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There is a wide variety of roofing materials available, and these vary in appearance, healthfulness, weight, life expectancy, and cost. The healthiest roofing materials are metal, slate, and concrete or clay tile, because they produce little or no outgassing.
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Some types of roofing are best installed over a solid roof deck. This is called solid sheathing. Most such decks are made of plywood or oriented-strand board, both of which contain a phenol-formaldehyde glue. A solid deck can also be made from 1x8 boards to avoid products with formaldehyde, but this will be more expensive.
Today most wood and slate shingles, as well as concrete and clay tiles, are installed over a solid deck, but in the past they were generally attached to boards spaced several inches apart. This is called “skip sheathing.” Skip sheathing enables wood shingles to last longer since they are able to dry out more easily than when they’re placed over a solid deck. The faster they dry, the less prone they are to rot. Sheet metal roofs are sometimes also attached to a solid deck, but more often they are mounted to horizontal purlins of either metal or wood. Wood 2x4s spaced 24" or more apart from the eave to the peak are often used as purlins.
Composition Shingles
Composition shingles are the most common residential roofing material on the market today. In the past they were made with asphalt-saturated felt and were called “asphalt shingles.” Today, most manufacturers have replaced the felt with a sturdier asphalt-saturated fiberglass mat, so composition shingles are also called “fiberglass shingles.”
Composition shingles are produced in a variety of different colors, are reasonably priced, and are easy to install. They will last from 15 to 30 years. How long they actually do last depends on several factors. Fiberglass lasts longer than felt. Heavier-weight or thicker shingles will last longer than thin ones. Dark-colored shingles can get hotter since they absorb more heat from the sun than light-colored ones; this excessive heat can shorten the shingle’s life. Most composition shingles are warranted to last a certain number of years. The longest-lived shingles can cost up to twice as much as those with shorter lives.
All composition shingles contain asphalt and when heated up by the sun can emit some asphalt odors. The amount of odor released is fairly small and rarely even bothers chemically sensitive people, but it is considered a pollutant. If a roof is well-separated from the living space, then the slight smell of asphalt should not reach the occupants. However, in a house with dormers overlooking a roof, as in a Cape Cod design, the odor could easily enter an open dormer window.
Wood Shingles
Wood is also a popular choice for roofs, especially on higher-priced homes. Wood shingles are available in two different styles: smooth sawn or roughly split. Split shingles have more texture and are usually called “shakes.” These products can last as long as 50 years, but if improperly installed can fail within 10 years. The best method of installing them is in conjunction with the skip sheathing mentioned earlier. While this can entail more labor than a solid plywood deck, it is a worthwhile investment. Most roofers are familiar with these techniques. Wood shingles will cost several times more than composition shingles.
By far, most of the wood shingles on the market are made of cedar. Redwood shingles are also available and pine shingles chemically treated against mold and rot have been introduced in recent years. Since wood burns easily, a wood roof can mean higher home insurance premiums. Some wood roofs are chemically treated to render them fire resistant.
Cedar shingles can be quite odorous. Moths are believed to avoid the natural aroma of cedar-lined clothes closets, however there is no evidence that cedar will actually repel moths. Many chemically sensitive people are bothered by the cedar odor. Avoiding certain chemical treatments may also be prudent because rainwater can leach the chemicals out of the wood and direct them down the gutters and downspouts into a yard or garden. Redwood shingles have a milder odor than cedar. Although not widely available, they have the same attractive look as other wood roofing products.
Slate Shingles and Clay Tiles
Slate shingles and clay or concrete roof tiles are non-toxic forms of roofing because they do not outgas, but they are quite heavy. The extra weight requires a stronger, costlier roof structure. These materials are sometimes expensive, but they are also extremely long-lasting. A slate roof can last well over 100 years, so the cost over the life of a house can be fairly reasonable. Synthetic slate is made of cement and wood fibers and is about half the cost of genuine slate. Synthetic slate can last for 50 years and is just as inert. All of these materials are often recommended for healthy houses, but their high initial cost usually restricts their use to more expensive homes.
Metal Roofing
Metal roofing has received an undeserved bad name over the years. This might be because of the cheap metal roofing seen in deteriorating condition on old barns. High-quality metal roofing has been common in Europe for some time and is now starting to catch on in our country.
Long-lasting, attractive metal roofing is available across the U.S. in the form of individual shingles or in large sheets.
Metal roofing is manufactured from steel, aluminum, copper, and even stainless steel. It is produced with a variety of durable baked-on finishes that are quite inert. The factory baking process hastens the curing of the paint so that the finished product produces virtually no outgassing. The life of a metal roof can be anywhere from 15 years for a bottom-of-the-line product to 100 years for a copper or stainless steel roof. Most metal roofing will last at least as long as a composition roof. Since stainless steel and copper are the longest-lasting, they are also the most expensive.
A painted metal roof can be periodically repainted to prolong its life, but repainting shouldn’t be necessary until a roof is twenty or thirty years old. Since a roof is exposed to a great deal of abuse from weather and ultraviolet light, it must be painted with a long-lasting, durable paint specifically designed for the purpose.
Metal Shingles
Individual metal shingles are made to resemble clay tiles, wood shakes, as well as embossed nineteenth-century styles. The wood shake pattern is the most popular. Metal shingles must generally be applied over a plywood roof deck, but it is possible to substitute solid wood 1x8s for plywood. Most metal shingles are made of steel with a baked-on finish, so they produce little outgassing.
Sheet Metal Roofing
Besides individual shingles, metal roofing is also sold in long sheets that are up to three feet wide. These panels can be obtained in custom-cut lengths that run in one piece from the ridge to the eave, giving the roof very simple and attractive lines. Sheet metal panels can be sealed to each other for watertight installation with standing seams or with lap joints.
Standing-seam roofing is a product that is attached with clips that are hidden once the roof is in place. The individual pieces fit together in vertical “standing seams” that shed water. Because of the hidden fasteners, this type of sheet metal roofing has a very uncluttered look. Many well-designed buildings are now using standing seam roofs because of their attractive appearance.
Sheet metal roofing having lap joints between the sheets is a lower cost product because it is generally lighter in weight and easier to install. It is often found on commercial and agricultural buildings, but it is now finding a niche in the residential market. The edges of the long panels lap over each other, and the fasteners (screws or nails) are visible. The cost of this type of metal roofing can be competitive with composition shingles.
(This article is from the archives of the original Healthy House Institute, and the information was believed accurate at the time of writing.)
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