Construction calculators

roof truss calculator

Updated Aug 6, 2026 By Jehan Wadia
Rate Formulas
Unit System
Truss Type
Building Dimensions
Span (wall to wall, no overhangs)
Measure the outside face of one bearing wall to the outside face of the opposite bearing wall — do not include the roof overhang.
Overhang (per side)
Horizontal distance the rafter tail projects past the wall — measured level, from the wall face to the fascia.
The length of the roof measured along the ridge direction — this sets how many trusses you need.
Vertical distance from the top of the bottom chord to the underside of the top chord at the wall — energy heels give room for full insulation depth.
Pitch, Spacing & Load
Roof Pitch (rise per 12 of run)
Pitch is written X/12 — the roof climbs X units vertically for every 12 units of horizontal run.
Filling this in overrides the buttons above.
Truss Spacing (on center)
On-center spacing is measured from the centerline of one truss to the centerline of the next.
Roof Load Category
Load category drives the assumed chord lumber size and therefore the board-foot and weight estimates.
Span
—
wall to wall
Trusses Needed
—
at spacing
Overall Width
—
span + both overhangs
Est. Weight Each
—
per truss
Detailed Member Measurements
Rise (vertical)—
Total Truss Height (rise + heel)—
Top Chord Length (wall to peak)—
Bottom Chord Length—
Rake Length (incl. overhang)—
Roof Slope—
Web Members—
Lumber per Truss—
Live Truss Diagram (scaled to your inputs)
Lumber Cut List
Member Nominal Size Qty per Truss Length Each Total Trusses Total Board Feet
Scroll the table sideways on small screens. Lengths are theoretical — add your own cutting/waste allowance.
Lumber Volume by Member (whole job)
Step-by-Step Solution

Introduction

A roof truss is the frame that holds up your roof. This roof truss calculator works out the size and number of trusses you need for a shed, garage, barn, or house. Just type in your span, pitch, overhang, and building length, and it does the math for you.

You can pick from six truss types: common (king), attic, scissor, fink (W-truss), king post, and shed/mono. Choose feet and inches or meters and centimeters. Then set your roof pitch, truss spacing, and load type.

The calculator gives you:

  • Rise, top chord, bottom chord, and rake length
  • How many trusses you need
  • Overall roof width and truss weight
  • A full lumber cut list with board feet
  • A scaled drawing of your truss
  • Step-by-step math so you can check the work

Use it to plan your build, order lumber, and cut down on waste. These numbers are estimates for planning. For a real build, have a truss maker or engineer sign off on the design and check your local building code.

How to use our Roof Truss Calculator

Enter your roof size, pitch, truss type, and spacing. The calculator shows your rise, top chord, bottom chord, rake length, how many trusses you need, the weight of each truss, a scaled truss diagram, and a full lumber cut list.

Unit System: Pick Imperial (feet and inches) or Metric (meters and centimeters). Your numbers change over on their own when you switch. If you need to convert a stray measurement by hand, the meters to feet calculator and feet and inches calculator can help.

Truss Type: Choose the truss shape you plan to build — Common (King), Attic, Scissor, Fink (W-Truss), King Post, or Shed/Mono. Each one uses a different web layout. For member forces in a general frame, see the truss calculator.

Span: Type the distance from the outside of one bearing wall to the outside of the other wall. Do not add the overhang here.

Overhang (per side): Type how far the rafter tail sticks past the wall, measured flat. Use 0 if you have no overhang.

Building Length: Type the length of the roof along the ridge. This sets how many trusses you need.

Raised Heel Height: Type the height at the wall between the bottom chord and top chord. Use 0 for a standard heel, or a larger number for an energy heel that leaves room for full-depth attic insulation.

Attic Room Width: This box shows only for Attic Trusses. Type the width of the flat ceiling inside the attic room. It must be at least 2 feet (61 cm) less than the span.

Roof Pitch: Tap a pitch button, like 6/12, to set how much the roof rises for every 12 of run. Not sure what your pitch is? Measure it first with the roof pitch calculator.

Custom Pitch: Need a pitch that is not listed? Type a number from 1 to 24. This overrides the pitch buttons.

