What Is a Chimney Cricket, and Does Your Roof Need One?

Why the uphill side of a chimney is a problem spot

The back of a chimney is the one place on most roofs where water has nowhere to go. Everywhere else it runs downhill and off the edge, but at a chimney a solid wall sits across its path.

Without a cricket, that water stalls in the corner where the roof meets the brick and leans on the flashing joint. Water pooling behind a chimney can seep into the roof structure, and once it reaches the decking and rafters the repair grows well beyond flashing.

Debris makes it worse. Leaves, needles and shingle granules wash down and stop at the brick, and that wet mat holds moisture against everything beneath it.

Does a cricket help with ice behind the chimney?

It helps, but it does not cure ice dams. A cricket gives meltwater a path down both sides, so the chimney stops acting like a snow fence that holds a drift against the masonry.

Ice dams themselves start with heat escaping into the attic, and insulation is a separate trade from roofing. On the roof side, ice-and-water shield in the cricket's valleys and against the chimney keeps trapped meltwater out of the deck.

When a chimney cricket is required

The residential building code calls for a cricket on the uphill side when a chimney is more than 30 inches wide, measured parallel to the ridge, and the chimney does not meet the ridge. A chimney that comes up through the ridge does not need one, because water already splits there and runs down both sides.

Requirements depend on the code edition your town has adopted, so confirm locally. A roofer should be able to show you the measurement behind any recommendation.

How to measure the 30 inches

Measure across the face of the chimney at its widest point, parallel to the ridge, not up the slope. A chimney 24 inches deep but 40 inches wide is a cricket chimney, while a narrow flue 20 inches across usually is not.

Think of the width as how much roof the chimney blocks. The wider the blockage, the more water it forces sideways.

Steeper roofs need taller crickets

A cricket gets taller as the roof gets steeper. On a low-slope roof the saddle can be fairly flat, but on a steep roof the same chimney needs a much taller peak so both sides drain at a usable angle.

That is why a cricket is framing work, not a shingle trick. The peak has to be high enough that both of its small valleys drain onto the roof below.

When it is not required but still worth adding

Some chimneys just under 30 inches wide still collect water and debris behind them. If you have had repeat leaks at the back of the chimney, or there is a permanent debris line and staining there, a cricket is worth considering.

Chimneys low on the slope, with a long run of roof above them, catch the most water. A chimney close to the ridge catches very little.

Signs your chimney needs a cricket

The clues show up at the back of the chimney, inside and out. Check from the attic and from the ground with binoculars, and stay off the roof itself.

• Water stains on the ceiling or attic sheathing directly behind the chimney
• Leaves and shingle grit that never wash clear of the back of the masonry
• Snow and ice that hang on behind the chimney after the rest of the roof has cleared
• Rusted, cracked or caulked flashing at the back corner, often patched more than once
• Shingles behind the chimney that look older, curled or stained compared with the rest
• Crumbling mortar or flaking brick on the uphill face, a sign it stays wet

A leak that shows up only in long, heavy storms is a common pooling symptom. Our article on why some roofs leak only in heavy rain explains how to narrow it down.

Signs an existing cricket is not doing its job

Some chimneys have something back there that was never a real cricket. These are the usual giveaways.

• Shingles bent up behind the chimney over a block of wood, with no framed peak
• A saddle narrower than the chimney, so one corner still collects water
• A peak so flat that the area stays dirty and slow to dry
• Roof cement smeared along the top of the flashing instead of counter flashing set into the mortar joints

How a chimney cricket is built and flashed

A cricket is real carpentry, framed on the roof deck and then covered and flashed like a small roof. Skipping any layer is how crickets leak.

1. Frame it. Ridge and valley framing is cut to the roof pitch and chimney width, set tight to the back of the chimney, and covered with decking.
2. Underlay it. The cricket and its two valleys get underlayment, with ice-and-water shield in the valleys and against the chimney.
3. Cover it. The surface is shingled to match the roof or covered in formed metal, which is common on small or steep crickets.
4. Flash it. Step flashing is woven into each course where the cricket meets the chimney, and counter flashing is set into the mortar joints and lapped over it.
5. Tie in the valleys. Both valleys must shed cleanly onto the roof below the chimney, because a cricket redirects water rather than stopping it.

Leaks at an existing cricket usually trace to worn or cracked flashing where it meets the chimney, or to poor installation in the first place. Surface caulk on brick is a temporary patch, not a flashing.

We build, flash and reflash crickets as part of our chimney flashing work. If the leak turns out to be at a skylight, sidewall or valley, that is our roof flashing repair work too, and our guide to chimney flashing repair covers the diagnosis in more depth.

Why this is not a do-it-yourself job

The framing is not the hard part. The leaks come from the flashing, which has to be woven into the shingles and set into the mortar in the right lap order.

Get that order wrong and the cricket funnels water into the decking instead of around the chimney. Working against masonry on a pitched roof is also dangerous, which is why this work belongs with a roofing crew.

Roofing work with one masonry overlap

The framing, decking, underlayment, flashing and shingle or metal work are all roofing, and that is the work we do. The overlap is the mortar, because counter flashing is set into the joints.

If the brick is flaking or the joints are crumbling, a mason should repair them first. Then the new flashing goes in, so it does not have to be torn out later.

The best time to add a cricket

Add the cricket while the shingles around the chimney are already off. The framing sits on clean decking, the underlayment runs continuously, and the step flashing goes in course by course as it should.

That makes a roof replacement the natural moment. Adding one later means cutting back good shingles to make room.

Watch for one shortcut on reroof bids. A thick bead of roof cement and a skirt of rolled material is not a cricket, and it tends to crack once winter arrives.

Ask any contractor two questions. Is the chimney wide enough to need a cricket under your town's code, and will the counter flashing be set into the mortar joints or only sealed on the surface?

If the chimney is no longer in use

Sometimes the better fix is removing the chimney. If nothing vents through it, taking it down below the roofline and decking over the opening removes the leak point and the cricket question together.

Our article on whether to remove an unused chimney before a new roof walks through that decision.

Looking after a cricket once it is in

A well-built cricket needs little care, but the flashing and mortar it ties into still age. Check the back of the chimney from the attic and the ground each spring and fall.

Look for rust streaks, open mortar joints, failed sealant at the counter flashing, and debris that still collects where it should now wash clear. Have the flashing checked after any chimney masonry work and after hail or high wind.

If debris keeps piling up behind a finished cricket, the peak or valleys are likely wrong, and that deserves a second look.

Get the back of your chimney checked

Buckshot Exteriors is family-owned, has served homeowners since 1979, and has offices in Madison, Wisconsin and Centerville, Iowa. We handle chimney flashing and crickets alongside the rest of your residential roofing, and our on-site inspection is free.

Call (608) 909-9109 to schedule your free inspection, or send us a message and we will set a time. If water is coming in right now, our 24-hour storm line takes emergency calls.