IRC R802.10 and ANSI/TPI 1: a truss is a sealed engineered design
Every other truss calculator will hand you a member size. None of them is entitled to. A metal-plate-connected truss is a delegated engineered design: IRC R802.10.1 requires a sealed drawing carrying thirteen specified items, approved by the building official before installation. This gives you everything that genuinely is arithmetic, and tells you exactly what to hand the truss supplier.
Truss quantities
ft
ft
in
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no.
Counts and geometry only. Member sizing is a sealed design and is not offered here.
Specification summary
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Reopen that address to reproduce these figures exactly.
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Result
What you can count, and what needs a sealed drawing
Worked example
A 28 ft span, 40 ft long roof, trusses at 24 in on centre, 6 in 12 pitch, 12 in overhang, two gable ends.
Count. 40 ft at 24 in is 20 bays, so 21 trusses, of which 2 are gable ends and 19 are commons.
Rise. A 14 ft run at 6 in 12 gives 7 ft at the peak.
Roof area.1,409 sq ft including the overhangs, or 14.1 squares.
Sheathing.49 sheets of 4x8 with 10% waste.
Member sizes. Not here, and not anywhere else that is honest. Those come from the sealed drawing.
The last line is the point of the page. You can order the right number of trusses from this, and buy the right amount of sheathing, and you still cannot tell anybody what the top chord is.
The formula
Counts and right-angle triangles. Nothing here touches the truss itself:
n = ⌈ 12L / s ⌉ + 1 rise = (W/2)(p/12) slope = √((W/2)² + rise²)
L
roof length, ft
s
truss spacing, in on centre
W
span bearing to bearing, ft
p
pitch, rise in 12
slope
half the top chord length before the overhang is added
The overhang is added along the slope, not horizontally, which is the small error that makes hand takeoffs come up short. A 12 in overhang on a 6 in 12 roof adds 13.4 in of top chord, not 12.
Why no honest calculator sizes a truss
A rafter can be sized from a table because it is one member in bending with a known span and a known load. A truss is a determinate frame of ten or twenty members, each carrying axial force, joined by proprietary metal connector plates whose capacity depends on plate size, tooth pattern and orientation. There is no table because there is no general case.
The code handles this by delegating. IRC R802.10.2 sends truss design to accepted engineering practice and ANSI/TPI 1, and R802.10.1 requires a truss design drawing, carrying thirteen specified items, approved by the building official before installation. Any tool that outputs a member size is producing a number nobody will seal.
What is genuinely useful, and what this page does, is the arithmetic either side of that design: how many trusses, at what spacing, what the roof geometry works out to, how much sheathing, and precisely what information you owe the truss supplier before they can start.
The gap worth knowing about is that the truss designer's scope stops at the individual truss. Somebody has to be responsible for the roof system, the bracing between trusses, and what the bearing points land on. On a lot of houses nobody has explicitly taken that job.
You cannot size a truss. Nobody can, from a table
Every other truss calculator on the internet will give you a member size. None of them is entitled to. A metal-plate-connected wood truss is a delegated engineered design, and the code says so:
Wood trusses shall be designed in accordance with accepted engineering practice. The design and manufacture of metal-plate-connected wood trusses shall comply with ANSI/TPI 1.
IRC R802.10.2
Truss design drawings, prepared in conformance to Section R802.10.1, shall be provided to the building official and approved prior to installation.
IRC R802.10.1
Approved prior to installation. The truss on your roof arrives with a drawing that a named designer has sealed, and that drawing has to carry all of this:
Slope or depth, span and spacing
Location of all joints
Required bearing widths
Design loads: top chord live and dead, bottom chord live and dead, concentrated loads, wind and earthquake loads
Adjustments to lumber and connector design values
Each reaction force and direction
Joint connector type, description and location
Lumber size, species and grade for each member
Connection requirements: truss to girder-truss, ply to ply, field splices
Calculated deflection ratio and maximum description
Maximum axial compression forces in truss members
Required permanent truss member bracing location
Ultimate design wind speed and exposure category
Thirteen items, including the axial force in every member and the location of every required permanent brace. That is not a table lookup. It is the output of proprietary analysis software run by somebody who signs for it.
And here is the gap nobody warns you about
It is important to understand that the Truss Designer's responsibility is limited to the design of the individual truss members, and not either the roof system or floor system.
Metal Plate Connected Wood Truss Design Process Summary, Structural Building Components Association
Read that twice. The truss designer designs the trusses. Not the roof. Not how the trusses are braced against each other, not how the load gets from the bearing points into the walls, not whether your header under the girder truss is big enough. Every individual truss can be perfect and the roof still be wrong.
The Building Designer is responsible for preparing construction documents that provide sufficient information, including required spans and loading information, for the Truss Manufacturer and Truss Designer.
TPI 1 responsibilities, via SBCA
That is the Building Designer's job, and on a great many houses nobody has explicitly taken it. The person you order trusses from is usually not it:
The truss technician is typically not a Registered Design Professional and is not performing design services.