Truss Spacing: Pick the on-center spacing between trusses — 12″, 16″, 19.2″, or 24″. Most homes use 24″ on center, the same layout used by the stud calculator for walls.

Roof Load Category: Pick Light for metal roofing and low snow, Standard for asphalt shingles, or Heavy for tile, slate, or deep snow. This sets the lumber size used in the cut list and weight.

Calculate and Reset: Press Calculate to see your results, cut list, and step-by-step math. Press Reset to start over with the default values.

What Is a Roof Truss?

A roof truss is a strong wooden frame that holds up a roof. It is built in a triangle shape because triangles do not bend or twist easily. Trusses are made in a shop, then lifted onto the walls and lined up in a row. Together they carry the weight of the sheathing, shingles, snow, and wind down into the walls.

Parts of a Truss

  • Top chord — the two sloped boards that form the roof line. The roof deck nails to these.
  • Bottom chord — the flat board across the base. It ties the walls together and often holds the ceiling.
  • Webs — the short boards inside that spread the load between the chords.
  • King post — a single upright web in the middle, from the peak down to the bottom chord.
  • Heel — the point where the top chord meets the bottom chord over the wall. A raised heel lifts the top chord higher so thick insulation fits above the wall.
  • Overhang (tail) — the part of the top chord that sticks out past the wall to make the eave and fascia.

Span, Pitch, and Rake

Span is the distance from the outside of one bearing wall to the outside of the other. It does not include the overhang. Run is half the span on a normal gable roof, because the roof climbs from each wall to the peak in the middle.

Pitch is written as X/12. A 6/12 pitch rises 6 inches for every 12 inches of run. Steeper pitch means more rise, longer boards, and a taller truss. Rise is how high the peak sits above the heel. If you would rather work in percent or degrees, the slope percentage calculator converts between them.

Rake is the full sloped length of the top chord, from the fascia at the eave all the way to the peak. It is longer than the run because it follows the slope, not the flat ground. You find it with the Pythagorean rule: the sloped length equals the run times the square root of 1 plus (pitch ÷ 12) squared. A stick-built roof uses the same math — see the rafter calculator.

Common Truss Types

  • Common (King) — a plain gable truss with a king post and two struts. The most used shape.
  • Fink (W-truss) — webs form a W. It is cheap and works well on longer spans.
  • King Post — just one center post. Best for short spans, porches, and open beam looks.
  • Attic truss — leaves an open room inside the roof for storage or living space.
  • Scissor truss — the bottom chords slope up and cross, giving a vaulted ceiling below.
  • Shed / Mono — one slope only, used on lean-tos, sheds, and additions.

Spacing and Truss Count

Trusses sit at an on-center spacing, measured from the middle of one truss to the middle of the next. Most houses use 24 inches on center. Heavier roofs, tile, or big snow loads may need 16 inches. Closer spacing means more trusses and more strength. The count is the building length divided by the spacing, plus one for the last truss at the end. The same layout rule drives the framing calculator and the floor joist calculator.

Roof Load

Load is the total weight the roof must hold. Dead load is the roof itself — trusses, sheathing, and shingles. Live load is added weight like snow, rain, wind, and workers. Light roofs, such as metal in warm areas, need smaller lumber. Heavy roofs with tile, slate, or deep snow need bigger chords, like 2×8 top chords instead of 2×6. Wind uplift on the roof plane can be checked with the wind load calculator, and beam sizing over open spans with the beam deflection calculator.

Why the Numbers Matter

Board lengths, board feet, and truss weight help you order lumber, plan cuts, and know how the trusses will be lifted. Small trusses can be set by hand; long or heavy ones need a crane. Always add a little extra length for cuts and waste. Once the trusses are up, use the roof area calculator to get the deck area, then the shingle calculator or the roofing calculator to price out the covering. For total board length on the order, the linear feet calculator is handy.

Get an Engineer's Stamp

These numbers are for planning and estimating. Real trusses must be designed and sealed by a truss engineer for your local snow, wind, and building code. Never cut, notch, or drill a finished truss, and never change a web without written approval — one cut board can weaken the whole roof.

