Three measurements before you come down, a four-mark layout, and the order the metal goes on in — the job around the one line of arithmetic
The arithmetic is one line. The job is everything around it.
Table R1003.20 is five rows long, and the calculator turns it into a cutting list in about ten seconds. Nothing on this page argues with any of that. What this page is about is the other ninety per cent: which dimension to put the tape on, what you do when the number comes out and the chimney turns out not to need a cricket at all, how to set the layout in four marks instead of forty, and the order the metal goes on in — because a cricket built to exactly the right height with the counterflashing lapped the wrong way is a leak with good paperwork.
The sequence below is survey, decide, strip, lay out, frame, membrane, metal, shingle, hand over. It is written for a reroof on a masonry chimney, which is where most crickets get built, with notes where new construction differs.
Before you come down: three measurements and one photograph
Everything downstream depends on getting these right while you are still up there, because none of them can be recovered from the ground.
| Take this | How | Why it matters |
|---|---|---|
| W — the chimney width parallel to the ridgeline | Tape across the finished masonry face, face to face, on the upslope side | It is the only dimension in Table R1003.20, and it is the one that decides whether a cricket is required at all |
| Roof slope of the plane the chimney sits in | Speed square and level on the deck, or an inclinometer; not the average of the whole roof | H scales directly with it, and it fixes the cricket’s own pitch |
| Headroom — deck from the chimney’s upslope face to the ridge or wall above | Tape flat on the deck, straight up the slope | A standard cricket needs a fixed amount of roof above the chimney. If it is not there, this is a design problem and you want to know now |
The photograph is of the chimney’s upslope face with the tape still on it, so that when somebody later asks “are you sure that was the ridge-parallel dimension?” there is an answer.
If you are not confident in the slope figure, take it properly — the roof pitch and slope guide covers measuring it on the deck, off a rafter and from the ground, and the pitch calculator converts between rise per 12, degrees and slope factor. An error of a whole pitch at 6:12 moves H by about 17 per cent.
Step 1 — Decide whether a cricket is actually required
Two conditions, joined by “and”, from R1003.20: the dimension parallel to the ridgeline is greater than 30 inches, and the chimney does not intersect the ridgeline. R903.2.2 and IBC 1503.6 get to the same place with “more than 30 inches wide… on the ridge side”.
| Situation | Cricket required? | What you build |
|---|---|---|
| W over 30 in, chimney in a roof plane | Yes | A cricket or saddle to Figure R1003.20 and Table R1003.20 |
| W exactly 30 in, chimney in a roof plane | No — the code says more than | Base and counterflashing, in practice a back pan |
| W 30 in or under | No | Base and counterflashing, in practice a back pan |
| Any width, chimney through the ridgeline | No — no roof above it | Base and counterflashing on all four sides |
| Wide unit skylight with a manufacturer flashing kit | No — named exception | The manufacturer’s kit, installed to their instructions |
“Not required” is not the same as “nothing goes there”. A 28 in chimney still needs base flashing and counterflashing on its upslope face, and on a low-slope roof under trees a cricket on a 28 in chimney is often the right call anyway — that trade-off is the whole subject of cricket vs back pan.
One more case worth naming: a chimney coming up through a hip is not a chimney through the ridgeline. Roof still drains toward it from at least one side, so the 30 in test still applies to whichever upslope face has roof above it.
Step 2 — Strip back and look at what the cricket will sit on
The area immediately upslope of a chimney is, structurally, the wettest square metre on most roofs. If there is no cricket there now, or there is one that has failed, you should expect to find something.
- Sheathing. Probe it. Soft or delaminated panels behind a chimney are normal, not exceptional, and framing a cricket onto rotten decking is work you will do twice.
- The old flashing. Note whether the previous counterflashing was let into a reglet, surface-mounted and sealed, or simply mortared in. That decides how much masonry work is in front of you, and it is the single biggest cost variable on a chimney.
- The masonry itself. Spalled brick, a cracked crown, a missing cap. A new cricket will not stop water that is coming down the inside of the flue or soaking through a failed crown, and this is the moment to say so to the customer rather than after the scaffold is down.
- Existing cricket geometry, if there is one. Measure its height before you pull it off. A previous installer’s undersized cricket is the most common reason a chimney leaks with a cricket already fitted.
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What turns the layout into a watertight chimney
A cricket is a little framing and a lot of metalwork. The height comes off the table; whether it still keeps water out in five years comes down to how the pan, the valleys and the counterflashing were formed and lapped.
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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Step 3 — The layout, in four marks
This is where the geometry earns its keep. Because Table R1003.20’s five rows are exactly H = (W ÷ 2) × (rise ÷ 12), the cricket in the figure has its valleys at 45 degrees in plan and its ridge dead level. That turns the layout into four marks:
- Centre mark. Halfway across the chimney’s upslope face, on the deck. Everything is symmetrical about it.
- Apex mark. Measure straight up the slope from the centre mark, along the deck, the distance the calculator prints as reaches up the deck. This is not half the chimney width — that is the plan figure. On a 48 in chimney at 10:12 the plan distance is 24 in but the tape reads 31¼ in. Using the plan figure puts the apex seven inches too low and throws both valleys.
- Two valley lines. Snap from the apex mark back to each upslope corner of the chimney. If your apex mark is right, these come in at 45 degrees in plan without you setting an angle.
- Ridge height mark. H up the chimney face on the centreline. Then check it: run a level from that mark back to the apex. It should be flat. If it is not, the apex distance is wrong.
