What the 30 inch rule requires, why the threshold exists, and the cases either side of it where the code minimum is not the right build
Two different details, one threshold between them
Behind every chimney that sits in a roof plane there is something diverting water around it. On a narrow chimney that something is usually a back pan: a single piece of formed metal with an upstand against the brick and a flange out onto the roof, slid under the course above. On a wide one the code requires a cricket (or saddle): a small double-sloped structure that sheds water sideways into two valleys instead of holding it across the whole back of the chimney.
The line between them, in both IRC R903.2.2 and IBC 1503.6, is 30 inches measured parallel to the ridgeline. Over that, a cricket. At or under it, no cricket is required — but something still has to be there.
| Back pan | Cricket / saddle | |
|---|---|---|
| What it is | One formed piece of metal, no framing | Two sloping planes meeting at a level ridge, framed or formed |
| How water leaves | Sideways along the chimney face, at whatever slope the pan has | Down two valleys at 45 degrees in plan, at the roof’s own pitch |
| Code position | Satisfies base and counterflashing where no cricket is required | Required over 30 in; built to Figure R1003.20 and Table R1003.20 |
| Height at the chimney | Whatever the upstand needs, minimum 4 in per R905.2.8.3 | H from Table R1003.20, which can be anything from 4 in to 30 in |
| Framing | None | Usually; sheet metal only on shallow ones |
| Debris behaviour | Collects, and the wider the chimney the longer it dwells | Sheds, because there is cross-slope all the way across |
| Fails by | Standing water and silt behind a wide chimney | Being undersized, or built with the apex in the wrong place |
Why the threshold exists at all
The code does not explain its 30 inches, but the physics is not mysterious and it is worth understanding, because it tells you which side of the line your judgement should fall on when the measurement is close.
A back pan has no cross-slope. Water arriving on it from the roof above has to travel along the chimney face to one end or the other, on a surface that is essentially level in that direction. The further it has to travel, the longer it dwells, the more likely it is to find a lap, and the more readily silt, pine needles and granules settle out of it and stay. Double the chimney width and you roughly double the worst-case travel distance while adding nothing to the slope driving it.
A cricket replaces that level run with two surfaces that slope at the roof’s own pitch, so every drop has a downhill path from wherever it lands. That is the reason the detail changes with width rather than with slope or with rainfall.
The decision, laid out
Code column and judgement column kept separate, deliberately — the first is what you must do, the second is what the detail wants.
| Chimney width parallel to ridge | Code requires | What usually wants building |
|---|---|---|
| Up to about 20 in | Base and counterflashing; no cricket | Back pan. A cricket here is over-building |
| 20 to 30 in, steep roof, open site | Base and counterflashing; no cricket | Back pan, confidently |
| 20 to 30 in, shallow roof, trees, or ice country | Base and counterflashing; no cricket | Consider a cricket anyway. This is the genuinely contested band |
| Exactly 30 in | No cricket — the code says more than 30 in | Build the cricket. You are one eighth of an inch from a requirement and the measurement is not that precise |
| Over 30 in, chimney in a roof plane | Cricket, to Table R1003.20 | Cricket. Not a discussion |
| Over 30 in, chimney through the ridgeline | No cricket — R1003.20 does not apply | Base and counterflashing all round. There is no roof above it |
| Over 30 in, but no roof above it to fit one in | Cricket — and it does not fit | Stop. Engineered detail or design change, not a squashed cricket |
| Wide unit skylight with a manufacturer kit | No cricket — named exception in both codes | The manufacturer’s flashing kit, to their instructions |
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The hardware either choice still depends on
The same metal rules govern a back pan and a cricket. Most chimney leaks live in the flashing, not in which detail was chosen.
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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Build a cricket even though you do not have to, when…
These are the cases in the 20–30 in band where the back pan is permitted and still the wrong answer. None of them is a code requirement; all of them are reasons a back pan will underperform.
