Symptom-led fixes for a chimney that leaks with or without a cricket, plus the plan-check and inspection failures that recur
Start from the symptom, not from the cricket
A chimney is the single most common source of roof leaks, and a cricket is only one of about six things behind the chimney that can let water in. Diagnosing in the wrong order means building a perfectly good cricket onto a chimney whose real problem is a cracked crown, and then owning the leak.
So this is symptom-led. Find the row that matches what the customer is describing, then work the column.
| Symptom | Most likely causes, in order | How you tell them apart |
|---|---|---|
| Stain on the ceiling directly below the chimney, in ordinary rain | No cricket on a chimney over 30 in; undersized cricket; counterflashing surface-sealed only and the sealant has gone | Measure W parallel to the ridge and measure the existing H. Then look at how the counterflashing enters the brick |
| Only leaks in winter, and only after snow | Ice damming in the cricket valleys, not a flashing failure at all | No leak in summer downpours. Ice visible behind the chimney. See the ice dam guide below |
| Only leaks in driving, wind-blown rain | Counterflashing or step flashing too short; lap sequence reversed somewhere | Direction matters: it tracks the wind, not the rainfall total |
| Debris dams behind the chimney even with a cricket fitted | Cricket too shallow for the chimney width; apex set too close to the chimney; valleys not actually at 45 degrees | Check H against the table for that W and slope. Run a level along the cricket ridge |
| Damp smell and staining inside the chimney breast, no ceiling stain | The masonry, not the roof: crown, cap, spalled brick, flue condensation | It is wet above the ceiling line, around the breast, rather than on the ceiling below the penetration |
| Sheathing soft behind the chimney but no active drip | A historic leak that has already been patched, or long-term condensation | Rot with no current water path. Probe the full area before you frame on it |
The undersized cricket, which is the commonest fault of all
A chimney with a cricket already on it that still leaks is, more often than not, a chimney with a cricket that is too shallow. It happens because the person who built it eyeballed the height, or built every cricket the same height regardless of the chimney, or — the frequent one — used half the chimney width as the apex distance measured along the roof instead of in plan.
The test is quick. Measure W parallel to the ridgeline, take the slope of that roof plane, and compare the actual H to the table:
| Roof slope | H should be | 36 in chimney | 48 in chimney | 60 in chimney |
|---|---|---|---|---|
| 3:12 | ⅛ of W | 4.5 in | 6.0 in | 7.5 in |
| 4:12 | ⅙ of W | 6.0 in | 8.0 in | 10.0 in |
| 6:12 | ¼ of W | 9.0 in | 12.0 in | 15.0 in |
| 8:12 | ⅓ of W | 12.0 in | 16.0 in | 20.0 in |
| 12:12 | ½ of W | 18.0 in | 24.0 in | 30.0 in |
Those are the figures Table R1003.20 publishes, and the pattern underneath them is H = (W ÷ 2) × (rise ÷ 12), which is what lets you check a slope the table does not print. A 48 in chimney on a 7:12 should have H = 14 in; find 8 in and you have found your fault.
Plan check rejections
Five that recur, and what the reviewer is actually asking for in each case.
“No cricket shown” on a chimney you measured as under 30 in. Almost always the dimension question: you gave the across-slope width, the reviewer read the up-slope depth off the plan, or vice versa. Dimension both faces on the drawing and label which one is parallel to the ridge. That annotation makes the rejection unanswerable in your favour.
“Cricket height not in accordance with Table R1003.20” at an untabulated slope. The table prints 3:12, 4:12, 6:12, 8:12 and 12:12. At 5:12 or 7:12 there is no row, and a bare number looks invented. Show the working: H = (W ÷ 2) × (rise ÷ 12), with a line demonstrating that the same expression reproduces all five published rows exactly. That is a derivation, not an extrapolation, and it reads as one.
“Cricket height less than minimum flashing height”. On a shallow roof with a chimney just over the threshold, H lands under the 4 in that R905.2.8.3 requires for base flashing against a vertical wall. Detail the flashing at 4 in and note that the greater of the two governs. The reviewer is not asking you to change the cricket; they are asking you to show you noticed.
“Cricket covering not specified”. R903.2.2 permits exactly two things: sheet metal, or the same material as the roof covering. Name which, and if metal, name the thickness — 0.019 in nominal minimum per R903.2.1.
“Structural support of cricket not shown”. This one is fair. The cricket sections give a shape and say nothing about structure. On a tall cricket, on a wide chimney, or in snow country, the reviewer is entitled to ask what carries it.
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What you need on the roof before you can diagnose any of this
Most of these faults are confirmed with a tape, a torpedo level and a probe. Only after that does anything get ordered.
