OSB (oriented strand board) that are attached to the roof rafters to cover the house.
  2. Roof Covering - Shingles, tiles, wood shakes, foam, rolled roofing, etc. These are the materials that protect the roof sheathing. There is often a layer of felt paper placed between the sheathing and the specific roof covering. This felt paper is primarily used to protect the sheathing from the effects of water.
  3. Roof Structure - The rafters and trusses that are constructed to support the sheathing. Roof incline is discussed below.
  4. Drainage - The specific features of the roof design, such as height, slope, materials and layout that affect the ability to shed water from rain and snow.
  5. Flashing - Sheet metal, aluminum, copper or other material installed at various joints, valleys, corners and other locations to control drainage and to prevent water intrusion.
Roof Incline is the final factor to consider in roofing structure. Incline is often called pitch, slope, or angle, and this factor impacts the selection of the roofing materials that may be properly installed.

A roof with little or no slope is called a low slope or flat roof. Common materials used are bituminous such as asphalt and gravel (built-up roofs); rolled roofing; and foam roofing. Low-sloped roofs present the homeowner with many unique issues. Water may pond in certain areas due to lack of slope, and tree debris will accumulate for the same reason. Typically, drainage is provided by roof drains or scuppers. Clogged drains may cause damage inside the building, especially if these protrusions are not properly sealed with flashing.

With low-sloped roofs, it is particularly important to understand how water changes its structure due to temperature. Water expands 700 times in volume from the liquid state to the vapor state due to heat and expands as much as 4 times when it freezes into ice. Moisture penetration into the felt layers of a built-up roof can cause bubbles and ripples, and also may result in delamination, especially on rolled asphalt roofing. Low-sloped roofs also suffer from poor ventilation, which can lead to deterioration of the sheathing material.

Since low-sloped roofs suffer due to lack of slope, architects now call for some slope on all roofs and tend to locate drainage systems on the perimeter of the building (using drains and scuppers) instead of through the building such as that found with non-perimeter roof drains. We have also seen fast deterioration of low-sloped roofs where drainage was installed on only one side of the roof. This is because all water from the furthest points of the roof must travel the entire distance of the roof to reach the drains.

Ultimately, roofs are adversely affected by five major foes:

Sun - Heat and ultraviolet rays from the sun cause roofing materials to dry out, overheat, crack, peel, blister and deteriorate. Oftentimes areas with South, Southwest or West exposure wear out faster than those facing the North or East. This is especially true if the roof is dark in color as darker colors absorb more heat. Interestingly, several recent studies regarding asphalt shingle deterioration have shown roof color to be even more important than attic ventilation as a key factor impacting shingle performance. Assuming all other factors to be equal, the darker the color of the shingle, the faster it deteriorates.

Rain - Rain exists in every part of the United States. Rain, including hail and freezing rain, can directly damage roof coverings. Water intrusion from rain can cause sheathing to rot, mildew or mold to grow and can destroy interior components such as insulation, drywall or sheetrock. Rain can also destroy electrical systems including wiring, receptacles, lighting fixtures and junction boxes.

Wind - Wind direction and speed vary more than you might imagine. Asphalt shingles on a roof can be lifted by one type of wind during one specific storm but not from any other storm, ever. Many clients who have lived in the same home for more than a decade, tell us that their shingles lifted up one time only during one specific storm. A strong wind can lift up shingles, tear them off or drive rain water under the edges of the material. It can easily knock tree branches into the material, causing scraping, puncturing or tearing. Wind also causes an increase in tree debris left on the roof, such as leaves, small branches and seed pods. All of these can impede proper drainage.

Snow/Ice - Depending on exterior temperatures, attic ventilation and insulation, melting snow often refreezes at the overhang areas (the eaves) and can block proper drainage into the gutter. Instead, the water backs up under the shingles and seeps into the interior of the home, often underneath flashings and then through window frames. Oftentimes gutters and downspouts fill with an ice/snow/water mixture, and can be damaged beyond repair or tear off the house.

Moss/Lichen - Wood, built-up and shingle roofs particularly can be damaged by moss and other simple structure plants such as lichen. The root systems serve as pathways for moisture to penetrate the roofing material. On built-up or low-sloped roofs, moss can impede water drainage. Plant structures can also cause nails to rust, causing shingles to loosen and detach.

In our next Newsletter, we will discuss roofing products, ventilation, leaks, maintenance and related issues.

Comments (1)

Allison Stewart
St.Cloud Homes - Saint Cloud, FL
St. Cloud Fl Realtor, Osceola County Real Estate 407-616-9904
Very well done.  Most of us Floridians got new roofs thanks to Charlie, Francis and Jeanne. For a while we were the official Blue tarp state.  But this is great information valuable to both buyers and sellers. Thanks for presenting it in a well written easy to understand format!
Jul 30, 2007 08:05 AM