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Wood-Shingled Roofs and Your Historic House | Wisconsin Historical Society

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Wood-Shingled Roofs and Your Historic House

Wood-Shingled Roofs and Your Historic House | Wisconsin Historical Society

There is a good chance that your historic house originally had a wood-shingled roof. Most houses built in Wisconsin before 1920 had wood-shingled roofs. The most common shingle wood was northern white cedar, logged right in Wisconsin. After 1915, asphalt roof shingles were readily available in many shapes and sizes. Fires from coal furnaces, wood-burning furnaces and stoves caused a lot of wood-shingle roof fires, so fire marshals were delighted with the more fire-resistant asphalt shingles. As a result, most original cedar roofs were covered with asphalt shingles after 1920.

Since coal was discontinued as a heating fuel, the risk of a roof fire has dropped to almost zero. If your historic house originally had a wood-shingled roof, you might consider installing a new cedar-shingled roof. This work requires some advanced skills, so you should hire a professional roofer with experience installing wood-shingled roofs. If you already have a wood-shingled roof and you just need to replace individual shingles, you may want to take that on as a do-it-yourself project.

Determine Whether Your House had Wood Shingles

A good way to determine if your house originally had cedar shingles is to look in your attic. All roof rafters were covered with some type of sheathing. Historic houses with wooden roofs had a sheathing of wood shingles laid over skipped decking. These 1- x 4-inch, 1- x 6-inch, or 1- x 8-inch boards were laid across the rafters and parallel with the eave. A gap of 1 to 3 inches was maintained between each board. These gaps allowed air in the attic to dry the shingles from the inside as well as the outside. Often you can see the original wood shingles in these gaps. Even if you can't see shingles in the gaps, you can assume your original roof was wood shingles if you see skipped decking.

Ignore Common Myths about Wood Shingles

Because of the historical risk of fire with wood shingles, certain myths about them persist.

You should ignore myths about wood shingles such as these:

Myth: "Cedar-shingled roofs do not meet the current International Building Codes."
Fact: Cedar shingles are not only legal; they are readily available on the market for residential homes.

Myth: "A wood-shingled roof cannot be insured."
Fact: Although you might need to pay a slightly higher insurance rate if you install a wood-shingled roof, the slightly higher rate is due to the higher material replacement cost of a wood-shingled roof over an asphalt-based shingle roof, not because of a potential fire hazard.

Choose the Appropriate Type of Wood Shingle

Cedar is highly rot-resistant and available throughout Wisconsin. If cedar shingles are installed properly, the shingles laid on a 6-12 pitched roof or steeper can last 50 years. Cedar shingles laid on very steep-pitched roofs with a 12-12 pitch or higher can last up to 75 years. (The pitch or steepness of your roof is determined by how much your roof rises within 12 vertical inches. Roof pitches are stated as 3-12, 6-12, 8-12, 12-12 etc. For example, a 4-12 roof pitch means that for every 12 inches measured horizontally, the vertical rise would be 4 inches.)

When you shop for cedar shingles, you will find several options. Use the following guidelines to help you choose appropriate cedar shingles for your roof: 

  • Grade. Premium roof-grade cedar shingles are made of straight-grained heartwood. There are two types of premium roof-grade cedar shingles: Western red cedar shingles are called “#1 Blue Label” and white cedar shingles are called “A-grade.” A square is a 10-foot by 10-foot section of your roof, or 100 square feet. Four bundles of premium cedar shingles make a square and cost about 50% more than an architectural-grade asphalt shingle.
  • Length. Cedar shingles are cut in random widths but uniform lengths and thickness. The standard cedar shingle lengths used on historic Wisconsin houses varied. The most common lengths were 16, 18, and 24 inches, but they could have been as long as 36 inches. The thicker split and rough-faced cedar shakes never would have been laid on historic house roofs in Wisconsin with the exception of hand-split shakes placed on some early cabins.
  • Reveal. Historic cedar roofs in Wisconsin have a reveal of 5 to 7 ½ inches. The ratio of shingle length to reveal is critical to the longevity of a roof. Your wood-shingle roof should have full double coverage, which means that the reveal you use should be no more than one-third the shingle length. For example, if the reveal on your roof is 5 inches, your shingles are probably 16 inches long. If the reveal is 7 ½ inches, your shingles are probably 24 inches long. If the one-third rule was not followed on your house, you should not repeat the mistake. However, you should match the historic reveal of your roof and increase your shingle length to match the one-third maximum exposure rule.  

Use Good Installation Practices

When your roofer is installing a cedar-shingle roof on your historic house, you should make certain your roofer uses these two installation practices:

  • Add a solid layer of sheathing. Prior to 1920, cedar-shingled residential roofs were installed on top of spaced decking boards. The 2009 International Building Code, which is the building code that currently applies in Wisconsin, requires a cedar-shingle roof to be installed on top of a solid sheathing layer. The solid layer is necessary because the average daily temperature in January is below 25 degrees Fahrenheit. Therefore, you should make certain your roofer replaces your spaced decking boards with a solid layer. If you intend to finish the attic space under your roof, you can add plywood sheathing over the original spaced decking.
  • Use corrosion-resistant fasteners. If you choose pressure-treated or fire-treated shingles, make certain your roofer fastens them with either hot-dipped, galvanized nails or stainless steel nails to avoid corrosion. Corroded nails will cause shingles to fall off your roof. Your roofer will require about two to four hours to hand-nail one square (100 square feet) of wood shingles in place.

Provide Adequate Ventilation

Cedar shingles require ventilation to remain dry, so make sure you provide some level of ventilation between your wood shingles and the solid roof deck. If you fail to provide ventilation under your wood shingle roof, you will significantly accelerate the deterioration of your shingles. Your 50-year roof could become a 20-year roof if the wood shingles cannot dry out properly.

To provide adequate ventilation for your cedar shingles, you should try to maintain a “cold” roof. You maintain a cold roof by preventing heat from escaping through your roof. Every house is different, so you should consult with a roofing professional to choose the best option for your situation. Here are three options for maintaining a cold roof:

  • Add insulation. You can prevent heat from escaping through your roof by installing enough insulation below your shingles that the insulation can serve as a vapor barrier.  
  • Install insulating panels. If you have a vaulted ceiling space that prevents you from putting insulation in your attic, you can install insulating panels. One type of insulating panel is a “cedar breather” layer that your roofer can install between the sheathing layer and the cedar shingles. If you install a cedar breather, you must also properly vent the ridge and eaves.  A second type of insulating panel is a top vent ridged insulation panel. This type of panel has a ventilation space built into it.
  • Apply a thick layer of spray-on insulation to your rafters. Your third option is to apply spray-on insulation to the underside of your rafters. If you spray on this type of insulation in a thick-enough layer, it will act as a vapor lock.