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Basis: IRC 2021 · R802 (Roof-Ceiling) · Table R802.5.1 (rafter spans) · ASCE 7 Ch. 7 (snow) · prescriptive tables only

Roof Rafter Calculator

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Roof Framing is fully usable — size the rafters, read the load analysis and the whole cut list on screen. Your inputs are kept in this browser automatically.

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Site profile

Choose your state to load site values. Nothing is assumed until you do.

Advancedcollar ties 48" o.c.

Choose your state to load site values.

Rafter size is selected against the ground snow load, and the snow load comes from where you are building. Until the site profile is settled there is nothing to size against — so the load analysis, the compliance checks and the takeoff below are not an answer for your roof.

Ridge — 6/12 pitch28.0' span + 12" overhangs
2x8rafters at 16" o.c.DOES NOT COMPLY
2x8 rafters at 16" o.c.Verification required · 79% used
Spans 17' 8"You need 13' 11 1/4"

Limited by bending strength.

Assumptions applied

IRC 2021 · Table R802.5.1 · NDS 2018 Supplement Table 4A/4B — AWC span methodology — simple span, uniform load, min of bending/shear/deflection — governed by bending

Code compliance

DOES NOT COMPLY

Something here does not comply.

InputsVERIFICATION REQUIRED

Every dimension needed for this calculation has been given and is usable.

Ridge design (Structural ridge beam — to be designed)ENGINEERING REQUIRED

With no rafter ties in the bottom third of the roof, the rafter pair has nothing resisting its outward thrust and the walls will spread. Either add rafter ties (commonly the ceiling joists, running parallel to the rafters), or carry the ridge on a structural beam and posts — which needs to be designed. A ridge BOARD will not do either job.

IRC 2021 R802.3 — ridge board and rafter connections· this rule is marked transcribed in our data, so it cannot return a PASS

Rafter size and span (2x8 at 16" o.c.)VERIFICATION REQUIRED
Your horizontal run: 13' 11 1/4"Maximum allowed: 17' 8"Margin: 3' 8 3/4"

Horizontal run of 13' 11 1/4", within the 17' 8" maximum we compute. That figure has not been verified against the printed table yet, so check it against your code book before you build.

IRC 2021 · Table R802.5.1 · NDS 2018 Supplement Table 4A/4B· this rule is marked unverified in our data, so it cannot return a PASS

Birdsmouth notch depth (2x8 over a 5.5" bearing)DOES NOT COMPLY
Your notch depth: 2 3/4"Maximum allowed: 1 13/16"

The birdsmouth removes 2.75" of the 7.25" actual depth — 38%, against a limit of 25% (1.81"). Three things change it: a DEEPER RAFTER (the limit is a fraction of depth, so a 2x10 gives you more), a NARROWER BEARING (the seat cut is as deep as the plate is wide — 5.5" here, and a 3.5" plate cuts shallower than a 5.5" one), or a SHALLOWER PITCH (the heel depth is the seat depth times the slope, so 6/12 cuts deeper than a 4/12 would). A bearing block under the rafter avoids the notch entirely.

IRC 2021 R802.7.1 — Maximum depth of a notch at the END of a rafter, as a fraction of its actual depth· this rule is marked unverified in our data, so it cannot return a PASS

Rafter ties — outward thrust at the wallNEEDS MORE INFO

No rafter ties. A conventional rafter pair pushes OUTWARD on the walls, and something has to resist that — normally the ceiling joists, running parallel to the rafters. Without a tie the walls spread and the ridge drops. Collar ties do NOT do this job.

IRC 2021 R802.4 — rafter ties and ceiling joists· this rule is marked transcribed in our data, so it cannot return a PASS

Site design values confirmed with the building departmentNEEDS MORE INFO

The 0 psf ground snow load is a state-level starting point, not a value for your address. ASCE 7 maps snow by county and elevation. Confirm it with your building department before this goes on a permit set.

ASCE 7-16 Ch. 7 — ground snow load Pg· this rule is marked unverified in our data, so it cannot return a PASS

Checked against IRC 2021 · NDS 2018 · ASCE 7-16. These checks are an aid, not an approval — your building department has the final say.

Not sure what rafter to use?

