Contractor Job Estimator
Price remodels and GC work, then send a quote with no internal numbers.
Excel or Google Sheets. Best on a computer.
$29.00
BuyChimney Cricket Calculator
One published table, five rows, and the geometry hiding inside it — height, valleys, ridge, flashing run and the metal blank
Any trade
Checkout opens in a new tab.
What turns those numbers into a watertight chimney
A cricket is a small amount of framing and a large amount 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 sheathing triangle above has three different edge lengths — none of them come pre-bent
- R903.2.2 lets the covering be sheet metal, 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 a flat sheet
- Both cricket valleys are the same length, which means 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 cricket valleys are the two places on a chimney that collect ice and debris
- A self-adhered membrane over the cricket and up the chimney face is cheap insurance
- Works out of the same roll you are already using at the eaves and in 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
As an Amazon Associate, TestTalkHQ earns from qualifying purchases. Prices and availability can change.
What the code actually says about crickets
There are three short passages, and between them they contain the whole requirement. It is worth reading them as written, because almost every argument about crickets on a job site comes from paraphrasing one of them.
| Section | Scope | What it says |
|---|---|---|
| IRC R903.2.2 | Roof assemblies, all penetrations | A cricket or saddle on the ridge side of any chimney or penetration more than 30 in wide measured perpendicular to the slope; covering to be sheet metal or the same material as the roof covering |
| IRC R1003.20 | Masonry chimneys | Crickets where the dimension parallel to the ridgeline is greater than 30 in and does not intersect the ridgeline; flashed and counterflashed as a normal roof-chimney intersection; built to Figure R1003.20 and Table R1003.20 |
| IBC 1503.6 | Everything outside the IRC | Word for word the R903.2.2 rule, same 30 in, same covering requirement, same unit-skylight exception |
Three things in that table do real work on a job.
“More than 30 inches” means more than. A chimney measuring exactly 30 in parallel to the ridgeline does not cross the threshold. A 30½ in chimney does. That is a narrow distinction and it is the code's, not ours — both R903.2.2 and IBC 1503.6 say “more than” and “greater than” respectively, not “30 inches or more”.
“Perpendicular to the slope” and “parallel to the ridgeline” are the same measurement. R903.2.2 words it one way and R1003.20 the other, and people read them as two different dimensions. They are not. Stand at the eave and look up at the chimney: the dimension you see across is the one that counts. The depth the chimney projects up the slope does not appear anywhere in the sizing.
Measuring the wrong face is the single most common cricket error. A chimney that is 24 in across the slope and 48 in up the slope does not need a cricket under either section — and a chimney that is 48 in across and 24 in up does. The numbers are identical; only the orientation differs, and the orientation is the whole rule. If the chimney is not square, write down which dimension is parallel to the ridge before you leave the roof.
The ridgeline condition is a genuine exemption. R1003.20 joins its two conditions with “and”. A chimney wider than 30 in that comes up through the ridge has no roof plane above it, nothing draining toward it, and no cricket requirement. R903.2.2 gets to the same place by requiring the cricket “on the ridge side” — if there is no ridge side, there is nothing to install.
Table R1003.20, and the formula hiding inside it
Here is the published table, exactly as the code prints it. W is the chimney width parallel to the ridgeline; H is the cricket height at the chimney.
| Roof slope | H | As a decimal of W | (W / 2) × (rise / 12) |
|---|---|---|---|
| 12:12 | ½ of W | 0.500000 | 0.500000 |
| 8:12 | ⅓ of W | 0.333333 | 0.333333 |
| 6:12 | ¼ of W | 0.250000 | 0.250000 |
| 4:12 | ⅙ of W | 0.166667 | 0.166667 |
| 3:12 | ⅛ of W | 0.125000 | 0.125000 |
The last two columns are identical in every row, to every digit. That is not a coincidence and it is not a curve fit through five points:
H = (W ÷ 2) × (rise ÷ 12)
It is worth being precise about what has been established and what has not. The formula is derived — it falls out of the figure the table is drawn from. The five rows then confirm the derivation, because a wrong derivation would not land on all five published fractions exactly. Those two facts together are why this page is willing to compute H at 5:12 and 7:12, slopes the code does not print: the rule being extended has already been checked against every case the code does print.
