What Is a Roof Truss?

What a roof truss is

Look up into the attic of most newer homes and you will see a row of wooden triangles. Each one is a roof truss. Together they form the skeleton of the roof.

The roof sheathing is nailed across the top of them, and the ceiling of the rooms below is often fastened to the bottom of them.

A truss has three kinds of parts. The top chords are the two sloped boards that form the roof line. The bottom chord is the flat board along the base, which often doubles as a ceiling beam.

The webs are the shorter boards inside the triangle that link the top and bottom. Where the pieces meet, flat steel plates with teeth are pressed into the wood to hold the joint. The code calls these metal-plate-connected wood trusses ¹.

Trusses are engineered. That word means someone ran the numbers for that exact shape, span and load before it was built.

The code says trusses must be designed in line with accepted engineering practice, and the design and making of plated wood trusses must follow a standard called ANSI/TPI 1 ¹.

That is very different from a board cut to fit on site.

The word truss is often used loosely for any roof framing. On this page it means the factory-built, engineered frame. When a roof is framed with single boards instead, those boards are rafters, and we cover them in the comparison below.

The diagram shows why no part is spare: each one is load-bearing.

How a truss carries the roof

A triangle is the one simple shape that cannot change form without a side changing length.

Push down on the peak of a wooden triangle, and the sloped sides press inward while the bottom board is pulled tight. As long as every joint holds, the shape stays put.

A truss is a chain of small triangles built on that rule.

Because the triangles share the work, each piece can be small. Many trusses are built from 2 by 4 lumber, yet they span the full width of a house with no wall in the middle.

A rafter of the same span usually needs deeper boards or a support wall below. That is the main reason trusses became common for new homes.

The trick only works if every piece stays whole. Each web and chord is part of a path that load follows to the walls.

Cut one web to make room for a duct, and the load has nowhere to go.

That is why the code bans cutting, notching, drilling or splicing a truss without approval from a registered design professional ¹.

It also says added loads such as a furnace or water heater that go past the truss's design load are not allowed unless someone checks that the truss can carry them ¹.

Trusses also need to be held upright. A row of tall, thin triangles can tip over like dominoes.

The code says trusses must be braced to stop rotation and keep them stable ¹.

Where the plans give no bracing details, the code points to the industry's BCSI guide for handling, installing and bracing trusses ¹.

Common truss shapes

Truss makers build many shapes. These are the ones homeowners see most. The names are trade terms, and each one is still designed for its own job.

Whatever the shape, each truss is made to its own truss design drawing. The code says those drawings must show the span, spacing, joint locations, bearing widths and design loads, including the top and bottom chord loads ¹.

Truss vs rafter

Rafters and trusses both hold up the roof deck. The table shows how they differ, based on the code rules for each ¹.

Sources: [1]

How rafters differ in practice

Rafters are the older way to frame a roof. A carpenter cuts each board to length and angle on site.

The tops of opposing rafters meet at a ridge board, which the code says must be at least 1 inch nominal thick and as deep as the cut end of the rafter ¹.

The code also says rafters on each side should be framed opposite each other, offset no more than 1 1/2 inches ¹.

A rafter roof pushes out on the walls. Left alone, the weight on the slopes would spread the walls apart at the top. The code calls for continuous ties across the house to stop that push.

Ceiling joists or rafter ties do that job, and rafter ties must be at least 2 by 4 inches ¹. In a truss, the bottom chord does this job inside the truss itself.

Rafters can be cut a little, but only within set limits. The code says structural roof members must not be cut, bored or notched past the limits in R802.7 ¹.

Those limits are tight. They are not a guide for homeowners, and any cut should be checked by the builder, an engineer or the building department first.

Hips and valleys on a rafter roof need their own heavier rafters. The code says hip and valley rafters must be at least 2 inches nominal thick and as deep as the cut end of the rafters that land on them ¹. Truss roofs handle hips with a set of stepped trusses instead.

Worked example: a duct in the attic

Say a heating crew wants to run a new duct through the attic, and a truss web is in the way. Here is how the code rules play out step by step ¹.

The lesson is simple. With trusses, the answer to "can we cut this?" is a design question, not a carpentry one. Ask before the saw comes out, not after.

Where trusses sit in the roof

Trusses run from one outside wall to the other and rest on the top plates of those walls.

