Repairing Chimney Flashing
- Procedure code:
- Hspg Prepared For Nps - Sero
- Thermal and Moisture Protection
- Sheet Metal Flashing And Trim
- Last Modified:
REPAIRING CHIMNEY FLASHING
A. This procedure includes guidance on repairing chimney
flashing that is missing, deteriorated or damaged.
B. Flashing is an integral part of a roofing system and
protects against water infiltration at roof hips and
valleys; penetrations such as chimneys; where roofing
meets a vertical surface such as a parapet or at a porch
roof; or at drip edges.
C. Copper, with copper nails, galvanized sheet metal with
hot-dipped galvanized nails, or terne-coated stainless
steel with stainless steel nails are the primary
materials used for flashing.
CAUTION: THE DIFFERENT METALS SHOULD NOT BE MIXED.
RAINWATER RUNOFF FROM COPPER FLASHING WILL CORRODE BOTH
GALVANIZED SHEET METAL AND STAINLESS STEEL THROUGH
GALVANIC ACTION. TAKE CARE TO USE A FLASHING MATERIAL
COMPATIBLE WITH THE REST OF THE ROOF SYSTEM.
D. Safety Precautions:
1. Wear rubber-soled shoes that have non-slip or grid
type tread (preferably high top sneakers for good
ankle support. Avoid wearing loose clothing.
2. Wear a safety belt or harness secured to a
substantial chimney or to a window on the opposite
side of the house. Leave only enough slack so you
can work comfortably in one area, and adjust the
slack as you work on other sections of the roof.
3. Be sure the roof is clear of debris and water.
4. Do not work on wet or snow covered roofs, or on a
brittle roof surface. Work on cleated walkboards.
5. On steep roofs:
a. Secure chicken ladders or cleats at the top
for adequate footing.
b. Safety lines should be tied and secured with
c. Carry a limited amount of materials so that
balance and footing are not impaired.
d. Provide scaffolding, ladders and working
platforms as required to execute this work.
D. Historic Structure Precautions: Examine, measure, and
record existing flashing application methods at eaves and
gable edges, ridges, chimneys, parapet vents, and other
roof penetrations. Take note of reglet details and
methods of fastening sheets together.
E. 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 before performing
this procedure and should be followed, when applicable,
along with recommendations from the Regional Historic
Preservation Officer (RHPO) or designated representative.
1.02 SYSTEM DESCRIPTION
A. Flashing is functioning as it was intended when there are
no cracks, splits or torn areas. It should not show any
signs of corrosion. Where flashing meets a vertical
surface, all cap flashing should be secure and the
flashing itself should be soundly anchored.
A. Check flashings as a part of the annual roof inspection
(preferably in late autumn). Repair defects immediately
because flashing is a critical part of the roof fabric.
Small leaks in the flashing which are neglected may lead
to major faults in the roof system.
A. Flashing material: 16 oz. or 20 oz. sheet copper; lead-
coated copper; 26 gauge galvanized steel. Metal should
match remaining metals on the roof.
B. Inorganic Zinc-Rich Primer: Zinc dust - zinc oxide
primer by 3M, Mobil, or Tnemic to paint galvanized
C. Sealant: Clear Silicone Rubber Sealant as manufactured
by Dow Corning, General Electric, or equivalent.
D. Nails used for fastening copper shall be copper or
hardware bronze of stronghold type or equal, with large
flat head. They shall not be smaller than No. 12 stubs
gauge (0.109 inches) and of sufficient length to
penetrate roofing boarding. For galvanized metal use hot
dipped galvanized nails.
E. Building Paper: For copper, use red rosin paper having
minimum weight of 2 pounds per square. For galvanized
metal, roofing felts may be used.
NOTE: THE USE OF ROOFING FELTS UNDER COPPER IS GENERALLY
NOT RECOMMENDED BECAUSE THE BITUMEN IN THE FELTS HAVE A
NEGATIVE REACTION WITH THE COPPER.
F. Mortar as directed by RHPO.
A. Safety belt or harness; protective gloves and gear.
B. Chicken ladder; ladders and scaffolding.
C. Straight snips for cutting straight or curved lines in
sheet metal 24 gauge or lighter.
D. Handy tongs for bending the edges of the solder.
E. Hawk, pointing trowel
A. Outside, inspect flashing for splits, holes or corrosion.
B. Inside, inspect the underside of the roof deck for
evidence such as water stains or damp wood; inspect the
ceiling and walls around fireplaces and chimney flues for
stained or spalling plaster or wallpaper, or peeling
A. Surface Preparation:
1. Examine, measure, and record existing flashing and
its method of application. Take note of reglet
details and methods of fastening sheets together.