- The roof is shallow. At 3:12 or 4:12 everything moves slowly and silt settles. The flatter the roof, the more the back pan’s lack of cross-slope matters.
- There is a long roof above the chimney. A chimney low down a long slope receives everything shed by the area above it. A chimney just under a ridge receives almost nothing.
- Trees. Needles and leaves do not drain. Once a back pan silts up it stops being a drainage detail and becomes a reservoir with a 4 in wall.
- Ice country. A level run of metal behind a chimney in a freeze-thaw climate backs up under exactly the conditions the ice dam guide describes.
- The customer is not going to maintain it. A back pan on a wide-ish chimney is a detail that assumes somebody clears it. Most do not.
- You are already stripping the deck. The marginal cost of framing a small cricket while the sheathing is open is far lower than coming back for it later.
The inverse also holds. On a 12:12 roof, a 26 in chimney a few feet below the ridge in an open, unwooded site, a properly formed and lapped back pan is a perfectly good detail and a cricket is cost without benefit.
Do not build a cricket when…
The chimney comes through the ridgeline. R1003.20’s two conditions are joined by “and”, and this fails the second. There is no upslope roof draining toward the chimney, so there is nothing for a cricket to divert. What this chimney needs is proper base and counterflashing on all four faces. Note that a chimney through a hip is a different case: roof still drains toward it on at least one side, and the 30 in test still applies there.
There is not enough roof above the chimney. A standard cricket projects W ÷ 2 up the slope in plan, which is further again measured along the deck. On a 60 in chimney that is 30 in in plan and about 35 in along the deck at 8:12. Where the ridge, a wall or a dormer is closer than that, the honest answer is that the tabulated detail does not fit. Shortening it changes H, unlevels the ridge and leaves you with something that is neither a cricket nor a back pan. This is an engineered detail.
The chimney is coming down. If the masonry is being rebuilt, reduced or removed within the roof’s life, the sequencing matters more than the detail. Building a cricket against brickwork that is about to be taken apart wastes both.
The real problem is the chimney. A cracked crown, a missing cap or saturated spalled brick all present as a leak at the chimney, and a new cricket fixes none of them. Diagnose first — the troubleshooting guide works through separating the two.
What does not change either way
Whichever detail you land on, the same code requirements apply to the metalwork, and most chimney leaks live here rather than in the choice itself.
| Requirement | Figure | Section |
|---|---|---|
| Flashed and counterflashed at the chimney | Two components, not one | IRC R1003.20 |
| Minimum metal thickness | 0.019 in nominal, No. 26 galvanized | IRC R903.2.1, R905.2.8.1 |
| Base flashing against a vertical wall | Minimum 4 in high, 4 in wide | IRC R905.2.8.3 |
| Covering material, where a cricket is built | Sheet metal, or the same material as the roof covering | IRC R903.2.2 |
| Open metal valley lining width | Minimum 24 in overall | IRC R905.2.8.2 |
Three practices follow from that list and apply to back pans and crickets equally: lap every layer over the one below it rather than sealing a reverse lap, keep fasteners out of any line water runs along, and stay inside one family of metal rather than lapping dissimilar ones together. The galvanic reasoning is the same as in aluminum vs galvanized steel drip edge, and it transfers straight across.
Putting the decision to bed in one visit
- Measure the chimney width parallel to the ridgeline. Over 30 in and the decision is made for you.
- Check whether the chimney intersects the ridgeline. If it does, neither detail’s sizing rules apply — flash all four faces.
- Take the slope of the roof plane the chimney sits in.
- Measure the deck available above the chimney. Confirm a tabulated cricket fits before promising one.
- Look up: how much roof drains toward this chimney, and is there anything shedding debris over it.
- Run the cricket calculator. If it says required, you are building one and now have the cutting list. If it says not required, you have the geometry anyway and can price both.
- If you land in the 20–30 in band, decide on the six judgement factors above rather than on the tape alone, and write down why — it is the answer to the question the customer asks in three years.