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Repairing the Slate Roof on Your Historic House | Wisconsin Historical Society

Guide or Instruction

Repairing the Slate Roof on Your Historic House

Repairing the Slate Roof on Your Historic House | Wisconsin Historical Society
EnlargeSlate roof

Milwaukee County. Slate roofs (as seen in this photo) are not only a character-defining feature of your house, but also have life spans of up to 100 years. Source: WHS - State Historic Preservation Office.

If your historic house has a slate roof, you have one of the most beautiful and long-lasting roofs in Wisconsin. However, your slate roof will not reach its long-term life expectancy if it has damage that is not addressed promptly. Slate roofs can suffer damage from storms, fallen tree limbs or poor-quality flashing.

You should hire an experienced professional to make repairs to your slate roof. When you consult with a slate roof professional, you will be able to make better decisions if you understand how your roof was installed and what types of repairs are possible.

Slate Roof Longevity

Most slate roofs in Wisconsin were installed on new houses built from the late 1800s through about 1920. Slate roofs can last between 60 and 100 years in Wisconsin. Therefore, most slate roofs have reached or are reaching the end of their functional life. The longevity of your slate roof will depend on:

  • The quality of your slate.
  • How your slates were fastened to the roof.
  • How steep your roof is.
  • How well your roof was originally flashed.
  • Whether your roof has been maintained over the years.

Slates are damaged from chemical exposure and environmental conditions. Slates absorb more moisture as they age, which causes them to swell and begin to delaminate with pieces of slate flaking off.

Slate Roof Repair and Replacement

You must decide whether to replace individual slates or your entire slate roof. You can assess the condition of your slate in two ways:

  • Thump your slate with your finger. If you hear a crisp sound, your slate is in good condition. If you hear a dull sound, your slate is likely at the end of its life.
  • Look in your gutters for flakes of slate. When your slate begins to flake, the rain will wash small bits of slate into your gutter. If you have noticeable pieces of slate in your gutter, your roof is nearing its life expectancy.

If less than 20% of your slate roof is damaged or worn out, you can hire a professional to replace individual slates. New slates are installed in two ways:

  • Fastening the new slate in place by a nail with copper flashing installed over it
  • Hanging the new slate in place with copper straps hooked under the butt edge of the slate

If 20% or more of your slate roof is damaged or worn, you will have to replace your entire roof and all the flashings. Your slate roof is a character-defining feature of your historic house, so your best choice is to replace your damaged slate roof with a new slate roof. However, if your original roof was not slate, you should not install a slate roof on your house.

New Slate Selection

Slate comes in various colors, textures and levels of durability based on the location where it is mined. The slate used for slate roofs in the United States is mined along the eastern seaboard around the Appalachian Mountains. When you choose new slate for your roof, be sure to buy American-mined slate. Although slate imported from other areas of the world can be excellent quality, it will not match the color and texture of your original slate. In most cases, American slate shingles are still manufactured by hand.

Slate Installation

When your roofer is installing a slate roof on your historic house, you should make certain your roofer follows these installation practices:

  • Use copper or stainless steel nails. Your roofer should install slate with copper or stainless steel nails. The original nails used on your slate roof could be failing. Cut or square nails used in the late 1800s rust and fail the most. Slates should have been installed using nonferrous metal nails, like copper.
  • Install appropriate flashing systems. Most slate roofs were installed with copper or lead-coated copper flashings. Your roofer should use only one type of flashing metal. Differing metals that come in contact can create galvanic action — an electric current that will erode the weaker of the two metals. Make certain your roofer uses copper sheets that are not less than 16 ounces. A 16-ounce copper sheet is a one-square-foot sheet of copper that weighs 16 ounces. The higher the number of ounces, the thicker the copper will be. In valleys and other high-water areas, your roofer should use 20- or 24-ounce copper. Your roofer should also install rubber membranes at all eaves and in all valleys, and copper W-valley flashing over the rubber membrane.