Chimney Flashing and Counterflashing Repair

Purpose

A chimney leak is almost never the chimney. It is a two-layer flashing system where one layer has been replaced by sealant, and the most common field repair in this trade - a bead across the top of a lip, or cement troweled over the whole intersection - is the one that guarantees the return visit. This procedure guarantees the flashing woven into the courses and the counterflashing anchored in the masonry are re-established as separate layers, and that the cricket question gets answered instead of avoided.

Scope

Covers repair or replacement of apron, step and back flashing plus counterflashing at a masonry chimney through a steep-slope asphalt shingle roof, residential and small commercial, including cutting a reglet into a mortar joint.

Does not cover masonry work (repointing, a failed crown, a spalled course, a liner), which belongs to a mason and is scheduled before the roofing repair. Does not cover designing or framing a cricket, a carpentry scope this SOP triggers but does not perform. Course lifting and hand sealing belong to the shingle repair and blend-in standard; finding the leak belongs to leak diagnosis by water testing.

Roles and responsibilities

Two layers, two jobs, and sealant is not one of them

The lower layer lives in the roof plane: an apron on the down-slope face, step flashing on the two side faces with one L-shaped piece per shingle course each lapping the one below, and a back pan or a cricket's pans on the up-slope face. Each step piece is nailed to the deck or to the masonry, never to both.

The upper layer lives in the wall. Counterflashing is let into a mortar joint - a reglet - or a through-wall receiver, locked or wedged, and hangs over the vertical leg of the flashing below without being fastened to it. That overlap is why the joint works: both layers can move and water crossing between them never meets an upward-facing seam. Sealant belongs in the reglet groove and nowhere else.

Two code items set the scope before any of that starts, and both belong to the edition your jurisdiction has adopted rather than to memory. The IRC sets a minimum height, width and end lap for step flashing at a vertical wall, and requires a cricket on the up-slope side of a chimney over a stated width measured perpendicular to the slope, commonly 30 in. Measure and pull the section; do not decide a cricket by eye.

Procedure

1. Set access, fall protection, and take the flue out of service. Access runs under the ladder setup standard and the fall protection setup SOP. Confirm with the occupant that the appliance served by this chimney is off and the flue is cold. Acceptance: anchorage set, everyone connected, appliance confirmed off, masonry cool to a gloved touch. Wrong looks like a crew flashing a chimney with a stove running below. Stop rule: a warm flue stops the work until the appliance is off and cooled. Hazard: an operating flue discharges carbon monoxide at head height on the roof, an inhalation route with no smell and no warning, so the appliance goes off first rather than being worked around.

2. Classify the failure face by face, and decide whether this is roofing work. Inspect all four faces plus the crown and the top three masonry courses. Acceptance: a written condition per face - apron, both step runs, back pan or cricket, counterflashing - plus a masonry note. Wrong looks like a scope written from the face that points at the driveway. Stop rule: open joints, a cracked crown, spalled brick or a displaced course make this a masonry job first, with the flashing repair scheduled after, in writing. Hazard: a chimney that rocks under hand pressure is a structural condition, so nobody works against it, nobody ties off to it, and the area below stays clear until it is assessed.

3. Measure the chimney width perpendicular to the slope and answer the cricket question. Take the dimension across the slope, not the one you can reach. Acceptance: a recorded width compared in writing against the cricket trigger in the adopted code edition. Wrong looks like a flat back pan behind a wide chimney, holding debris and snow against the masonry. Stop rule: at or over the trigger width with no cricket, the flashing scope stops, no back pan goes in as a substitute, and the job returns for a customer decision as framing plus roofing. Hazard: standing water and trapped ice behind a wide chimney is the load that pushes a back pan open, so installing one anyway is the failure this step prevents.

4. Open the field and remove the old flashing whole. Lift the courses along both side faces and above the chimney per the shingle repair and blend-in standard, then remove the old step, apron and back flashing intact. Acceptance: every piece out and counted, courses below the chimney undisturbed. Wrong looks like new material laid over the old, which doubles the thickness and lifts the counterflashing off the wall. Stop rule: cracked mat takes the stop rule in that SOP. Hazard: old flashing edges are sharp and usually rusted through the galvanising, so cut-resistant gloves go on and pieces go into a bucket rather than onto a slope they will slide down.

5. Cut the reglet under dust control, or decide against cutting. Cut into a mortar joint, not through brick, to the depth the counterflashing profile needs. Acceptance: a continuous groove in a single joint on each face, consistent depth, cleaned of dust. Wrong looks like a groove wandering across brick faces, or a surface-mounted flashing caulked to the wall and called a reglet. Stop rule: where a reglet cannot be cut, install surface-mounted counterflashing with a sealed top edge and tell the customer in writing that it is a maintenance item on an inspection cycle, not a permanent detail. Hazard: grinding mortar releases respirable crystalline silica, an inhalation carcinogen that no glove or face shield addresses. Mortar removal is a listed task under 29 CFR 1926.1153 Table 1, which pairs a shroud and dust collector with respiratory protection at any duration, under a written program per 29 CFR 1926.103. If the collector stops pulling mid-cut, stop cutting; the job does not finish dry.

