Historic Preservation - Technical Procedures

Minor Repairs To Asphalt Roll-Roofing Or Built-Up Roofing
Procedure code:
Hspg Prepared For Nps - Sero
Thermal And Moisture Protection
Built-Up Asphalt Roofing System
Last Modified:
Minor Repairs To Asphalt Roll-Roofing Or Built-Up Roofing
Last Modified:




    A.   This procedure includes guidance on making minor repairs
         to asphalt and built-up roofing membranes.  Some problems
         include open lap joints, blisters, splits, holes, ridges,
         undulations, wrinkles and cracks.

         1.   Ridges:


              a.   Ridges are firm and do not yield under
                   pressure.  They may result from the substrate
                   being uneven before the felt was laid, or from
                   moisture distorting the substrate.

              b.   Ridging usually occurs over or near joints
                   between boards.  The ridges expand as
                   entrapped moisture vaporizes.  Ridges become a
                   problem when they interfere with drainage or
                   when the roof begins to leak.

         2.   Undulations:

              a.   Unlike ridging, undulations will yield to

              b.   Distortions of felts may result from the way
                   the rolls were stored (flat instead of on
                   end), inadequate pressure applied while
                   laying, poor application of bitumen compound,
                   or if entrapped moisture becomes vaporized.

         3.   Blisters:

              a.   Blisters will yield to pressure.

              b.   Blisters begin with the expansion of trapped
                   air and/or moisture pockets and can result
                   from inadequate pressure during laying.  

              c.   Full Membrane Blistering:  Usually occurs when
                   no separating layer is provided between the
                   asphalt and the substrate.

              d.   Inter-layer Blistering:  

                   a.   When large blisters result from
                        additional air and water drawn in from
                        outside through the top layer of the

                   b.   Not a common problem, but can result if
                        the 2nd layer does not adhere well to the
                        1st layer due to the accumulation of dirt
                        or dust.


                        If a blow develops in the 1st layer, the
                        2nd layer become thin at that point and
                        allow air and moisture to penetrate
                        between the layers.  This can create the
                        potential for a blister to develop.

                   c.   A blister ususally develops under the cap
                        sheet of the membrane.

                   d.   Inter-layer blistering is most common
                        with unsurfaced or mineral surfaced felts
                        on vertical or sloping surfaces.
                        However, high performance membranes can
                        reduce the incidence of this occurring.

         4.   Top Pitting:  


              a.   Miniature surface blistering of the bitumen
                   coating (especially with BS747 Type 1E and 2E
                   mineral-surfaced felts).

              b.   The blisters may range in size from 1mm to

              c.   Top pitting may be caused by:

                   1)   Trapped air and/or moisture in the
                        manufacture of the material.

                   2)   Oliensis:  Incompatibility of the
                        saturating bitumen with the coating
                        bitumen (i.e. oils from the coating
                        separate as an incompatible reaction with
                        the saturant bitumen).

              d.   If numerous across the surface, the coating
                   may separate from the membrane and result in a
                   loss of surfacing.

         5.   Cockling:  

              a.   Rounded ridging usually in line with the
                   length of the material.

              b.   Cockles sometimes develop as a result of
                   thermal expansion (especially with Polyester
                   felts, which are heat sensitive).  The heat
                   from the bonding bitumen during application
                   can create minor wrinkling or cockles which
                   usually disappears as the material settles.

              c.   Most common in fiber and asbestos base felts
                   as they tend to expand when their moisture
                   content increases.

              d.   Cockling is less likely in coated felts
                   especially those with a glass or polyester

         6.   Blowing:

              a.   Blows are bubbles that develop on the surface
                   from gas moving through the asphalt.  The heat
                   of the asphalt during application causes the
                   air to expand and creates steam by raising the
                   temperature of the moisture.

              b.   Blowing is most common in vertical application
                   where there is no separating layer between the
                   asphalt and the substrate.

    B.   Problems with mineral-surfaced, asphalt roll roofing and
         built-up roofing membranes are usually related to sun
         exposure, and excessive moisture.  Exposure to the sun
         may cause the asphalt to dry out, resulting in cracking
         of the surface, separation of plies, and may eventually
         require replacement of the entire membrane.

