Roof trusses: the parts, the common types, how many you need, and the code's rules
A roof truss is a triangle of lumber built in a plant to an engineered design: a top chord along the roof slope, a bottom chord that makes the ceiling, and webs between them, joined by metal connector plates. The code requires truss design drawings approved before they go up, and bars cutting, notching or drilling a truss without a registered design professional's approval (IRC R802.10.1, R802.10.4).
Anthony Hicks30+ years in construction. Updated November 5, 2026.
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The parts of a truss
| Part | What it is |
|---|---|
| Top chords | The two sloped members along the roof; the decking nails across them |
| Bottom chord | The level member across the bottom; the ceiling hangs from it |
| Webs | The members between the chords that make the triangles |
| Connector plates | The toothed metal plates pressed into each joint |
| Heel | Where the top and bottom chords meet over the wall |
| Overhang | The top chord’s tail past the wall, under the eave |
A truss bears on the outside walls and carries the roof and the ceiling across the span between them. The roof rafters guide compares the two side by side.
Common truss shapes
| Shape | What sets it apart |
|---|---|
| King post | One upright web from the bottom chord to the peak |
| Fink | Webs in a W between the chords |
| Howe | Upright webs with diagonals between them |
| Scissor | A sloped bottom chord, for a vaulted ceiling |
| Attic | An open box in the middle, for a room or storage |
| Mono | One slope, high on one wall and low on the other |
| Gable end | The truss at the end wall, with closely spaced uprights for siding or sheathing |
| Girder | A heavier truss, which can be several plies, that carries other trusses |
Which shape a roof gets is the truss designer’s call from the span, the pitch, the loads and the ceiling.
What the code asks
For metal-plate-connected wood trusses, the residential code says:
- Design: they’re designed by accepted engineering practice and made to ANSI/TPI 1, with the drawings prepared by a registered design professional where the jurisdiction requires it (R802.10.2).
- Drawings first: truss design drawings go to the building official and are approved before the trusses go up, and they ship with the trusses to the site. They list the slope, span and spacing, the bearing widths, the design loads, the lumber for each member and the plates (R802.10.1).
- Bracing: trusses are braced as the construction documents and the truss drawings call for, or by accepted industry practice such as SBCA’s BCSI guide (R802.10.3).
- No changes on site: a truss member can’t be cut, notched, drilled or spliced without a registered design professional’s approval, and added loads such as HVAC equipment or a water heater over the truss’s design load need verification first (R802.10.4).
How many trusses a roof needs
Trusses are counted along the length of the house: the length divided by the spacing, plus one for the end. A 40 foot house at 24 inches on center is 40 ÷ 2 + 1 = 21 trusses; at 16 inches, 40 ÷ 1.333 + 1 = 31. The truss drawings set the spacing; on a gable roof, the two end trusses are gable end trusses.
The roof’s area comes from the span, the pitch and the overhangs; the roofing calculator works it out, and the rafter calculator gives the rafter length for the same pitch.
Common questions
What is a roof truss?
A prebuilt triangle of top chords, a bottom chord and webs, joined with metal plates, that frames the roof and the ceiling in one piece.
Can you cut a roof truss?
Not without a registered design professional’s approval: the code bars cutting, notching, drilling or splicing a truss member otherwise (IRC R802.10.4).
What spacing are roof trusses?
The spacing the truss designer chose is on the truss design drawings (IRC R802.10.1), and the trusses go up at that spacing.
Trusses or rafters?
Trusses come from a plant to an engineered design and fill much of the attic with webs; rafters are cut on site and leave the attic open. The roof rafters guide goes through both.