Cut dimensions

Building width28'
Horizontal runwall face to ridge centreline13' 11 1/4"
Pitch6/12 (26.6°)
Ridge heightabove the top plate6' 11 5/8"
Rafter spacing16" o.c.

Common rafter

Length, wall to ridgeno overhang15' 7"
Overhanghorizontal projection1'
Overhang, along the slopelonger than the projection1' 1 7/16"
Total cut lengthwhat you cut16' 8 7/16"

Cuts

Plumb cutridge and heel, from square26.6°
Seat cutlevel cut on the plate63.4°
Birdsmouth seat depthbearing width5 1/2"
Birdsmouth plumb depthheel cut2 3/4"
Rafter left above the notch4 1/2" (62%)

The maximum permitted notch depth is not checked here — verify the birdsmouth against R802.7.1.

Tie zones

Rafter ties — bottom thirdresist outward thrustup to 2' 3 7/8"
Collar ties — upper thirdresist uplift at the ridgeabove 4' 7 3/4"

Heights above the top plate. These are two different members doing two different jobs — a collar tie fitted low does not become a rafter tie.

Snow load coefficients

These multiply into the load your rafter is sized against. Anything marked assumed is our value, not yours — override it if your building department or engineer gives you a different one.

Pg — Ground snow load
Yours0

From your building department or the ASCE 7 Hazard Tool. ASCE maps this by county and elevation — a state-level figure is a starting point, not your address.

Ce — Exposure factor
Assumed

ASSUMED 0.9 — Exposure C, partially exposed, typical suburban. A sheltered site among trees or buildings holds more snow (up to 1.2); a windswept open site holds less.

Ct — Thermal factor
Assumed

ASSUMED 1.0 — a heated building. An unheated structure or a well-ventilated cold roof holds snow longer and takes 1.1 to 1.3.

Is — Importance factor
Assumed

ASSUMED 1.0 — Risk Category II, an ordinary house. Buildings whose failure endangers many people take more.

Cs — Roof slope factor
Assumed

COMPUTED from a 26.6° slope on a non-slippery surface with Ct = 1: full snow load is held to 30°, then falls linearly to zero at 70°. This is our reading of the ASCE 7 Fig. 7.4-1 graph, not a tabulated value.

The 20 psf minimum roof live load governs — your snow load works out lower. The rafter is checked against 20 psf.

Not for purchase — design does not comply

One or more checks failed. The quantities below show what this design would take, so you can see how far off it is — they are not an order. Fix the failing check first.

Material takeoff

Common rafters@ 16' 8 7/16"62 × 2x8
Structural ridge beam — to be designed@ 40'2x10
Collar ties@ 6' 11 5/8", 48" o.c. — upper third, uplift11 × 2x6
Rafter tiesbottom third — outward thrustNONE — see checks
Roof sheathing1336.1 sq ft ÷ 32 sq ft per 4×842 sheets
Framing lumber, as stock1112.25 lf
Waste allowance @ 10%+ 111.23 lf
Total to buy1223.47 lf

Waste is its own line. Sheathing is counted separately in sheets.

⚠ Site design values are not confirmed — this result is NOT for permit submittal. Verify each value below with your building department or the ASCE Hazard Tool, then check the confirm box in the site profile.

⚠ Ground snow load — not available. Choose your state to load site values.

⚠ Rafters — 20 psf ground snow, 10 psf dead, L/180

⚠ Ground snow load 0 psf rounded up to the 20 psf table band (the conservative direction).

⚠ Rafter spans are measured horizontally (the level run from wall to ridge), not along the slope.

⚠ The load duration factor for this table (CD = snow) is not yet confirmed against the printed table.

16.7 ft
Rafter length
62
Rafter count
0 psf
Ground snow load
30 psf
Governing load

ASCE 7 load analysis

Flat roof snow load (Pf): 0 psf

Sloped roof snow load (Ps): 0 psf

Dead load: 10 psf · Live/snow load: 20 psf

Cut list

Not for purchase — design does not comply

One or more checks failed. The quantities below show what this design would take, so you can see how far off it is — they are not an order. Fix the failing check first.