Metal Plate Connected Wood Truss Design Process Summary
The truss technician at the supplier converts your requirements into a job file. They are not designing anything, and they will tell you so if asked. The Truss Designer is often employed by the connector-plate supplier and is a registered design professional. Neither of them is designing your roof system.
Where the Legal Requirements mandate a Registered Design Professional for buildings, the Building Designer and the Truss Designer shall be Registered Design Professionals.
ANSI/TPI 1 Section 2.3
So what can this page legitimately give you? Counts, spacing, quantities and the list of what you have to hand the truss supplier. Everything below the line is arithmetic. Everything above it needs a seal.
What to hand the truss supplier
The truss designer cannot start without this, and every delay in a truss order traces back to one of these being missing or wrong:
Span, bearing to bearing. Not the outside of the wall, not the footprint. Where the truss actually sits.
Spacing and quantity, and where the girder trusses are.
Pitch and overhang, and whether the overhang is level or plumb cut.
Ground snow load and ultimate design wind speed with exposure category. These are jurisdiction values, not guesses.
Bottom chord live load, which is not the same for a plain ceiling, a storage attic and an attic truss with a floor in it.
Point loads: HVAC units, solar arrays, a ridge beam landing on a truss, anything hanging from the bottom chord.
Getting the point loads in late is the classic and expensive one. A truss designed without knowing about the air handler is not a truss you can hang an air handler from.
Bracing is not optional and it is not on the truss
Item 12 of the required drawing contents is the location of required permanent truss member bracing. That is bracing the design depends on: without it the compression members are not restrained and the truss does not have the capacity the drawing claims.
Temporary bracing during erection is a separate problem again, and it is where people actually get hurt. A partially erected truss roof with inadequate temporary bracing is one of the most dangerous conditions in residential framing.
Neither of these is the truss designer's responsibility to install, and neither shows up in a truss price.
Never cut, notch or drill a truss
IRC R802.10.4 covers alterations, and the short version is that a truss may not be cut, notched, drilled, spliced or otherwise altered without the approval of a registered design professional. Every member is carrying a calculated axial force and there is no spare capacity designed in for a plumber's convenience.
This is the single most common way a correctly designed truss roof ends up compromised, usually by a trade who has no idea the rule exists.
If a duct or a waste line has to pass through the roof space, that is a design input, and it goes to the truss designer before the trusses are made.
Frequently asked questions
Can a calculator tell me what size truss members I need?
No, and any that claims to is producing a number nobody will seal. IRC R802.10.2 sends truss design to accepted engineering practice and ANSI/TPI 1, and R802.10.1 requires a truss design drawing approved by the building official before installation.
How many trusses do I need?
Roof length divided by the spacing, rounded up, plus one. A 40 ft roof at 24 in on centre is 21 trusses. That includes the gable ends, which are a different truss and usually cost more.
What information does a truss supplier need from me?
Span bearing to bearing, spacing, pitch, overhang, ground snow load, ultimate design wind speed and exposure category, bottom chord live load, and every point load including HVAC and solar. Providing that is the Building Designer's responsibility under TPI 1.
Who is responsible for the roof system if the truss designer is not?
The Building Designer. SBCA states plainly that the truss designer's responsibility is limited to the individual truss members and not the roof or floor system. On many houses nobody has explicitly taken the Building Designer role, and that is a real gap.
Can I cut or notch a roof truss?
No, not without approval from a registered design professional. IRC R802.10.4 covers alterations. Every member carries a calculated axial force and there is no spare capacity for running a duct through.
What is a girder truss?
A truss that carries other trusses, taking a concentrated load at each connection. It is usually multi-ply and what it bears on must be sized for that point load. Do not count girder trusses as common trusses when ordering.
Do truss design drawings have to be sealed?
Where the statutes of the jurisdiction require a registered design professional, yes. IRC R802.10.2 says the drawings shall be prepared by a registered design professional where the jurisdiction requires it, and the IBC requires a seal and signature where the building official or the jurisdiction requires it.
Is the person at the truss yard designing my trusses?
Usually not. SBCA describes the truss technician as typically not a registered design professional and not performing design services. They convey your specifications to the truss designer, who is often employed by the connector-plate supplier.
Check these numbers yourself
IRC R802.10.1, truss design drawingsUpCodes. The thirteen items a truss design drawing must carry, and the requirement that it be approved before installation. Confirm the edition it serves you.
Metal Plate Connected Wood Truss Design Process SummaryStructural Building Components Association. Who is responsible for what under ANSI/TPI 1, and the statement that the truss designer's scope stops at the individual truss.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.
This page does not size trusses and will not. Member sizes, web layout, plate sizes and permanent bracing come from a truss design drawing prepared under ANSI/TPI 1 and approved by the building official before installation. The counts and geometry here are arithmetic from your dimensions. Never cut, notch or drill a truss without approval from a registered design professional.