Formulas used

Rise from pitch
\text{Rise} = \frac{P}{12} \times \text{Run}, \quad \text{Run} = \frac{\text{Span}}{2}\ (\text{Span for mono})
Top chord length (wall to peak)
L_{TC} = \sqrt{\text{Run}^2 + \text{Rise}^2}
Rake length (including overhang)
L_{rake} = \left(\text{Run} + \text{OH}\right)\sqrt{1 + \left(\frac{P}{12}\right)^2}
Overall width and total truss height
W_{overall} = \text{Span} + 2\,\text{OH}, \qquad H_{total} = \text{Rise} + \text{Heel}
Roof slope angle
\theta = \arctan\!\left(\frac{P}{12}\right) \times \frac{180}{\pi}
Number of trusses
N = \left\lfloor \frac{L_{building}}{S_{oc}} \right\rfloor + 1
Board feet and estimated truss weight
V = \sum \frac{q \times L_{ft} \times w_{nom} \times d_{nom}}{12}, \qquad W = V_{actual} \times 2.3 \times 1.07
Attic room geometry
H_{ceil} = \text{Heel} + \frac{P}{12}\left(\frac{\text{Span}}{2} - \frac{W_{flat}}{2}\right), \qquad W_{5ft} = 2\left(\frac{\text{Span}}{2} - \frac{(60 - \text{Heel})\times 12}{P}\right)

Frequently asked questions

How many trusses do I need for my building?

Divide your building length by the truss spacing, then add one. The extra truss is for the far end wall.

Example: a 40 ft building at 24 in (2 ft) on center is 40 ÷ 2 = 20, plus 1 = 21 trusses. The calculator does this for you and shows it in the summary.

Why does the calculator add one extra truss?

Spacing only counts the gaps between trusses. You still need one truss to close off the last gap at the end of the roof.

Think of a fence: 20 sections of fence need 21 posts.

What if my building length does not divide evenly by the spacing?

The calculator rounds down and adds one, so the last bay ends up a little tighter than the rest. That is normal and safe.

Never leave a bay wider than your chosen spacing. A short bay is fine, a long bay is not.

Does the overhang change how many trusses I need?

No. The overhang sticks out sideways over the walls, not along the ridge. Truss count only depends on building length and spacing.

The overhang does make each top chord longer, which shows up in the rake length and the cut list.

Why is the rake length longer than the top chord length?

The top chord length is measured from the wall to the peak. The rake adds the overhang tail, so it runs from the fascia all the way to the peak.

Order lumber by the rake length, not the top chord length, or your tails will come up short.

Should I add extra length for waste?

Yes. The cut list gives exact math lengths with no cutting allowance.

Add about 3 to 6 inches per board for angle cuts, and order 5 to 10 percent extra lumber overall for mistakes and bad boards.

What lumber size does the calculator use?

It picks sizes from your load choice:

  • Light — 2×4 top chord, 2×4 bottom chord, 2×4 webs
  • Standard — 2×6 top chord, 2×4 bottom chord, 2×4 webs
  • Heavy — 2×8 top chord, 2×6 bottom chord, 2×6 webs

These are typical sizes for planning. A truss engineer sets the real sizes for your span and code.

How accurate is the truss weight number?

It is a close estimate. The calculator uses the real dressed size of each board, about 2.3 lb per board foot for dry softwood, plus 7 percent for metal plates and fasteners.

Green or wet lumber can weigh 20 to 30 percent more. Use the number to plan lifting, not for crane rigging math.

Do I need a crane to set my trusses?

It depends on weight and length. Trusses under about 100 lb and 20 ft can often be set by hand with a few helpers.

Longer or heavier trusses bend easily and should be lifted with a crane or telehandler. Check the weight number in the summary before you plan the day.

What is a raised heel and do I need one?

A raised heel lifts the top chord higher above the wall so full-depth insulation fits over the top plate.

Without it, insulation gets squashed at the eave and the corner gets cold. Many builders use a 6 to 12 inch energy heel. Enter 0 if you want a standard heel.

Can I use this calculator for a hip roof?

Not directly. This tool covers gable, mono, scissor, and attic shapes with the same profile all the way down the roof.