    C.   The repairs described in this procedure are ONLY
         temporary.  For roofs that are near the end of their
         useful life, these repairs may provide an extra 3 to 4
         years of service, enough time to start planning for a new
         roof.  For roofs that are relatively new, these
         techniques will provide protection while the cause of the
         problem is being investigated and more permanent repairs
         are planned.

    D.   See 01100-07-S for general project guidelines to be
         reviewed along with this procedure.  These guidelines
         cover the following sections:

         1.   Safety Precautions

         2.   Historic Structures Precautions

         3.   Submittals

         4.   Quality Assurance

         5.   Delivery, Storage and Handling

         6.   Project/Site Conditions

         7.   Sequencing and Scheduling

         8.   General Protection (Surface and Surrounding)

         These guidelines should be reviewed prior to performing
         this procedure and should be followed, when applicable,
         along with recommendations from the Regional Historic
         Preservation Officer (RHPO).

    E.   For additional information on factors contributing to
         roof deterioration, see 07500-02-S.

    F.   For guidance on inspecting for sources of flat roof
         failures, see 07500-01-S.


    A.   American Society for Testing and Materials (ASTM)


    A.   Asphalt--A dark brown to black semi-solid hydrocarbon
         obtained from crude petroleum.  See ASTM D 312-78.

    B.   Asphalt cement--Trowelable black asphaltic mastic used
         for flashing and roof repairs.  Contains fiber additives
         to thicken the mixture so that it will not run down
         vertical surfaces.  Also called roofing cement, flashing
         cement, plastic asphalt cement, plastic cement, roofing
         tar, bitumen and elastic cement.

    C.   Bitumen--A semisolid mixture of complex hydrocarbons
         derived from coal or petroleum, as coal-tar pitch or
         asphalt; before application, usually dissolved in a
         solvent, emulsified, or heated to a liquid state.

    D.   Built-up roofing--A continuous roof covering made up of
         laminations or plies of saturated or coated roofing
         felts, alternated with layers of asphalt or coal-tar
         pitch and surfaced with a layer of gravel or slag in a
         heavy coat of asphalt or coal-tar pitch or finished with
         a cap sheet; generally used on flat or low-pitched roofs.

    E.   Bituminous coating--An asphalt or tar compound used to
         provide a protective finish for a surface.

    F.   Coal tar bitumen--A dark brown to black, semi-solid
         hydrocarbon, a residue distillation of coal tar.  It is
         used for the construction of low-pitch built-up roofs.
         It differs from coal tar pitch because of a different
         volatility.  See ASTM D, 450, Type III.

    G.   Coal tar pitch--Similar to coal tar bitumen.  Used for
         dead-level or low-slope built-up roofs.  See ASTM D 450,
         Type I.

    H.   Lap cement--Thinner than asphalt cement, this is used to
         make watertight joints at the laps of roll-roofing.

    I.   Roof restorant--The restorant penetrates the existing
         roofing felts and plasticizes the age hardened bitumen to
         its originally installed state.  It will not, however,
         stop or seal leaks.  In addition, a deep coating on the
         surface provides a weathering surface and a binder for
         the replacement aggregate.  Also called roof coating or
         roof paint.


    A.   Storage and Protection:

         1.   Store roof system materials in a dry location.
              Outside, they should be placed on platforms off the
              ground or roof deck, covered with waterproof
              coverings which will not produce condensation.  

         2.   Store roll materials on end to prevent their
              becoming deformed or damaged.  Remove moisture,
              dirt, snow or ice from roofing bitumens before they
              are heated.  

         3.   Replace lids on cans of material stored on the job

         4.   Protect water based materials from freezing.


    A.   Environmental Requirements:

         1.   It is extremely important that the roof deck be dry
              at the time roofing operations commence, to avoid
              later problems with water vapor trapped under the
              membrane.  A deck should not be roofed when rain,
              snow, or frost is present in or on the deck

         2.   Cold temperatures:  Apply roofing materials only
              when correct bitumen temperatures can be
              maintained.  "Wind chill" and surface temperature
              affects application temperatures.  Keep surface
              temperature and asphalt thermometers at the work

         3.   Hot temperatures:  In hot weather the bitumen will
              cool slowly, which can lead to sticking, making the
              membrane susceptible to physical damage from
              mechanical equipment and foot traffic.