ComponentQtySizeLength
common rafter622x816' 8 7/16"
Estimated total board footage: 1380.8 bd ft

Not on the shopping list

These members are waiting on a compliance check. Nothing here has a quantity you should order — buying against an unresolved check is how the wrong part ends up on site.

ridge boardStructural ridge beam — size by design professional, not included in takeoff
collar tie11 × 2x6 at 6' 11 5/8"Unresolved input

Provisional — depends on the “roof.rafter-ties” check, which has not been settled.

Optimized cut list (minimized waste)

73
Boards to buy
1204 ft
Total stock length
91.8 ft
Estimated waste
7.6%
Waste rate

Roof Rafter Calculator

This calculator is for STICK-FRAMED COMMON RAFTERS. If you are ordering engineered trusses, the plant designs and seals them and their sealed drawing is what your building department wants — nothing here substitutes for it.

Size common rafters and get every dimension you cut to. Enter the building width, the pitch and your rafter spacing and the calculator returns the horizontal run, the rise, the sloped rafter length with and without the overhang, the plumb and seat cut angles, and the birdsmouth seat and heel depths.

Rafter spans are checked the way the code measures them: the HORIZONTAL run from the bearing wall to the ridge centreline, not the sloped length and not including the overhang. Getting that wrong is one of the most common rafter errors, and it goes the unsafe way when the sloped length is used as the span.

Snow load runs through the ASCE 7 flat-roof and sloped-roof calculation, and every coefficient — Ce, Ct, Is and Cs — is shown on screen with its value, whether it is a default or yours, and what would change it. The rafter is checked against the governing load, which is the greater of the snow load and the minimum roof live load.

Which code tables this works from

IRC Table R802.5.1 — rafter spans
Rafter spans by species, grade, spacing, ground snow band and dead load. Spans are the horizontal run, and the snow bands are 20, 30, 50 and 70 psf.
IRC R802.3 — ridge board and rafter connections
Below a 3:12 pitch the ridge must be a structural beam, not a ridge board, and the calculator refuses to produce a conventional design there.
ASCE 7 Chapter 7 — snow loads
Flat roof snow load Pf = 0.7 x Ce x Ct x Is x Pg, sloped roof load Ps = Cs x Pf, and the minimum roof snow load Pm on low-slope roofs.

The inputs that matter

Roof pitch
Sets the rise, the cut angles and the snow slope factor. Below 3:12 the whole configuration leaves conventional framing and needs a designed ridge beam.
Ground snow load
The single biggest driver of rafter size in snow country. ASCE 7 maps it by county and elevation — a state-level figure is a starting point, not your address.
Rafter ties
Rafter ties in the bottom third resist the outward thrust that spreads the walls. Without them a ridge board will not do — you need a structural ridge beam.
Ceiling attached to the rafters
A drywall ceiling nailed to the rafters tightens the deflection limit from L/180 to L/240, which can mean a deeper rafter.

Common questions

How do I calculate rafter length from the span and pitch?

The horizontal run is half the building width less half the ridge thickness. The rise is the run times the pitch over 12. The rafter length is the hypotenuse of those two. The overhang is a horizontal projection, so along the slope it adds more than its own length — on a 12/12 roof a 12 inch overhang adds about 17 inches of rafter.

What is a birdsmouth cut and how deep should it be?

A birdsmouth is the notch where the rafter sits on the top plate: a level seat cut as deep as the plate is wide, and a vertical heel cut at the back of it. On a 6/12 roof with a 5-1/2 inch plate the heel cut is 2-3/4 inches deep. The calculator gives both dimensions and the rafter depth left above the notch.

What is the difference between a collar tie and a rafter tie?

They are different members doing different jobs. A rafter tie sits low, in the bottom third of the roof height, and resists the outward thrust that pushes the walls apart — usually the ceiling joists. A collar tie sits high, in the upper third, and resists the rafter pair separating at the ridge under wind uplift. A collar tie fitted low does not become a rafter tie.

Do I need a ridge beam or a ridge board?

A ridge board is non-structural — it just aligns the rafters, and the rafter ties at the bottom are what hold the walls together. A ridge beam is structural and carries half the roof to posts. You need a beam if the pitch is below 3:12, or if there are no rafter ties. The calculator makes this determination and will not produce a ridge-board design where a beam is required.

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