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Low-Slope Roof Cricket Taper Calculator

Cricket thickness at a curb on a nearly flat roof, plus the tapered board courses that build it and whether the run available is enough.

  • Answers as you type
  • Every formula cited
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Imperial · sales tax
Width of the curb or obstruction measured across the direction water runs.

Measure the face the water arrives at, including any counterflashing or equipment overhang that would trap debris. Where two curbs sit close together across the flow, it is usually cheaper to build one cricket over the pair than two crickets with a dead area between them.

The slope built into each face of the cricket, as rise per twelve of run.

A cricket is normally built steeper than the field it drains into, because its job is to move water sideways across a surface that is already barely moving it. Steeper means thicker at the curb for the same width, so this input and the obstruction width together decide how much build-up the detail needs.

The slope of the main roof surface, as rise per twelve of run.

A quarter per twelve is the classic low-slope minimum and the case this page was written for. It matters here because the field is rising as it runs away from the curb, and the cricket has to shed its whole thickness against that rise before it can die out into the field.

Run upslope of the curb before the cricket has to die out into the field.

Bounded by whatever comes next upslope — another curb, a drain, a wall, an expansion joint. This is the length the ridge has to lose its whole thickness over, so a short run forces a steep ridge taper and can leave the cricket falling back toward the curb.

Length of one tapered board measured along the direction of its taper.

Tapered insulation is supplied in a standard board length with a stated rise across it, and the courses are laid with the taper running from the cricket's ridge down to its valley. Board length and slope together set how much thickness one course gains.

Cricket thickness at the curb

2 in

High confidence
Half-width the taper runs across
4 ft
Tapered board courses per side
1 course
Thickness gained per board course
2 in
Ridge taper over the available run
0.2 rise per 12
Net rise of the cricket ridge going upslope
0.05 rise per 12
Field rise over the same run
2.5 in
Cricket slope as a multiple of the field slope
2 x field slope
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • Thickness at the curb = half the obstruction width × the cricket's slope ratio. Board courses per side = half width ÷ taper board length, each course gaining board length × the slope ratio. Plane geometry; no code figure is involved.
  • NRCA Roofing Manual: Membrane Roof Systems — the reference describing crickets and saddles behind curbs on low-slope roofs. The cricket slope itself is a project specification and is entered here rather than assumed.
  • The ridge check is arithmetic on the two slopes entered: the ridge must lose its full thickness over the run available, so its taper rate is thickness ÷ run, and what is left of the field slope after that is the ridge's own rise going upslope.

Inputs used

Obstruction Width Across the Flow
8 ft
Cricket Slope (Rise per 12 of Run)
0.5
Field Slope (Rise per 12 of Run)
0.25
Run Available Upslope
10 ft
Tapered Board Length
4 ft

Intermediate steps

Half-width the taper runs across
4 ft
Tapered board courses per side
1 course
Thickness gained per board course
2 in
Ridge taper over the available run
0.2 rise per 12
Net rise of the cricket ridge going upslope
0.05 rise per 12
Field rise over the same run
2.5 in
Cricket slope as a multiple of the field slope
2 x field slope
Final result2 in

What this calculation does not cover

  • This is the build-up geometry only. Board layout at the valleys, the flat stock beneath a thick cricket, and the fastening pattern through it are separate exercises.
  • It does not check the membrane manufacturer's minimum thickness over the deck at the cricket's thinnest point.

Add the equipment this sizes

This result is a specification — 2 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

2.4°10 ft0.50.5/12
Schematic, drawn to the proportions you entered — not to scale on screen.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations3
  1. Thickness at the curb = half the obstruction width × the cricket's slope ratio. Board courses per side = half width ÷ taper board length, each course gaining board length × the slope ratio. Plane geometry; no code figure is involved.
  2. NRCA Roofing Manual: Membrane Roof Systems — the reference describing crickets and saddles behind curbs on low-slope roofs. The cricket slope itself is a project specification and is entered here rather than assumed.
  3. The ridge check is arithmetic on the two slopes entered: the ridge must lose its full thickness over the run available, so its taper rate is thickness ÷ run, and what is left of the field slope after that is the ridge's own rise going upslope.
Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Called something else where you work? Fall and gradient — the term in each market, how close the equivalence really is, and the standard that governs it.

How to calculate low-slope roof cricket taper in 6 steps

  1. Obstruction Width Across the FlowWidth of the curb or obstruction measured across the direction water runs.
  2. Cricket Slope (Rise per 12 of Run)The slope built into each face of the cricket, as rise per twelve of run.
  3. Field Slope (Rise per 12 of Run)The slope of the main roof surface, as rise per twelve of run.
  4. Run Available UpslopeRun upslope of the curb before the cricket has to die out into the field.
  5. Tapered Board LengthLength of one tapered board measured along the direction of its taper.
  6. Cricket thickness at the curbThe tool computes the cricket thickness at the curb from those figures and shows the formula, its sources, and a confidence rating alongside it.

Cricket thickness at the curb by obstruction width across the flow

Page defaults, not your figures above.

Obstruction Width Across the FlowCricket thickness at the curb (in)
4 ft1
6 ft1.5
8 ft2
10 ft2.5
12 ft3
14 ft3.5

Frequently asked questions

Why can the existing roof cricket calculator not do this?
Its pitch field will not go below one rise per twelve, which is four times steeper than the quarter per twelve a low-slope field commonly runs at. Entering the nearest permitted value would have quadrupled the slope and returned a thickness with nothing to do with the roof — a refusal, and then a separate page, was the honest answer.
How steep should the cricket itself be?
Steeper than the field, and the specification decides by how much. The reason is drainage speed: a cricket has to carry water sideways across a surface that is barely draining in the first place, so matching the field slope leaves it running very slowly around the curb. That is a design decision, which is why it is an input here.
What does the net ridge rise line actually tell me?
Whether the cricket dies out cleanly. The ridge has to lose all of its thickness over the run you have, while the deck beneath it is rising. If it sheds thickness faster than the deck rises, the ridge tips back toward the curb and you have built a shallow basin instead of a diverter — the fix is more run, a narrower span or a shallower cricket.
Can I build a cricket out of flat stock instead of tapered board?
In small sizes, by stepping flat boards and shaving the steps, though it is slow and the result is only as good as the shaving. Once the thickness at the curb runs to several courses, tapered stock cut to a shop drawing is faster and far more consistent — and the course count above is what that drawing starts from.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.