Replacing the Flat Roof on Your Historic Building | Wisconsin Historical Society

General Information

Replacing the Flat Roof on Your Historic Building

Replacing the Flat Roof on Your Historic Building | Wisconsin Historical Society

If your historic house has a flat roof, you will have some unique considerations to address if your roof needs to be replaced. Since this work is complex, you should hire a professional roofer to replace your historic flat roof.

The first thing you should know about your flat roof is that it is not really flat. Even roofs that look flat have some pitch for water run-off. A more accurate term for your roof is “low-slope roof.” You should inspect your low-slope roof once a year to make sure the drains are clear and operating effectively.

Low-Slope Roof Design

Low-slope roofs typically appear on the top portion of Mansard style roofs, porch roofs, bay windows and historic structures with parapet walls. Mansard, porch and bay roofs usually have a low-pitched, hip roof design. Parapet walls are generally used on commercial buildings, but a few appear on the rear additions of historic houses.

Some low-pitched roofs were originally covered with a soldered flat-seam metal roof. After 1915, hot tarred roofing was used on some house roofs as well as asphalt-based rolled roofing materials. Low-pitched metal roofs were often covered with tar or a rolled roofing material. If your low-pitched metal roof was tarred or covered with rolled roofing, it will have tongue and grooved (T&G) 1 x 6-inch or 1 x 8-inch decking boards.

Alternative to a New Roof

If the cost of a new low-slope metal roof is beyond your budget, you may want to opt for the best alternative today: a synthetic rubber membrane called EPDM (ethylene propylene diene monomer). EPDM has a 40-year track record on low-slope roofs in Wisconsin. The material can be made in very large sheets, so your roof will have very few seams. When two pieces of EPDM need to be joined, a special tape and adhesive bonds the two sheets together.

Typical EPDM membrane thicknesses are 40 and 60 mil. A “mil” is one-thousandth of an inch in thickness. Some manufacturers are now offering a 90-mil membrane that is even more puncture-resistant than the tough 40-mil and 60-mil products. You should use 60-mil EPDM for your house, because the heavier 90-mil EPDM will be more difficult to install.

Since heating is an important issue in Wisconsin, choose black EPDM instead of white. Black will absorb heat and white will reflect heat away from your roof.

Working with Your Roofing Contractor

Here are some additional tips for working with a professional roofer to replace your low-slope roof:

  • If your low-slope roof has two or more layers of tar and rolled roofing, you should insist that your roofer remove these layers down to the wood decking. These layers place excess weight on your roof.
  • Your roofer will need to add ½-inch exterior-grade plywood decking over your old T&G decking. The plywood should not be pressure-treated (because the glue won't adhere to it properly) and must have one smooth face on which to glue the EPDM.
  • Before laying the plywood decking, your roofer should repair the old T&G decking and nail it tightly to the rafters below with ring shank, hot-dipped galvanized, #5 box nails. The nails should be placed every 8 to 9 inches so they are flush, not below, the smooth surface of the plywood.
  • Your roofer should apply residential EPDM with a “total glue down.” A total glue down means the approved adhesive is applied to both the EPDM membrane and the deck. For the best adhesion, your roofer should use an approved oil-based adhesive over a water-based adhesive.
  • Some roofers will want to mechanically attach the EPDM membrane around the perimeter of the roof, leaving it lying loosely over most of the roof and on top of foam insulation board. You should avoid this practice because this type of installation is prone to wind damage, wrinkling and stretching from contraction and expansion.

The information presented here is not intended to provide comprehensive technical advice or instructions on solving historic preservation issues. Any information contained or referenced is meant to provide a basic understanding of historic preservation practices. Read full disclaimer.