Worked: a 36 in chimney on a 6:12 roofTable R1003.20 at 6:12 gives H = ¼ of W.
¼ of 36 in = 9 in.
Closed form: (36 ÷ 2) × (6 ÷ 12) = 18 × 0.5 = 9 in.
Same number, which is the point.
¼ of 36 in = 9 in.
Closed form: (36 ÷ 2) × (6 ÷ 12) = 18 × 0.5 = 9 in.
Same number, which is the point.
Why the formula means the cricket ridge is level
This is the part that saves time on the roof, so it is worth following once.
Set up coordinates on the deck. Put the chimney's upslope face on the line y = 0, with y running up the slope in plan and x running across it. Let m = rise ÷ 12, so the deck surface is the plane z = my, measuring heights from the deck line at the chimney. Centre the chimney on x = 0, so its two upslope corners are at x = ±W÷2.
Now run the two valleys at 45 degrees in plan from those corners, the way Figure R1003.20 draws them. Two lines starting W÷2 apart on each side of the centreline and closing at 45 degrees meet on the centreline after travelling W÷2 up the slope in plan. So the cricket apex sits at plan position (0, W÷2), and since it is a point on the deck its height is m × W÷2.
The cricket ridge runs from that apex down to the chimney face. Its height at the chimney is H. Its height at the apex is mW÷2. And the table says H = mW÷2. The two ends of the ridge are at the same height.
So the cricket ridge is level, and you can set it with a torpedo level. Mark the apex up the deck, strike the two 45 degree valley lines back to the chimney corners, and run the ridge board dead level off the chimney face. If the ridge comes out level, the cricket is the one the table describes. If it does not, something upstream is wrong — usually the apex distance.
Three more consequences fall out of the same figure, and all three are things you would otherwise have to work out on the roof:
- The valleys lie flat in the main roof plane. Both end points are on the deck, and the line between two points of a plane is in that plane. So there is no twist to cut into the valley — the cricket sheathing lands on the deck cleanly on both sides.
- The cricket's own planes slope at exactly the same pitch as the roof. A cricket on an 8:12 roof is itself an 8:12 surface, just running sideways. You already own the speed square setting.
- The apex is W÷2 up the slope in plan, so measured along the deck with a tape it is a little further: W÷2 × the square root of (1 + m²). On a 12:12 that is 41 per cent further than the plan figure, which is more than enough to put a cricket in the wrong place if you confuse the two. The calculator prints both.
Every length on one cricket, in one place
Using m = rise ÷ 12 throughout:
| Dimension | Formula | 36 in chimney, 6:12 | 48 in chimney, 10:12 |
|---|---|---|---|
| Cricket height at the chimney, H | (W ÷ 2) × m | 9.000 in | 20.000 in |
| Apex: run up the slope, in plan | W ÷ 2 | 18.000 in | 24.000 in |
| Apex: run measured along the deck | (W ÷ 2) √(1 + m²) | 20.125 in | 31.241 in |
| Cricket ridge, level, true length | W ÷ 2 | 18.000 in | 24.000 in |
| Valley true length, each of two | (W ÷ 2) √(2 + m²) | 26.077 in | 39.395 in |
| Chimney-face edge of each plane | (W ÷ 2) √(1 + m²) | 20.125 in | 31.241 in |
| Counterflashing run across the face | W √(1 + m²) | 40.249 in | 62.482 in |
| Cricket area, both planes | (W² ÷ 4) √(1 + m²) | 362.2 sq in | 749.8 sq in |
| Slope of the cricket's own planes | rise : 12 (unchanged) | 6:12 | 10:12 |
| Valley plan angle | 45 degrees | 45 deg | 45 deg |
Note how differently those grow. H scales with W and with slope, so it climbs fastest: a 48 in chimney on a 10:12 wants a cricket more than twice as tall as a 36 in chimney on a 6:12. The valleys and the ridge, by contrast, only scale with W and barely notice the slope. That is why wide chimneys on steep roofs look so different from the sketch in everyone's head — a 20 in tall cricket is a structure, not a bit of flashing.