         4.   Wind:  Wind can blow hot materials, mastics and
              coatings causing damage to surrounding property and
              making the handling of roll materials difficult.



    A.   Sand and/or gravel to match existing or historic

    B.   Asphalt Roof Felts:

         1.   BS747 Type 1 fiber base felts - suffer from inter-
              layer blistering.

         2.   Type 2 Asbestos base felts - inter-layer blistering
              is less common.

         3.   Type 3 Glass fiber felts - Extremely resistant to

         4.   Polyester felts - some resistance to blistering

    C.   Asphalt cement (also called, among others, flashing
         cement or roofing cement

    D.   Roof Coating (also called roof paint and roof restorant)

    E.   1-1/2" Flathead galvanized roofing nails

    F.   Turpentine


    A.   Utility knife or hook-nosed linoleum knife

    B.   Hammer

    C.   Trowel

    D.   Stiff bristle brushes (non-metallic)

    E.   Long handled brush or broom

    F.   Ladders, scaffolding as required



    A.   Inspect the surface for obvious cracks, blisters, or
         ridges in the membrane.

    B.   Check also for bald spots, areas where the gravel (built-
         up roof) or mineral granules (roll roofing) are missing.

    C.   Be especially diligent inspecting where the roof meets
         parapets, chimneys, vents, and other vertical surfaces.

    D.   If possible, check the underside of the rafters and
         decking for water stains or rot which would indicate
         water infiltration, realizing that water can migrate
         through layers of roof plies from distant points.

    E.   Check the interior, especially ceilings and around
         fireplaces and vent pipe locations, for clues as to the
         location of any leaks.

    F.   See also roofing section of 01800-01-S, "Checklist for
         the Routine Inspection of Buildings".


    A.   Protection:

         1.   When transporting liquid bitumen to the roof,
              protect adjacent surfaces and permanent equipment
              by providing masking or covering.  Secure coverings
              without the use of adhesive backed tape or nails.
              Impervious sheeting which produces condensation
              should not be used.

         2.   Keep a portable fire extinguisher on hand where
              work is being done.  

         3.   At the end of each work day provide temporary
              roofing when existing roof is being opened for
              repair or replacement.  Provide an effective way to
              divert water runoff away from open roof.

    B.   Surface Preparation:

         1.   Sweep exposed felts clean of loose aggregate, dirt
              and silt with stiff bristled brush or broom.  Sweep
              at least six inches into embedded aggregate in all
              directions.  Clean all roof surfaces, including
              parapets, copings, and flashings.  Sometimes this
              can be done using a long-handled push broom.  In
              the case of a tar and gravel roof, you may have to
              cut the gravel off by hand or use a gravel removing
              machine.  Parapet cleaning should include brushing
              of any efflorescence from the brickwork using
              stiff, non-metallic bristle brushes.  Do not begin
              repair of flashings until completion of all
              pointing or resetting of parapet units and repair
              of coping joints.

         2.   Go over the entire roof area with roofer's chalk,
              marking areas where repair work is to be done.


    A.   Repairing open lap joints on roll roofing:

         1.   Force lap cement or asphalt cement under open seam.
              Weight it down with heavy weight until it is dry.

         2.   If it pops again, slit wrinkle being careful not to
              cut sound layers of roofing below.

              a.   Using 1-1/2 inch galvanized roofing nails
                   secure both sides of the slit.  

              b.   Using 90-lb. roofing felt, cut a patch large
                   enough to cover the nail heads with a 2 inch
                   overlap on all sides.

              c.   Coat the back of the patch with asphalt cement
                   and press it into place over the nails.

              d.   Nail the edges of the patch with nails spaced
                   about 1 inch apart and cover the nail heads
                   with more asphalt cement.

              e.   Finish the patch by sprinkling light colored
                   fine gravel or sand over the wet cement.  DO
                   OUT PROCESS.

