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Historic Preservation - Technical Procedures

Spectitle:

Repairing Chimney Flashing

Procedure code:

0762002R

Source:

Hspg Prepared For Nps - Sero

Division:

Thermal And Moisture Protection

Section:

Sheet Metal Flashing And Trim

Last Modified:

02/24/2012

Details:

Repairing Chimney Flashing



REPAIRING CHIMNEY FLASHING


PART 1---GENERAL

1.01 SUMMARY

    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
                   manila rope.

              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

         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 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.

1.03 MAINTENANCE

    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.


PART 2---PRODUCTS

2.01 MATERIALS

    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
         flashing.

    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.

2.02 EQUIPMENT

    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


PART 3---EXECUTION

3.01 EXAMINATION

    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
         paint.

3.02 PREPARATION

    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
              across.

              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
                   the brick.

              c.   Use a cold chisel to knock out mortar between
                   cuts.

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
              for reinstallation.

         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
              by 2".

         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
              stiffening.

         3.   Overlap the base flashing a minimum of 4".  It can
              however, extend all the way down the vertical
              surface.

         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
              is galvanized.

    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
              desired.

         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
              center.

         5.   The final piece of cap flashing, at the rear of the
              chimney, should include a minimum 2" return at each
              end.

    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