The counterflashing run is longer than the chimney is wide. It follows a V up and back down the chimney face, so on a 48 in chimney at 10:12 that is 62 in of reglet or surface-mount counterflashing across a 48 in face — plus whatever returns onto the chimney's sides. Ordering 48 in because the chimney is 48 in is how you end up driving back to the supplier.
Where the 4 inch flashing minimum beats the table
H gets small quickly on a low-slope roof. R905.2.8.3 does not care: “Base flashing against a vertical sidewall shall be continuous or step flashing and shall be a minimum of 4 inches in height and 4 inches in width.”
| Chimney width | Slope | H from Table R1003.20 | Which one governs |
|---|---|---|---|
| 31 in | 3:12 | 3.875 in | The 4 in flashing minimum |
| 36 in | 3:12 | 4.500 in | The cricket height |
| 32 in | 3:12 | 4.000 in | Exactly on the line |
| 48 in | 3:12 | 6.000 in | The cricket height |
| 36 in | 6:12 | 9.000 in | The cricket height, comfortably |
So on the shallowest slopes the code tabulates, a chimney only just over the 30 in threshold produces a cricket shallower than the flashing that has to cover it. There is no contradiction in the code — one section sets a shape, the other a minimum metal dimension, and the larger of the two is what you build to. But it does mean that at 3:12 a cricket on a 31 in chimney is almost entirely a flashing exercise.
The calculator flags this case rather than silently printing a figure you cannot build a flashing to.
What the code tells you to cover it with
R903.2.2 is specific and short: “Cricket or saddle coverings shall be sheet metal or of the same material as the roof covering.” Two permitted options, and nothing else. Mortar is not one of them, and neither is roof cement.
If you go the sheet metal route, the thickness minimum comes from R903.2.1 — corrosion-resistant metal of not less than 0.019 in nominal, which is No. 26 galvanized sheet. R905.2.8.1 repeats the same 0.019 in for base and cap flashing on asphalt shingle roofs, and allows mineral surfaced roll roofing weighing at least 77 lb per 100 sq ft as a base flashing alternative — but cap flashing has to be metal.
The cricket's two valleys are valleys, so the valley lining rules apply to them. For an open valley lined with metal, R905.2.8.2 requires the lining to be at least 24 in wide and made from one of the metals in Table R905.2.8.2:
| Valley lining material | Minimum thickness | Gage | Weight |
|---|---|---|---|
| Cold-rolled copper | 0.0216 in nominal | — | ASTM B 370, 16 oz per sq ft |
| Lead-coated copper | 0.0216 in nominal | — | ASTM B 101, 16 oz per sq ft |
| High-yield copper | 0.0162 in nominal | — | ASTM B 370, 12 oz per sq ft |
| Lead-coated high-yield copper | 0.0162 in nominal | — | ASTM B 101, 12 oz per sq ft |
| Aluminum | 0.024 in | — | — |
| Stainless steel | — | 28 | — |
| Galvanized steel | 0.0179 in | 26 (zinc coated G90) | — |
| Zinc alloy | 0.027 in | — | — |
| Lead | — | — | 2.5 lb |
| Painted terne | — | — | 20 lb |
For a closed valley — shingles woven or cut over the top, which is how most residential crickets finish — R905.2.8.2 item 3 permits one ply of smooth roll roofing at least 36 in wide, or either of the open-valley linings, or a self-adhering polymer modified bitumen underlayment complying with ASTM D 1970 in lieu of the lining.
The galvanized steel row is the useful cross-check. Table R905.2.8.2 calls for 0.0179 in, 26 gage, zinc coated G90 as a valley lining, while R903.2.1 calls No. 26 galvanized 0.019 in nominal. Those are the same sheet described two ways — nominal versus minimum — and not two conflicting requirements. If a plan reviewer queries it, that is the answer.