Hip roofs use a stepped set of shorter trusses at each end. Use this tool for the common trusses in the middle, then have a truss shop lay out the hip set.

What is the biggest span a wood truss can handle?

Plated wood trusses are commonly built up to about 60 ft, and some go past 80 ft. Most homes stay under 40 ft.

Long spans need deeper chords, more webs, and careful bracing. Anything over about 40 ft should be shop built and engineered.

Can I build trusses myself with plywood gussets?

Small trusses for sheds and barns are often site built with plywood gussets and glue. Many codes allow it for non-living buildings.

Trusses for a house usually need pressed metal plates and an engineer's stamp. Check with your local building office before you cut.

Why do the web counts go up on long spans?

Longer chords bend more. Adding webs breaks the chord into shorter braced sections so it stays straight under load.

The calculator adds two extra webs for every 10 ft of span past 24 ft on common, fink, attic, and scissor trusses.

What is 19.2 inch on center spacing for?

It splits an 8 ft sheet into five equal parts instead of four. That means fewer trusses than 16 in spacing, but more support than 24 in.

It is used on engineered floor and roof layouts where 24 in is a bit too wide.

How do I measure the span on a building that is already framed?

Measure from the outside face of one bearing wall to the outside face of the other wall. Measure at the top plate, not the foundation.

Do not include the eave or overhang. Check at both ends of the building, since walls are rarely perfect.

Why does the scissor truss show a longer bottom chord?

On a scissor truss the bottom chords slope up to meet in the middle, so they follow a slope instead of a flat line.

The calculator uses half the roof pitch for the ceiling slope, which is the common rule, and measures the true sloped length.

How much headroom will my attic truss give me?

The calculator shows the clear height at the flat ceiling and the width where you still have 5 ft of headroom.

For a usable room you want at least 7 ft of clear height over most of the floor. If the numbers come up short, raise the pitch or add a taller heel.

What is a board foot and why is it in the cut list?

A board foot is a piece of lumber 1 ft long, 1 ft wide, and 1 in thick. Lumber yards price and sell by it.

The cut list totals board feet so you can quote the whole job fast. You can double check any board with the board foot calculator.

Does the cut list include metal plates, nails, or bracing?

No. It lists chord and web lumber only.

You still need connector plates or gussets, fasteners, hurricane ties, temporary bracing, and permanent lateral bracing. Budget for those on top of the lumber.

Can I cut or drill a truss on site to fit a pipe or duct?

No. Never cut, notch, or drill a chord or web without written approval from the truss engineer.

Every board in a truss carries load. One cut can weaken the whole roof. Plan pipe and duct paths before you order.

What roof pitch should I pick?

Most homes use 4/12 to 8/12. Low pitch saves lumber and is easier to walk. Steep pitch sheds snow and rain better and gives more attic room.

Most shingles need at least 2/12, and 4/12 or more is safer. Measure an existing roof with the roof pitch calculator.

How do I get the roof area for shingles from these numbers?

Multiply the rake length by 2 for a gable roof, then multiply by the building length plus your gable overhangs.

That gives the deck area. The roof area calculator and shingle calculator handle the rest.

Does the calculator check if my truss design is strong enough?

No. It gives geometry, member lengths, counts, and lumber volume. It does not run a load or stress check.

Real trusses must be designed and sealed by a truss engineer using your local snow, wind, and code numbers.

Related Construction calculators

All Construction calculators
Construction

Rafter Calculator

Use our free rafter calculator to find rafter length, pitch, rise, and run. Get accurate roof rafter measurements for your construction project instantly.

Open Construction

Roof Area Calculator

Calculate your roof area instantly with our free roof area calculator. Enter dimensions, pitch, and slope to estimate roofing materials needed accurately.

Open Construction

Roof Pitch Calculator

Use our free roof pitch calculator to quickly determine roof angle, slope ratio, and rise over run. Perfect for roofing projects and construction planning.

Open Construction

Roofing Calculator

Use our free roofing calculator to estimate materials, costs, and square footage for your roof project. Get accurate results for shingles, metal & more.

Open Construction

Shingle Calculator

Use our free shingle calculator to estimate how many bundles and squares of shingles you need for your roof. Get accurate roofing material estimates instantly.

Open