Malco S2R 3" Hand Seamer
- Breaks the flange and hems the edges on a site-formed cricket pan and counterflashing
- The cricket sheathing triangle has three different edge lengths and none arrive pre-bent
- R903.2.2 permits sheet metal as the covering, and site-formed metal is how most crickets get covered
Malco M2006 Offset Aviation Snips
- Offset handles keep your knuckles clear while cutting a long diagonal in flat sheet
- Both cricket valleys are the same length, so it is two identical mirrored cuts
- Left and right offsets exist because one of those two cuts always runs the wrong way
TAJIMA Chalk‑Rite CR301JF Jam Free Chalk Line
- The whole layout is one mark on the centreline and two chalk lines back to the corners
- A fine braided line snaps a crisp diagonal on OSB instead of a fat blue smear
- Get the apex distance right and the 45 degree valleys set themselves
Ice & Water Shield Roll
- The two cricket valleys are the places on a chimney that collect ice and debris
- A self-adhered membrane over the cricket and up the chimney face is cheap insurance
- Comes off the same roll already in the van for the eaves and the valleys
Guardian 00815 Bucket of Safe-Tie Rooftop Kit
- Chimney work is slow, two-handed, and happens on the steepest part of the roof
- A cricket on a 10:12 or 12:12 is well past where a roof is comfortably walkable
- Harness, rope grab, lanyard and anchor in one bucket rather than four purchases
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Inspection failures on a cricket that was designed correctly
- Roof cement instead of flashing. A trowelled bed of mastic where the cricket meets the brick is the most-cited chimney failure there is, and R903.2.2 does not list it as a covering. It also has a service life measured against sun exposure, not against the roof.
- Mortar instead of counterflashing. Pointing the base flashing into the joint with mortar and calling it counterflashed. R1003.20 requires the intersection to be flashed and counterflashed; those are two components.
- Metal too thin. Builder’s-merchant coil that measures under 0.019 in. It passes a glance and fails a micrometer.
- Base flashing under 4 in against the chimney. Easy to lose when the cricket is shallow and the installer is trying to keep the metal below a shingle course.
- Counterflashing ordered to the chimney width. It runs as a V and is longer than the face. Short pieces get butted mid-face, and a butt joint at the bottom of a V is a leak.
- Mixed metals in the valley. Galvanized fasteners into copper, aluminum against treated lumber, two different flashings lapped together. Table R905.2.8.2 gives you a list; staying inside one family of it is free.
- Fasteners through the valley centre. Nails in the lowest line of the only drainage path on the detail.
The ones that pass plan check and inspection and still fail
Right height, wrong apex. H can be correct at the chimney while the apex sits at the wrong distance up the roof. The valleys then are not at 45 degrees, the ridge is not level, and the cricket sheds unevenly — one valley carrying more than it was shaped for. Nothing on the drawing is wrong; the thing built is not what was drawn. The level on the ridge finds it in two seconds.
Twisted sheathing at the valleys. On the figure, both valleys lie wholly inside the main roof plane, so the cricket sheathing meets the deck flat. If it has been forced — packed on one side, or screwed down hard to close a gap — the valley metal above it will not sit either, and the installer will usually face-nail it to make it lie down. That is the leak.
A cricket built where there was no room for one. A standard cricket needs a fixed amount of roof above the chimney: W ÷ 2 in plan, more along the deck. Where the chimney sits close under a ridge, installers squash the cricket to fit. The result is shorter than the table, and squashing it also unlevels the ridge. This case needs an engineered detail or a design change, not a trim.
A debris trap by geometry. Under heavy tree cover, a correctly sized cricket on a shallow roof can still silt up, because the whole detail only has the slope the roof has. That is not a defect; it is a maintenance requirement, and it belongs in the handover rather than in a warranty claim two autumns later.
What to measure before you price the repair
- W, parallel to the ridgeline. Decides whether a cricket is required at all. Photograph the tape in place.
- The slope of that roof plane. Not an average of the roof.
- Existing H, if there is a cricket. Compare against the table for that W and slope.
- Level on the existing cricket ridge. Pass or fail, no interpretation needed.
- Deck available above the chimney. Confirms a correctly sized cricket will actually fit.
- How the counterflashing enters the masonry. Reglet, surface-mount or mortared. The biggest single cost variable on the job.
- Crown, cap and brick face condition. So that what you are not fixing is on the record.
- Sheathing soundness over the whole area the cricket will cover. Probe it, do not tap it.
Those eight, taken on one visit, are enough to produce a price that does not move. The calculator turns items 1, 2 and 5 into the target geometry, and comparing it to items 3 and 4 tells you whether you are repairing a cricket or replacing one.