2. Select sheetmetal type to be used making sure it is
compatible with the remaining flashing on the roof,
the gutters and the downspouts.
3. Cutting and Bending:
a. Mark all bends, cuts, and notches which will
be required using existing pieces as patterns.
Regular metal cutting shears can cut 16-oz.
copper, or 26 gauge galvanized steel.
b. To aid in forming the bends on the base and
counterflashing aprons, clamp a 2 x 4 over the
flashing piece, with the bending line at the
edge of the worktable. Use an additional
piece of 2 x 4 as blocking to place against
flashing and bend it into a 90o bend by
striking it with a hammer. To create a hem,
unclamp and continue hammering against a 2 x 4
to bend metal edge over again.
c. Cut base flashing pipes so that they will
extend under the shingles a minimum of 4", and
up the chimney face a minimum of 4". The
total length of each piece will depend on the
exposure of the shingles.
d. Counterflashing pieces must overlap the base
flashing by a minimum of 4". At corners there
should be a 2" for double overlap.
4. Rosin paper should be laid on the roof decking
beneath the base flashing if none exists. Use only
copper nails with copper flashing, or hot-dipped
galvanized nails if using galvanized sheet metal.
The nails should be sized to allow for at least 1"
penetration into roof deck.
5. The reglet is the slot cut into the chimney mortar
joints into which the cap flashing is set. It goes
straight across the front and is stepped along the
two sides. If there is not a cricket at the back
of the chimney the reglet slot is also straight
a. Cut the slot by hand using a cold chisel.
Experienced operators may also use a diamond
blade with a water spray attachment in a hand-
held circular saw, a portable grinder, or a
circular saw with a carbide masonry blade.
b. Cut the top and bottom portions of the joint
to a depth of 1-1/2" being careful not to cut
c. Use a cold chisel to knock out mortar between
3.03 EXECUTION, INSTALLATION, APPLICATION
A. Apron Base Flashing:
1. Remove shingles on the back and sides of the
chimney only. Remove only as far as the next full
shingle beyond the 4" minimum. Save the shingles
2. Install apron flashing over shingles on lower
(front) slope with a hem at the bottom edge.
3. Place one nail at each top corner where it will be
covered by the first piece of side base flashing.
B. Stepped Base Flashing:
1. Lay the first piece of base flashing so that its
bottom edge covers the nail of the apron flashing.
2. Nail the base piece at its upper edge, close to the
chimney. If the roofing material is barrel tile,
bend the free edge of the base piece up about 1/2"
to act as a trough to direct water down the slope
of the roof.
3. Reinstall the first shingle. Nail so the bottom
edge of the shingle is 1/2" below the bottom edge
of the base flashing and the top edge of the
shingle is 2" below the top edge of the base
flashing. Make sure the lap of the reinstalled
shingle matches the lap of the remaining roof.
4. Continue to interweave base flashing, and shingles
until both sides are complete, making sure to match
exposure of the remainder of the roof.
5. At the rear of the chimney, install a continuous
strip of base flashing whose ends wrap each corner
6. Relay shingles over this piece of base flashing
making sure the nails of each shingle are covered
by the lap of the shingle above. The lap should
match the remainder of the roof.
C. Apron Counterflashing:
1. For each piece of counter flashing, bend a 1/2" lip
on the edge that slips into the reglet.
2. Slide the apron counter flashing into the reglet,
overlapping the corners about 4". A hem can be
bent into this piece as well to provide extra
3. Overlap the base flashing a minimum of 4". It can
however, extend all the way down the vertical
4. Set rolled metal wedges, no more than 12" apart, to
hold cap flashing into the reglet. Use lead or
copper if flashing is copper. Use lead if flashing
D. Stepped Counterflashing:
1. Insert pieces of counter flashing, overlapping each
succeeding piece 2". Trim the bottom edge of each
piece to follow the slope on the roof.
2. Each piece should step up above the previous piece
a maximum of three courses of brick.
3. Overlap base flashing by at least 4", or allow it
to come all the way down the vertical wall if
4. Insert rolled metal wedges to hold the counter
flashing tight. Insert a minimum of two wedges per
piece of counter flashing, no more then 12" on
5. The final piece of cap flashing, at the rear of the
chimney, should include a minimum 2" return at each
E. Repoint the reglet with a mortar that matches the
original in composition, color, and shape of joint.
Caulk can be also used, but this will require annual
inspection and maintenance.
F. If it is not possible to cut a reglet in the wall or
chimney, secure each piece of counter flashing with brass
screws driven into lead anchors. Flare the top of the
flashing 1/4" to receive a bead of high quality, long
lasting, UV resistant exterior caulk. This solution will
require annual inspections and periodic replacement of
the caulk seal.
END OF SECTION