What this calculator will not do
It does not size the framing. Table R1003.20 gives a shape, not a structure. Ridge board, valley members and sheathing thickness are not in the table and nothing here should be read as a lumber schedule. On a cricket 20 in tall carrying snow at the top of a 10:12, that is a real question for the designer rather than a rule of thumb.
It does not do wind, snow or dead load. A cricket collects drifted snow behind a chimney by design — diverting water is the whole point and snow does not read the drawing. None of that is in the sections captured here. Use the roof snow load calculator for the load side.
It does not compute a non-standard cricket. Everything on this page assumes the 45 degree valley plan angle that the five tabulated rows imply. Stretch the cricket further up the roof or squash it shorter and H changes, the ridge stops being level, and you are no longer building the detail the table describes. The honest answer there is an engineered detail, not a different number from the same tool.
It does not do the masonry. Clearance from combustibles, chimney height above the roof, flue sizing, caps, crowns and the condition of the existing brickwork are separate chapters and separate problems. A perfect cricket on a chimney that is spalling will still leak.
It does not pick a product or price the job. The takeoff figures are geometry with your own waste percentage applied. They are not a quote, and the manufacturer's installation instructions for your shingle or membrane may ask for laps and terminations that change the quantities.
Frequently asked questions
When is a chimney cricket required by code?
IRC Section R903.2.2 requires a cricket or saddle on the ridge side of any chimney or penetration more than 30 inches wide as measured perpendicular to the slope. IRC Section R1003.20 states the same threshold for masonry chimneys as the dimension parallel to the ridgeline, and adds a second condition: the chimney must also not intersect the ridgeline. IBC Section 1503.6 carries the identical 30 inch rule for buildings outside the scope of the IRC. A chimney measuring exactly 30 inches does not cross the threshold, because both sections say more than and greater than rather than 30 inches or more.
How do you calculate the height of a chimney cricket?
IRC Table R1003.20 gives the cricket height H as a fraction of the chimney width W measured parallel to the ridgeline: one half of W at a 12:12 roof slope, one third at 8:12, one quarter at 6:12, one sixth at 4:12 and one eighth at 3:12. All five of those rows are exactly H equals W divided by two, times rise divided by twelve. So a 36 inch chimney on a 6:12 roof takes a cricket 9 inches tall at the chimney, which is both one quarter of 36 and 18 multiplied by 0.5.
Which chimney dimension do you measure for a cricket?
The dimension parallel to the ridgeline, which is the same thing as the dimension perpendicular to the slope. It is the width of the chimney as you look up the roof at it, not the depth it projects up the slope. A chimney 24 inches across the slope and 48 inches up the slope needs no cricket under either section, while a chimney 48 inches across and 24 inches up does. The numbers are the same and only the orientation differs, so write down which dimension is parallel to the ridge before leaving the roof.
Is the cricket ridge level or sloped?
Level. Because Table R1003.20 sets H equal to W divided by two times the slope ratio, and because the cricket apex sits a plan distance of W divided by two up the slope on the chimney centreline, both ends of the cricket ridge end up the same height above the deck line at the chimney. That makes the ridge horizontal, and it is the easiest field check there is: mark the apex, snap the two 45 degree valleys, and run the ridge board dead level off the chimney face.
How far up the roof does a chimney cricket extend?
Half the chimney width, measured horizontally. Along the roof deck, which is what a tape measures, that becomes W divided by two multiplied by the square root of one plus the slope ratio squared. For a 48 inch chimney on a 10:12 roof the plan distance is 24 inches but the tape reads 31.24 inches, a difference of more than seven inches. Confusing the two puts the apex in the wrong place and throws both valleys out.
What angle are the cricket valleys?
Forty-five degrees in plan on both sides, which is what makes the cricket apex land exactly half the chimney width up the slope and is what gives Table R1003.20 its particular set of fractions. The true three-dimensional length of each valley is W divided by two multiplied by the square root of two plus the slope ratio squared. Both valleys lie wholly within the main roof plane, because both of their end points do, so there is no twist to cut into the cricket sheathing where it meets the deck.