They are set side by side, often 24 inches apart, though the spacing is set on each truss design drawing ¹. The spacing matters to the roofer, because it decides how thick the sheathing must be.

Our sheathing guide shows how panel span ratings line up with framing spacing.

At the bottom, the truss ends sit on the walls and often stick out past them to form the eaves. Above them come the fascia board, the drip edge and the first course of shingles.

At the top, the peaks of the trusses form the ridge line. On many roofs a slot is cut in the sheathing there, never in the trusses, so attic air can leave through a ridge vent.

The ends of each truss are tied down to the wall. The code requires roofs to resist uplift, the force of wind trying to lift the roof ¹.

In many areas that means metal straps or clips at each truss end. A simple nailed connection is allowed only in limited cases, such as where the uplift per truss is 200 pounds or less ¹.

Why builders use trusses

• Long clear spans with no wall in the middle, using smaller lumber
• Each truss is engineered for its span and load, so the design is written down ¹
• Built in a factory, so pieces are cut and pressed the same each time
• Fast to set on site, which shortens the time a house is open to weather
• Truss design drawings ship with the trusses, so the building official can check them ¹
• The bottom chord can carry the ceiling, so fewer separate ceiling joists are needed

Limitations

• You cannot cut, notch or drill a truss without a design professional's approval ¹.
• The webs fill most of the attic, which limits storage and makes a later attic room hard.
• Adding heavy items, such as a water heater, needs proof the truss can carry it ¹.
• Trusses are designed for set loads. A change of roofing to something much heavier is a design question.
• Repairs need a written design, which takes time, as the Simpson Strong-Tie truss repair article explains ⁴.

What drives the cost of truss work

We do not list prices. Truss work is rare on a normal reroof, and when it comes up it is priced job by job. These are the things that move the number. For the roof itself, try our roof cost calculator.

• Repair design. An engineer or the truss maker must design the fix, and that design work is its own charge ⁴.
• How many trusses. One cracked web costs less to fix than a row of broken chords.
• Repair or replace. Repairs in place, such as added wood or plates, are common. Fire damage often leads designers to suggest replacement ⁴.
• Access. Tight attics, low pitch and finished ceilings make the work slower.
• Permit and inspection. Many towns want a permit for structural work. Ask your building department.
• Roof over it. If the roof must come off to reach the trusses, the reroof adds to the job. See roof replacement.

How a damaged truss is handled

This is the path a damaged truss should follow. A licensed contractor and a design professional do the work. These steps are what you can expect and check, not a how-to.

• Someone spots the problem: a roofer at tear-off, a home inspector in the attic, or a crew after a storm.
• The damage is photographed and measured, and nothing is cut or removed.
• If the roof is unsafe, a pro shores it up for the short term. Do not stand under or on a sagging roof.
• The truss maker, if known, or a structural engineer is asked for a repair. The Simpson Strong-Tie article says the original truss designer can often open the design file and issue a repair quickly ⁴.
• The designer issues a written repair detail. Common fixes add wood or plywood pieces, or steel plates, over the damaged part ⁴.
• A contractor builds the repair to the detail, and the building department inspects it where a permit is required.

Looking after your trusses

Trusses need little care if they stay dry and whole. The main threat is water. A slow roof leak can soak a chord or rot the wood around a plate, and the plate teeth then lose their grip. Fix leaks quickly, and keep the flashing and shingles in good shape.

Moist attic air is the second threat. Good attic airflow carries damp air out before it can soak the wood. Our roof ventilation guide covers how much vent area the code asks for.

Once a year, look at the trusses from the attic hatch with a flashlight.

The code requires an attic access opening, at least 22 by 30 inches, where the attic is 30 inches or taller over at least 30 square feet ². Look without climbing across the ceiling.

Stains, dark wood, loose or rusted plates, cracked boards or a bow in the roof line are all worth a call to a pro.

Do not hang heavy items from trusses or stack boxes on the bottom chords unless you know the trusses were designed for it. The design loads are on the truss design drawing ¹. If you do not have the drawing, ask a builder or engineer before adding weight.