Two further facts fall out of the same figure and both save time. The cricket’s own planes slope at the same rise per 12 as the main roof, so the speed square setting already in your hand is the right one. And both valleys lie wholly within the main roof plane, because both of their end points do — so the cricket sheathing lands flat on the deck on both sides with no twist to cut in.
Step 4 — Framing it
A warning first, and it is not a formality: Table R1003.20 gives a shape, not a structure. Nothing in the cricket sections specifies a member size, a span, a bearing detail or a fastener schedule. Everything in this section is ordinary carpentry practice, and on anything tall, wide, or in snow country it is a question for the designer rather than a rule of thumb. A 72 in chimney on a 10:12 wants a 30 in tall cricket; that is a structure, and it will collect drifted snow by design. Run the roof snow load calculator before you decide it is a flashing detail.
In practice there are three ways crickets get built, and the choice is mostly about H:
| Method | Typical when | Notes |
|---|---|---|
| Formed sheet metal pan, no framing | H is small — a modest chimney on a shallow slope | Permitted outright: R903.2.2 allows sheet metal as the covering. Nothing to rot. Needs support if it can be stood on |
| Solid bevel-cut blocking, sheathed over | H small to moderate | Quick and stiff, no cavity to trap moisture, heavier on material |
| Framed: level ridge board plus valley members and jacks, sheathed | H is substantial, or the cricket is wide | The conventional build. Ventilate or seal the cavity deliberately rather than by accident |
Whichever you use, three things hold across all of them. The ridge runs level off the chimney face. The valleys bear on the deck, not on air, because that is the line all the water will travel. And nothing is fastened through the valley line itself if it can possibly be avoided — see the metal section below.
Step 5 — Membrane, in the right order
The order is the point. Water runs downhill, so every layer must lap over the one below it, and that sequence has to be decided before the first piece goes on rather than patched afterwards.
- Membrane out onto the main roof deck first, well beyond where the valleys will land.
- Membrane over the cricket itself, both planes, lapped over the main-roof layer below.
- Membrane turned up the chimney face, lapped over the cricket layer.
- Only then any metal.
Self-adhered polymer modified bitumen complying with ASTM D 1970 is explicitly contemplated by the code here: R905.2.8.2 item 3 permits it in lieu of a closed-valley lining. For how much of it to buy and where else on the roof it belongs, the live ice and water shield coverage calculator and the ice dam guide already cover the ground and there is no sense repeating it here. The choice between membrane types is covered in ice and water shield vs synthetic felt.
Step 6 — The metal, and the dimensions the code actually sets
Four numbers come straight out of the code and are worth having in your head:
| Requirement | Figure | Section |
|---|---|---|
| Minimum metal flashing thickness | 0.019 in nominal (No. 26 galvanized) | IRC R903.2.1, repeated for base and cap flashing in R905.2.8.1 |
| Base flashing against a vertical sidewall | Minimum 4 in high and 4 in wide | IRC R905.2.8.3 |
| Open valley lined with metal | Minimum 24 in wide overall | IRC R905.2.8.2 item 1 |
| Closed valley lining | One ply smooth roll roofing at least 36 in wide, or an open-valley lining, or ASTM D 1970 membrane | IRC R905.2.8.2 item 3 |
Three practical consequences.
The counterflashing run is longer than the chimney is wide. It follows a V up the chimney face from each upslope corner to the ridge height, so the length across the face is W × √(1 + slope ratio²), plus the returns onto the chimney sides. On a 48 in chimney at 10:12 that is a little over 62 in across a 48 in face. Ordering 48 in because the chimney is 48 in is a second trip to the supplier.
On a shallow roof the 4 in minimum can be taller than the cricket. A 31 in chimney on a 3:12 gives H = 3⅞ in, which is under the R905.2.8.3 minimum. There is no conflict — one section sets a shape and the other a minimum metal dimension, and you build to the larger. But it does mean that at the shallow end a cricket is almost entirely a metalwork exercise.
Mind what the metal is touching. Dissimilar metals in a wet valley is a real failure mode rather than a theoretical one, and the compatibility question is the same one covered in aluminum vs galvanized steel drip edge — the reasoning transfers directly. Table R905.2.8.2 lists copper, lead-coated copper, high-yield copper, aluminum at 0.024 in, 28 gage stainless, 26 gage G90 galvanized steel at 0.0179 in, zinc alloy at 0.027 in, lead and painted terne. Pick one family and stay in it.
Step 7 — Shingling in, and the handover
The cricket’s two valleys get finished exactly like any other valley on the roof, open or closed. Closed is more common on residential crickets simply because the valleys are short. Whichever you choose, the valley centre stays clear of fasteners and the shingle courses run through cleanly rather than being cut to a wandering line.
Before the scaffold comes down, four photographs are worth having: the bare framed cricket with the level on the ridge, the membrane sequence before the metal went on, the counterflashing where it enters the masonry, and the finished detail. On a chimney, the thing that leaks is usually the thing nobody can see any more.
What an inspector will look at, from the sections quoted above:
- Is there a cricket at all, where W is over 30 in and the chimney does not break the ridge.
- Is the covering sheet metal or the same material as the roof covering — not roof cement, not mortar.
- Is the intersection flashed and counterflashed, as R1003.20 requires.
- Does the metal meet 0.019 in nominal, and does base flashing against the chimney make 4 in.
- Does the shape match the table — which, on site, means: is H right for W and the slope, and is the ridge level.