What pitch should the cricket itself be?
The same rise per twelve as the main roof. That falls out of the geometry of Figure R1003.20 rather than being a separate rule: the two cricket planes slope at exactly the roof slope, just running sideways across it instead of up it. A cricket on an 8:12 roof is itself an 8:12 surface, so the speed square setting you are already using for the rafters is the one you need for the cricket.
What can a chimney cricket be covered with?
IRC Section R903.2.2 permits two options and no others: sheet metal, or the same material as the roof covering. Where metal is used, Section R903.2.1 requires corrosion-resistant metal of not less than 0.019 inch nominal thickness, which is No. 26 galvanized sheet, and Section R905.2.8.1 repeats that minimum for base and cap flashing on asphalt shingle roofs. Mortar and roof cement are not permitted coverings. The cricket valleys are valleys, so the valley lining provisions of Section R905.2.8.2 and Table R905.2.8.2 apply to them as well.
Does a chimney that comes through the ridge need a cricket?
No. IRC Section R1003.20 joins its two conditions with and: the chimney must be wider than 30 inches parallel to the ridgeline and must not intersect the ridgeline. A chimney that straddles or cuts the ridge has no roof plane above it and nothing draining toward it, so there is nothing for a cricket to divert. Section R903.2.2 reaches the same conclusion by requiring the cricket on the ridge side, which does not exist in that configuration. A chimney through a hip is a different case, because roof still drains toward it on at least one side.
What if the cricket height comes out less than 4 inches?
Then the flashing minimum governs rather than the cricket height. IRC Section R905.2.8.3 requires base flashing against a vertical sidewall to be a minimum of 4 inches in height and 4 inches in width, and on the shallowest slopes the code tabulates a chimney only just over the 30 inch threshold produces a smaller H than that. A 31 inch chimney on a 3:12 roof gives H equal to 3.875 inches, below the minimum. There is no conflict in the code; one section sets a shape and the other a minimum metal dimension, and you build to the larger of the two.
Can I make a chimney cricket shorter so it fits under a ridge?
Not and still be building the detail Table R1003.20 describes. The table's five fractions are what they are because the valleys run at 45 degrees in plan, which puts the apex half the chimney width up the slope. Pull the apex closer to the chimney and H changes, the ridge stops being level, and the figures no longer come from the table. Where a wide chimney sits too close under a ridge for a standard cricket to fit, that is an engineered detail or a design change, not a trimming exercise. The calculator checks the available roof above the chimney and says so rather than quietly resizing.
Is a saddle the same thing as a cricket?
The code treats them as alternatives for the same purpose. Both R903.2.2 and IBC 1503.6 say cricket or saddle throughout, and R1003.20 is headed Chimney crickets while referring to the same detail. In the field the two words usually describe the same small double-sloped structure behind a chimney, with saddle sometimes reserved for a sheet metal version and cricket for a framed one. Nothing in the sizing changes either way, and the covering rule applies to both.
What about a skylight wider than 30 inches?
Both R903.2.2 and IBC 1503.6 apply the 30 inch rule to any chimney or penetration, so a wide skylight curb is caught by the same words. Both sections then carve out an exception: unit skylights installed in accordance with the unit skylight provisions and flashed in accordance with the manufacturer's instructions are permitted to be installed without a cricket or saddle. That exception is specific to unit skylights with manufacturer flashing kits, and it does not extend to site-built curbs or to other wide penetrations.
How much counterflashing does a cricket need?
More than the chimney is wide. The cricket meets the chimney along a V that rises from each upslope corner to the ridge height H at the centreline, so the length of that line across the face is W multiplied by the square root of one plus the slope ratio squared, plus whatever returns you run onto the chimney sides. On a 48 inch chimney at 10:12 that is a little over 62 inches across a 48 inch face. Section R1003.20 requires the cricket-to-chimney intersection to be flashed and counterflashed in the same manner as a normal roof-chimney intersection.