Signs of a truss problem

• A sag, dip or wave in the roof line seen from the street
• A web or chord that is cut, cracked, split or missing
• Metal plates that are bent, rusted, loose or pulled away from the wood
• Dark, soft or stained wood around a leak
• Ceiling cracks that line up with the trusses, or doors that start to stick upstairs
• Trusses that lean or twist out of line with their neighbors

These are warning signs, not a diagnosis. A professional inspection can point to the cause, and an engineer can say how serious it is. Do not climb on the roof or across attic framing to check.

What the code requires

These rules come from the 2021 International Residential Code, as adopted in Maryland and read at UpCodes. Your state or city may use a different edition or change parts of it. Larger buildings fall under the International Building Code, which has a similar rule ³.

Sources: [1] [2] [3]

Trusses vs other roof parts

Common misconceptions

Myth One small cut in a web is fine.

Reality The code bars any cut, notch or drill in a truss without a design professional's approval ¹. Every web is part of the load path.

Myth A damaged truss must be torn out.

Reality Many damaged trusses can be repaired in place to a designer's detail ⁴. Replacement is more common after fire.

Myth Trusses and rafters are the same thing.

Reality Both hold up the roof, but trusses are engineered frames and rafters are single boards with their own code tables ¹.

Myth The attic floor can hold anything.

Reality Bottom chords are designed for set loads shown on the truss drawing ¹. Heavy storage or equipment needs a check.

Myth A roofer can fix a broken truss on the spot.

Reality A roofer can spot it and protect the roof. The fix itself needs a written design from an engineer or the truss maker ⁴.

Local rules and climate

Each state or city adopts its own edition of the code and can change it.

Your building department decides which rules apply, whether truss work needs a permit, and whether a state law calls for an engineer's seal on the truss drawings.

The code itself says the drawings must be prepared by a registered design professional where state law requires it ¹.

Climate shapes truss design. Snow areas need trusses designed for heavier roof loads, and the IRC truss rules have limits tied to snow loads and building size ¹. High-wind areas need stronger tie-downs at each truss end to resist uplift ¹.

If you live in a snow belt or near the coast, expect the truss drawings and the tie-down hardware to reflect that. Ask your builder or building department which local loads apply.

When it matters and what to ask

Trusses rarely come up on a plain reroof. They matter when there is damage, a remodel, or new equipment in the attic.

• Do we have trusses or rafters, and does that change anything about this roof job?
• Did you see any damaged, cut or wet trusses at tear-off? Can I see the photos?
• If a truss is damaged, who will design the repair: the truss maker or an engineer?
• Does the repair need a permit and an inspection here?
• Will the new roofing weigh much more than the old, and is that a design question for our trusses?
• Before anyone cuts or adds load in the attic, who signs off in writing?

Keep going

• Roof inspectionHave a pro check sags and attic damage
• How a roofing project worksWhere the framing check fits in the job
• Repair or replaceDecide what your roof needs
• Choosing a rooferAsk how they handle damage they find

Related terms

Roof Rafter

A sloped framing member that runs from the ridge to the wall and supports the roof deck. Rafters are cut and assembled on site, unlike trusses.

Roof Sheathing

Roof sheathing is the layer of plywood, OSB or wood boards nailed across the rafters or trusses.

Roof Sagging

A visible dip in a roof line or plane. It can mean a wet or failed deck or a structural problem and needs a professional look.

Roof Rot

Decay in the wooden deck, fascia or framing after long exposure to moisture. Rotten deck must be replaced before new roofing goes on.

Roof Ridge

The horizontal line at the top of a roof where two planes meet. It is capped with ridge shingles and is the usual place for a ridge vent.

Hip Roof

A roof whose planes slope down to every wall, so it has no vertical gable ends. The sloped external corners are called hips.

Gable Roof

A roof with two planes sloping down from a central ridge, leaving a triangular wall (the gable) at each end.

Roof Wind Uplift

Roof wind uplift is the upward pull that wind puts on a roof as it flows over and around a building, lifting shingles, panels or the whole deck away from the framing below.

Roof Ventilation

Roof ventilation is the system of vents that lets outside air enter an attic low, near the eaves, and leave high, near the ridge, so heat and moisture do not build up under the roof deck.

Reroofing

Reroofing is the work of putting a new roof covering on a house that already has one, either by tearing the old covering off down to the deck (a replacement) or by laying the new covering over the old one (a recover, often called an overlay).

Also referenced from: Roof Sheathing