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:
MINOR REPAIRS TO ASPHALT ROLL-ROOFING OR BUILT-UP ROOFING
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.
NOTE: RIDGES SHOULD BE REPAIRED AS SOON AS
POSSIBLE. CONTINUAL STRESS ON A RIDGE CAN LEAD TO
SPLITTING OF THE MATERIAL.
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.
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.
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
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:
NOTE: NO REMEDIAL TREATMENT IS RECOMMENDED, AS
THIS TYPE OF BLISTERING SHOULD NOT AFFECT THE
EFFICIENCY OF THE WATERPROOFING.
a. Miniature surface blistering of the bitumen
coating (especially with BS747 Type 1E and 2E
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.
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
d. Cockling is less likely in coated felts
especially those with a glass or polyester
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
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
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,
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
1.04 DELIVERY, STORAGE AND HANDLING
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
3. Replace lids on cans of material stored on the job
4. Protect water based materials from freezing.
1.05 PROJECT/SITE CONDITIONS
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-
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
A. Utility knife or hook-nosed linoleum knife
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".
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.
3.03 EXECUTION, INSTALLATION, APPLICATION
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
NOT ELIMINATE THIS LAST STEP. THE GRAVEL OR
SAND REFLECTS SUNLIGHT THUS SLOWING THE DRYING
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
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
END OF SECTION