6. Set the apron, then the steps, one piece per course, from the bottom up. Install the apron over the course below it, then work up each side face setting one step piece per course, each lapping the one under it with the shingle above covering its horizontal leg. Acceptance: one piece per course at the lap and leg dimensions the adopted code section requires, every fastener covered, no piece nailed to both deck and masonry. Wrong looks like a single long bent pan run up the side. Stop rule: a piece that will not lie flat means the previous one is out of sequence, so back up rather than forcing it. Hazard: cut metal edges are a laceration route and offcuts slide, so trim at the ground where possible and pouch every scrap.

7. Set the counterflashing into the reglet, lock it, then seal the groove only. Bend the top leg into the groove, wedge or lock it mechanically, and let the skirt hang over the vertical leg below without fastening the two together. Acceptance: mechanically held before any sealant, skirt overlapping the flashing below by the manufacturer's stated minimum, sealant only in the groove. Wrong looks like a skirt caulked to the step flashing, tying two layers that have to move independently. Stop rule: counterflashing that stays up only because of sealant is not installed - re-cut or re-wedge it. Hazard: masonry sealants carry vapor and skin routes, so read that label for ventilation and gloves and keep the pail off a hot slope, where solvent flashes off faster than the label assumed.

8. Reseal the field, restore the flue, and prove it drafts. Hand seal every shingle unsealed in step 4 per the shingle repair and blend-in standard, remove any cover placed over the flue during cutting, and reconcile covers out against covers in. Then have the occupant restore the appliance and confirm it drafts. Acceptance: covers reconciled to zero remaining, tabs tug-tested and holding, appliance lighting and drafting normally, working CO alarm confirmed on the level it serves. Wrong looks like a crew leaving with a plate still on the flue. Stop rule: an appliance that will not draft, or spillage at the draft hood, stops the handover - it stays off and the customer calls the trade that owns it. Hazard: this step puts a combustion path back in service, and the draft check plus the CO alarm are the only field evidence the protection interrupted at step 1 has been restored.

The record this produces

A chimney entry carrying: the appliance served and confirmation it was off and cold; the per-face condition from step 2 with the masonry note and any referral issued; the chimney width perpendicular to the slope and the cricket comparison; step pieces per side and the exposure they were spaced to; reglet length cut and the dust control used; whether counterflashing is reglet-set or surface-mounted, with the maintenance note if surface-mounted; covers in and out; and the draft and CO alarm result.

The estimator reads the cricket line when this address comes back, because a chimney that triggered a cricket and did not get one keeps leaking whatever the flashing looks like. The office reads the surface-mounted line to schedule the inspection cycle it was sold with.

Worked pass: 42 in chimney on an 8:12 slope, stain at the up-slope side

Step 1: appliance is a gas furnace, off at the switch, masonry cool, anchorage set. Step 2: apron sound, both step runs corroded through at the third course, back pan holding debris, counterflashing surface-mounted with a failed top bead, mortar joints tight and crown intact, so masonry is not the blocker.

Step 3 fails. The chimney measures 42 in across the slope against the 30 in cricket trigger in the adopted edition, and there is no cricket. The stop rule fires: no back pan goes in as a substitute, and the job returns to the estimator as framing plus roofing. The customer authorizes it, a framer builds the cricket to the height and plane slopes taken from the roofing manufacturer's cricket table for a 42 in chimney at 8:12 rather than by eye, and the crew comes back.

On the return the chimney measures 28 in deep along the slope, in a laminated field at 5-5/8 in exposure. 28 divided by 5.625 is 4.98, so five step pieces per side, ten in total, plus the apron and the cricket's two pans. Reglet length is two faces at 42 in plus two at 28 in, so 140 in, or 11.7 lin ft - well under a half shift of cutting, and the Table 1 row is run with shroud, dust collector and respirator throughout regardless.

Step 6: ten step pieces set, none nailed to both deck and masonry. Step 7: counterflashing locked into the reglet on all four faces before any sealant. Step 8: fourteen shingles hand-sealed and tug-tested, one flue cover in and one out, furnace restored by the occupant, drafting clean, CO alarm on that level tested and sounding.

The cricket is the finding. The visible defect was corroded step flashing, and the reason both runs corroded through at the same course is that everything shed off the up-slope face stood behind the chimney for years. Replacing the flashing without the cricket reproduces the failure on a new part.

References

• 29 CFR 1926.1153 and its Table 1 entry for handheld grinders used for mortar removal, which pairs a shroud and dust collector with respiratory protection, under a written program per 29 CFR 1926.103
• 29 CFR 1926.501(b)(13) and 1926.502(d), the fall protection duty and arrest criteria governing roof access for this work
• IRC Chapter 9, in the edition your jurisdiction has adopted, for step flashing dimensions and end lap at a vertical wall and for the chimney width that triggers a cricket
• The roofing manufacturer's cricket sizing table and flashing details for the covering installed
• See related: the shingle repair and blend-in standard, the flashing and penetration detail inspection SOP, and the leak diagnosis by water testing SOP