    B.   Repairing Small Blisters:

         1.   If no leaks are evident, apply a solar reflective
              treatment (see Section 3.03 B.3. below).


              Starcut the blister with a hook-nosed linoleum
              knife or a utility knife to release trapped air;

              a.   If the felt layers beneath the surface are dry
                   proceed with the repair.  

              b.   If they are damp, deepen the cut down to the
                   wood sheathing and let the roofing dry out
                   before proceeding.  A portable electric
                   heater-fan or hair dryer can be used to speed
                   up the process.

              c.   Patch the area and apply a solar reflective
                   treatment (see Section 3.03 B.3. below).


              Release entrapped moisture by installing drying
              units with evaporation tubes (see manufacturer's
              instructions for installation procedures).

         2.   If leaks are present, allow entrapped moisture to
              dry and then remove the felts and replace with new.

         3.   To Prevent Blistering:  Apply chippings over the
              surface (a form a solar reflective treatment):
              Chippings are usually light colored, of limestone,
              granite, gravel, calcite or feldspar, 6-10 mm in
              size set in bitumen compound after the asphalt is

              a.   Advantages of chippings:

                   1)   Helps cool the membrane.

                   2)   Helps hold down the membrane.

              b.   Disadvantages of chippings:

                   1)   Outlets may become blocked.

                   2)   Leaks in roof are difficult to locate and
                        bonded chippings are difficult to remove.

              c.   Alternatives of chippings include mineral-
                   surfaced roofing and liquid-applied surface
                   coatings (very effective on top of glass-base
                   felts, but not so effective with others).

    C.   Repairing Undulations:

         1.   One alternative is to remove and relay the roof
              with new felts.

         2.   If not a serious problem, that is the problem will
              not be exacerbated by foot traffic on the roof,
              then an application of solar reflective treatment
              should aid in reducing the spread of undulations.

    D.   Repairing larger damaged areas on either built-up or roll

         1.   Cut out damaged plies leaving sound layers in tact.

         2.   Dip knife in turpentine periodically while cutting
              to keep the blade free of tar and felt fibers, and
              pull out the layers of felt individually.

         3.   If water has soaked the felt, remove all the
              roofing within the rectangle, down to the sheathing
              and dry the area thoroughly.

         4.   With the damaged area removed, apply asphalt cement
              under loose edges of cuts and over the entire cut
              out area.  

         5.   Using 90-lb. roofing felt, cut out patches the same
              size as the cut-out.  Press first patch into
              asphalt cement.  Apply another coating of asphalt
              cement to cut-out and press another patch into
              place.  Repeat process until the patch is even with
              the surrounding roof.  The number of layers will
              depend on the depth of the cut-out.

         6.   Cut a final patch of either roofing felt or mineral
              roll roofing which will overlap the cut-out area by
              2 inches on all sides.  Cover the bottom of the
              final patch with asphalt cement, press in place,
              and nail edges with nails spaced about 3/4 to 1
              inch apart.  Cover nail heads with asphalt cement
              and sprinkle fine gravel or sand over, as in A.3.

    E.   Repairing cracks in Asphalt Flashing:

         1.   Clean the area of any gravel or other debris.

         2.   Coat area around crack with asphalt cement
              extending 6 inches beyond crack in all directions.

         3.   Lay a piece of 90-lb. roofing felt in the cement
              maintaining the 6 inch overlap.

         4.   Repeat steps (2) and (3).

         5.   Finish patch with a final coat of asphalt cement
              and lay sand or gravel on the horizontal surfaces.

    F.   If there are simply bald spots on the surface, sweep up
         any dirt and debris; paint area with roof coating and re-
         spread or add more gravel.

         NOTE:  DO NOT use reflective paints instead of sand or
         gravel, except on vertical portions of flashing.  Though
         it is not fully understood why, reflective paints
         encourage wrinkling, a potentially serious form of

    G.   Inspect patch at least twice a year to make sure it is
         still sound.

                             END OF SECTION

Last Reviewed 2015-06-09