Repair and Maintenance of Historic Roofs

Asphalt shingles and other synthetic materials have become typical for modern pitched roofs, but quite a few older houses still retain their original, character-defining slate, clay tile or wood shingle roofs. The roofing material – whatever the material may be – on an older home is an important architectural feature that should be preserved whenever possible. Most clay, slate and tile roofs can have a life span of over a hundred years when properly maintained.

Slate is a quarried stone that has been used for roofs in the United States since the 17th Century, with quarrying and manufacturing reaching a high at the turn of the 20th Century. Many architectural styles use slate as the roofing material, most notably the Second Empire, Gothic revival and Tudor revival styles.

Slate is cut in an array of textures, colors, shapes and sizes. Slate roof patterns are often decorative, vary widely, and are a defining feature of a building. A slate roof, properly maintained, can be expected to last 60-125 years. Some slate has been known to last over 200 years. Often the fastener elements break down before the slate meaning nails and pins may need to be replaced before the slate tile.

Slate is susceptible to weather and elements which can cause chipping and scaling. Deterioration of slates often happens in the break-down of the mineral deposits which initially formed the slate. Natural freeze/thaw cycles can contribute to the delamination of the slates. Signs of this occur on the back as well as the front of the slate, and means that slate cannot be turned over and reused.

Poor installation and fabrication of the tiles can cause additional problems and deterioration. Regular and timely inspection of your slate roof is an important part of your maintenance plan. It is important to not walk on the roof during inspection to prevent damaging the slate.

When performing regular inspection of your roof, be sure to look for evidence of moisture infiltration. Often, moisture is a result of problems with the gutters, flashing or condensation. Moisture in the roof rafters can be a sign of delamination. Old slates will hold water and cause adjacent rafters to become soft. Unsound slates will be brittle and crack under the pressure of your hand. Using your fingertips or knuckles to tap the slate can lend a clue to the condition of a tile. If tapped, the slate will sound dull once it is old. Slate which is in good condition will resonate with a clear, solid sound.

Signs of a failing slate roof include:

  • Missing, loose or broken slates
  • Delaminated slate tiles
  • Slate tiles are brittle to the touch, and can flake apart
  • Exposed sheathing
  • Presence of moss, mildew or mold

If your roof is in need of repair, inspect the slates to determine if they need replacing, or if it is the hardware or the sheathing. Often, the hardware or the sheathing will corrode or rot before the slate begins to delaminate. If the latter is the case, save the slate to be reused during the repair. If the slate is chipping or scaling, assess how much of the roof needs to be replaced. The National Park Service recommends that if less than 20% of the roof is damaged, repairs can be made. If more of the roof is delaminating, the entire roof will need replacement.

While replacing a slate roof can be costly, it is an investment which will last longer than most replacement materials. Purchasing new slates through a building supply company or salvage yard can save money. Replacements, made from fiber glass or architectural cement, are an option if real slate proves to be cost-prohibitive. The historic character of the roof is defined by its materials and pattern as is often the case with slate. Maintaining, repairing or replacing your slate roof will retain an important character defining feature of your home.

Clay tile roofs are often the roofing material of choice for Romanesque, Italianate and revival style architecture, which includes Mission, Spanish and Georgian, and is a character-defining element of a house. The tiles usually last between 50-80 years, sometimes 100 or more, if properly maintained.

Clay tile is distinguished by size, shape, style, color, and texture. The clay tiles either overlap or interlock to create a roof system. The tiles are usually wired or nailed to a structural underlayment, some have mortar. Clay tiles often outlast the attachment and may be preserved by a repair or for use with new attachments.

Signs of clay tile roof problems:

  • Broken, loose or missing tiles
  • Crazed or spalling tiles
  • Brittle to touch or falling apart
  • Exposed sheathing
  • Presence of moss, mildew or mold
  • Interior water damage caused by a leak

Water and moisture problems are the number one cause of deteriorating clay tiles. Ensure proper drainage and working gutter systems to best preserve a tile roof. The break-down of the support structure such as sheathing or roof underlayment can often occur before the tile begins to show signs of stress. Our climate’s freeze/thaw cycle has the greatest impact. Branches and hail can damage tiles. It is important to not walk on the roof during inspection to prevent damaging the clay tiles.

Tiles ought to be replaced with compatible, historically accurate materials. You can save money by getting them through building supply companies or salvage yards. If replacing tiles with original material proves too costly, use new tiles that are made of fiberglass or architectural cement. Should tiles need to be removed to complete a repair, they should be saved and reused if at all possible.

Wood Shingle roofs, like clay tile and slate, have been used for centuries in North America. A wide range of architectural styles were designed with wood shingle roofs, including 17th century colonial, and later 19th century revivals including the Queen Anne, Stick and Shingle styles. Shingles can be made by hand or machine and are expected to last 15-30 years. The installation technique on the roof creates a specific pattern which determines the design of the roof. Accuracy in the replacement of shingles is key to maintaining the historic character and architectural integrity of the home.

Regular inspection of your wood shingle roof can identify potential problems. It is important to not walk on the roof during inspection to prevent damaging the wood shingles. The National Trust for Historic Preservation recommends looking for the following signs of aging:

Thinness from erosion

Split and warping wood

Moss, mildew and mold on the surface

When matching the original wood shingles to new shingles, be sure to replicate the size and shape, staining, installation pattern and decorative features. In some cases the type of wood may not match the original, but be sure to select a high quality species with similar texture. Differences may occur in the fabrication process, treatments or fire retardants, ventilation or nailing method. This is acceptable as long as it is not visually disruptive to the original roof. Highly textured wood surfaces and irregular ends, prefabricated details, or shingles which are too wide should be avoided, unless those features existed as part of the original roof.

The life of a shingle roof can be extended by keeping the roof free of debris and moss or plant material which could introduce moisture. Coatings and treatments can also aid in the preservation of the roof. Fire retardant coatings, water and UV repellant sealants, and fungicides are sometimes applied. It is important to check before combining any of these treatments as they may be toxic when used alone or in combination. Properly maintained downspouts and gutters will help keep water draining off the roof and contribute to the longevity of the roof.

Historic roofing practices should be employed when repairing or replacing a roof. Power tools such as staplers and nail guns can damage or split shingles. It is important to employ a roofer who has knowledge of historic roofing techniques. A preservation architect may need to be consulted as to appropriate historic materials and construction. Using the highest quality materials in conjunction with professional historic installation, regular maintenance and repair will prolong the life of your wood shingle roof.