Should You Caulk Chimney Flashing?

How flashing is supposed to keep water out

Good chimney flashing works like shingles: pieces of metal overlap so water always runs from one piece onto the next, downhill and away from the joint. Step flashing is woven into the shingle courses along the sides, an apron covers the low face, and a back pan or cricket handles the high side. Counter flashing let into the brick laps down over all of it.

Nothing in that system depends on caulk to stay dry. The metal sheds the water, and the overlap means small movements between the roof and the chimney do not open a path. Sealant only finishes a few details.

Where caulk belongs

There are a few legitimate places for sealant in a chimney flashing job, and in each the sealant is a supporting detail rather than the main barrier.

• The groove, or reglet, where counter flashing is inserted into a ground mortar joint, sealed with mortar or a compatible sealant
• Small corner and end details where two pieces of flashing meet
• Fastener heads on surface-mounted metal, where they cannot be avoided
• A short-term seal on a small gap until a proper repair can be made

Why caulk and tar patches fail

A chimney and a roof move independently with temperature, moisture and settling. A bead of caulk or a layer of roofing cement bridging the joint between them is stretched and squeezed every day. Add sun, which breaks down many sealants, and freeze-thaw, which pries at any gap, and the bead cracks, peels from the brick or splits along its length.

Tar has a second problem. When it is smeared up the face of the brick, it traps water in the masonry behind it and hides what is happening underneath. Many leaking chimneys we open up have several layers of tar over flashing that has rusted through, with wet brick and rotted decking hidden beneath.

• Sealing the reglet on inset counter flashing is appropriate, because the sealant supports a mechanical overlap
• A bead along the top of surface-mounted counter flashing works for a time, but everything depends on the bead, so check it yearly
• Tar smeared over the roof-to-brick joint is a poor fix that cracks with movement and traps water
• Caulk over rusted or missing flashing covers the problem without fixing it

Surface-mounted counter flashing

Many chimneys have counter flashing screwed or nailed to the face of the brick with a bead of caulk along the top edge, instead of being set into a ground joint. It is quicker to install and common on newer work and quick repairs. It can keep water out for a while, but everything depends on that top bead and on the fasteners staying tight in the masonry.

If your chimney has this detail, check it from the ground each year for gaps along the top edge and for pieces pulling away from the brick. Resealing it is reasonable maintenance. When it is due for replacement, setting new counter flashing into a ground mortar joint gives a more durable result.

Flat roofs: coating is not flashing

On flat roofs, the shortcut takes a different form. Instead of caulk, the joint between the roof and the chimney is covered with roof coating or roofing cement, often brushed a few inches up the brick and renewed every time the roof is recoated. Over the years the layers thicken, crack and pull away from the masonry, and water runs behind them into the wall and down the flue.

The proper detail is a base flashing made from roofing membrane compatible with the roof, turned up the brick and sealed to it, with counter flashing set into a mortar joint above and lapped over the top edge. Coating can protect the membrane, but it is not the seal.

Choosing a sealant for the right jobs

Where sealant is appropriate, the product matters. It should be made for exterior masonry and metal, stay flexible through heat and cold, bond to clean, dry surfaces and be compatible with the flashing metal and roofing. Many professionals use polyurethane or specialty flashing sealants for these details. Plain acrylic caulk and interior products are not made for this exposure. The sealant maker's instructions set surface preparation and temperature limits.

When caulk is not enough

If the flashing is rusted through, has missing pieces, was installed as a single bent strip with no step flashing, or is buried under layers of tar, sealant will not make it right. Neither will it fix a leak that is coming from a cracked crown, a capless flue or porous brick above the flashing, which is often the real source when caulking the flashing seemed to make no difference.

The lasting fix is new flashing: step flashing woven into the roof, an apron and back pan or cricket, and counter flashing let into the mortar. On flat roofs, that means membrane base flashing turned up the brick with counter flashing lapped over it.

What a proper reflash involves

A proper repair starts by removing the old flashing, tar and caulk so the brick and roof deck can be seen. Any soft decking is replaced and the brick at the roof line is repointed so the counter flashing has sound joints to sit in. On a shingle roof, new step flashing is woven in with each course, with an apron across the low face and a back pan or cricket on the high side. A groove is then ground into a mortar joint above, and counter flashing is bent into it, sealed, and lapped down over the step flashing.

Sealant shows up only in the groove and a few corner details. Everything else is metal overlapping metal, which is why a properly flashed chimney stays dry for many years without anyone touching it.

What you can safely check

With binoculars from the yard, study the joint between the roof and the brick. Black tar smeared on the brick, cracked or peeling beads of caulk, metal strips lifting from the face, and rust streaks are all signs the flashing needs a proper look. Inside, check the attic around the chimney after a storm for damp wood. Leave the sealing itself to a roofer or chimney crew; working at the chimney means working at the steepest, most awkward part of the roof.

Note when leaks appear, too: a drip that starts only in wind-driven rain often points to the flashing on the face taking that wind.