Restoration Guide: Low-Slope Roofing

Jim Mallery

Editor's Note: This is article 9 of 13 in the Roofs Chapter of the Old House Web's Home Restoration Guide. This guide was developed and edited for old homes from original material in the U.S. Housing and Urban Development (HUD) Rehab Guide.

9. Low-Slope Roofing


Because low-slope roofs shed water more slowly than high-slope roofs, they are more prone to leaks. Besides poor maintenance, several factors can lead to leaks, including:

  • Deficient design or installation
  • Weathering and exposure to ultraviolet light
  • Extreme weather, including snow, hail, extreme wind and torrential rain, all of which might let water rise above the flashing.
  • Structural deficiencies or changes in the structure
  • Excessive thermal expansion and contraction

Tracking Down Leaks

An important thing to remember about leaks, especially on low slopes: the moisture may appear a considerable distance from the actual leak. Inspection for leaks should begin inside the structure, looking for stains or deterioration on the roof deck and structure, ceiling and walls. If there is any sign of water, the roof should be checked by a qualified professional to detail the needed repairs.

Methods and Materials

This section will discuss built-up roof membranes, modified bitumen membranes and (briefly) thermoset/thermoplastic single-ply membranes.

  1. Repairing Built-up Roof (BUR) Membrane

    One thing to know about repairing low-slope roofs, whether it's in your old house or in the restaurant down the street: such work should be left to professionals.

    Whatever the cause of damage, spot repair of BUR involves the same process, which is:

    • Remove debris or contaminants
    • Inspect for water damage to the insulation or decking (depending on design of roof)
    • Repair insulation or decking, if necessary
    • Remove damaged membrane
    • Install new plies of membrane (the same number as removed) in either hot bitumen or cold-applied adhesive.

    There are a number of top coatings to help extend the life of your BUR roof, including some that have pigments that reflect solar radiation to help cool a building in hot climates (the traditional black coating will absorb heat).

    The National Roofing Contractors Association has comprehensive publications on maintenance and repair of such roofs.

    Remember, while spot repairs can be a cheap way to keep a roof living longer, extensive damage probably means the roof should be replaced before water problems develop that endanger the preservation of your building.

  2. New BUR Membrane

    Built-up roofing systems are just that: multiple overlapping layers of roofing felt (with fiberglass or polyester-fabric reinforcement) coated in either asphalt or coal tar pitch. They have been around for over 150 years, and it is not uncommon for a 50-year-old roof still to be in service. BUR systems are durable and fairly easy to maintain and repair.

    The two systems (asphalt and coal tar) have different characteristics and generally are not interchangeable (though asphalt products will be used with flashing of coal tar BURs.

    Coal tar

    Coal tar comes in three viscosities, with Type I the most common. It is for the lowest slopes, ¼" for every 12" (this drops to 1/8" per foot in warm climates). It has "cold-flow" properties, which means it can mend itself at temperatures around 60 degrees, which a roof will warm to even in colder climates.


    Asphalt roofing also comes in different viscosities and softening points. Type I asphalt softens at 135 degrees and is used only on roofs that are virtually flat. The most common asphalt, type III, sometimes called "steep asphalt," has a softening point between 185 degrees and 205 degrees. It can be used on roofs sloped up to 3" per foot.

    Asphalt BURs can be applied in either a hot or cold process. The cold process uses a mix of petroleum distillates, polymers, fibers and fillers; it is applied with brush or sprayer, instead of hand-mopped as with a hot process. The cold process also requires fewer plies and does not generate the undesirable fumes of hot application.

  3. Repairing a Modified Bitumen (MB) Membrane

    If inspection shows leaks, modified bitumen membrane roofs can be repaired in much the same way as BUR systems. Any damaged or wet insulation needs to be removed and replaced, as would deteriorated roof decking. The surface then needs to be primed and a similar roofing material, 8" overlapping the damage, installed according to manufacturer's specifications.

    As with most other systems, spot repair can be cheap and effective; however, if the problems persist, or if damage is widespread, the roof may need to be replaced for the preservation of your structure.

  4. New Modified Bitumen (MB) Membrane

    Modified Bitumen membranes are asphalt or coal tar bitumen that has been modified with various polymers to improve weatherability, flexibility, strength and resistance to flow at high temperature. They usually are combined with fiberglass or polyester and come in 100-square-foot rolls that are installed in multiple layers.

    Depending on the system, MB roofing is applied with heat or a cold-application process. Some come in a peel-and-stick form.

    They can come with various coatings, such as ceramic granule or mineral surface akin to asphalt shingles, which add to the roof's lifetime. Nowadays, MB membranes often are laid over a couple plies of BUR membrane in a hybrid system, combining the tensile strength of BUR with the flexibility of MB roofing.

    MB systems are easier to install than BUR systems, though special care needs to be given to seams. It is easier to track down leaks in MB. They can accommodate some walking, but they are more sensitive to ponding water.

  5. Thermoset and Thermoplastic Single-Ply Membrane

    Products that came into use in the late 1950s, thermoset and thermoplastic membranes are used mostly with large commercial projects, though they can be found in multi-family and townhouse construction. It is unlikely you will find them on your old house.


About the Author

Jim Mallery, a semi-retired journalist and onetime registered contractor, has extensive experience remodeling, repairing, and rebuilding homes.

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