How to Analyze a Roof Truss — Step-by-Step with the Free Truss Calculator
A practical walkthrough of the free truss calculator: choosing the type, putting loads where they belong, picking combinations, setting bracing, and reading the results like a reviewer. Each step names the mistake it prevents.
Pick the truss type and geometry
Pitched roof trusses: Fink (W), king post, queen post, Howe and Pratt roof (choose the panel count), scissor (with a separate bottom-chord pitch) and mono-pitch. Parallel-chord trusses (Pratt, Howe, Warren, Warren with verticals) take a depth and a panel count instead of a pitch. Spans are measured between bearings, and pitch is in inches of rise per 12 in of run.
Enter loads as pressures on the horizontal projection
Enter TCDL, TCLL, BCDL and BCLL in psf on plan, plus the truss spacing. The calculator multiplies pressure by spacing and tributary length and lumps the result at the panel points. Loads at the heels go directly into the bearings. Set "TCLL is" to snow, roof live or floor live: that sets the load-duration factor in wood and the combination factors.
Wind is entered as a net uplift pressure on the top chord, the case that reverses member forces. It is not a full wind-pressure model: inward pressure on the windward slope and drag on the truss are not included. Member self-weight is optional; for light wood trusses it is usually already part of TCDL and BCDL.
Choose ASD or LRFD combinations
ASD runs D, D + L, D + 0.6W and 0.6D + 0.6W (ASCE 7 §2.4). LRFD runs 1.4D, 1.2D + 1.6L, 1.2D + 1.0W + 0.5L and 0.9D + 1.0W (§2.3). Each member is checked under every combination, and the governing one is reported. Wood design uses ASD (NDS), and each ASD combination carries its own CD. That is why dead load alone can govern; see the Fink example, step 5.
Choose the member check and sizes
- Sawn lumber (NDS ASD). Choose a species and grade (Douglas Fir-Larch No. 2 and Select Structural, Spruce-Pine-Fir No. 1/No. 2, Hem-Fir No. 2, or custom values), then the size and plies of the top chord, bottom chord and webs. The check applies CD, CF, CP about both axes and Cr on chords, and combines axial force with panel bending in the chords.
- Steel (AISC 360 Chapters D and E). Enter A and r for the chords and webs, Fy, E and K. Tension is checked as gross-section yielding (D2) and compression as flexural buckling (E3). Net-section rupture, connections and chord panel bending are not checked.
- Forces only. Enter A and E to get the forces and deflection without a member check.
Set bracing to match the building
| Member | Options | Typical |
|---|---|---|
| Top chord | Continuous (sheathing / purlins) · panel points only | Continuous: sheathing nailed to the chord |
| Bottom chord | Continuous (ceiling) · panel points only | Continuous under a drywall ceiling; panel points in garages and storage buildings |
| Webs | Panel points only · lateral restraint at mid-length (CLR) | Panel points, plus a CLR on long compression webs |
Bracing sets the out-of-plane buckling length, about the 1.5 in face of a 2x member. When a compression member's le/d exceeds 50, the NDS doesn't allow it, and the calculator reports the violation instead of a ratio. The uplift example shows how a bottom chord with no ceiling, and long webs under uplift, trip this limit.
Read the results like a reviewer
- Stability line. It should read "Stable" and say whether the truss is determinate (m + r = 2j). A mechanism means a missing member or support; the calculator names the joint.
- Total vertical load. Check it against your own (pressure × spacing × span) before you look at anything else.
- Member table. Each member's service force, its maximum tension and compression with the combination that causes each, and the demand/capacity ratio with the governing check. Switch the diagram between forces, D/C ratio, deflection and loads.
- Deflection. Total and live, as span/Δ, against the limits you set. These are pin-jointed elastic values; metal-plate trusses deflect more in service (joint slip, creep).
- Reactions. Maximum down and maximum uplift per bearing, for the wall, the girder or the uplift connector.
Edit joints and members for anything non-standard
Every template opens into editable joint, member and point-load tables. Add a hip girder's concentrated load, an attic room, a cantilevered heel or an extra web, then set the template to "Custom — keep my edited geometry" so your changes stay. Export the member forces as CSV, or copy a share link that reproduces the whole model for a reviewer.
Know what the tool does not do
- Joints are pins, and chord panel moments use simple spans. ANSI/TPI 1 analyzes continuous chords with semi-rigid joints.
- No metal connector plate design, bearing (Cb) or heel checks, or repetitive-member and temporary bracing design.
- No automatic unbalanced or drift snow (add it as point loads), and wind is net uplift only.
- Steel members are checked for axial force only.
Pre-engineered trusses are designed by the truss manufacturer's engineer. Use this tool to size a site-built truss, check a truss design, plan a repair or a modification, or learn how the forces flow.
Worked examples
Fink roof truss, NDS check · Wind uplift and bracing · Pratt footbridge, method of sections
Frequently asked questions
How do you calculate the loads on a roof truss?
Multiply each pressure (dead, snow or roof live, measured on the horizontal projection) by the truss spacing to get a line load in plf, then lump it at the panel points by tributary length: half the panel on each side of a joint. Top-chord loads go to the top joints, ceiling loads to the bottom joints, and the half-panels at the heels go directly into the bearings.
Is a roof truss determinate?
Most standard roof trusses (king post, queen post, Fink, Howe, Pratt) are statically determinate: members plus reaction components equals twice the number of joints. That means the member forces depend only on the loads and geometry, not on the member sizes. Adding redundant members or a third bearing makes the truss indeterminate, and the stiffness method is needed.
Can I use this instead of truss design software?
For site-built trusses, checks, repairs and teaching, yes, within its limits: pinned joints, simple-span chord bending, net-uplift wind only and no connector plate design. Pre-engineered metal-plate-connected trusses are designed to ANSI/TPI 1 by the manufacturer's engineer.