Roof Trusses: Types, Design, Repair and Cost
Most American roofs stand on trusses, factory-engineered triangles that frame a house in a day. Here is how trusses work, the types and what they cost, the attic tradeoff, and the one rule that must never be broken.
What a roof truss is
A roof truss is a complete structural triangle, top chords forming the slopes, a bottom chord tying them, and a web of interior members distributing loads, engineered by software, cut and pressed together in a factory with toothed steel gang plates, and delivered as finished units that a crane sets at two-foot intervals down the house. Trussed roofs frame in hours, use smaller lumber than stick framing (the web turns spans into short compression and tension runs), and arrive with engineering stamped and warranted, which is why the large majority of new American homes are trussed, the economics our trusses vs rafters guide weighs against the open-attic alternative.
The design brilliance is also the ownership catch: every member exists because calculation put it there, the structure is a system, not a pile of boards, and that fact drives the never-cut rule and the repair discipline below.
Truss types
Common (Fink) truss
The suburban standard: W-pattern webbing, efficient to the mid-30-foot spans of ordinary homes, the triangle behind most gable roofs in the country, per our gable guide.
Attic (room-in-attic) truss
Webbing pushed outboard to leave a framed room in the middle, the truss answer to the storage-and-bonus-space loss, spanning garage and cape designs, at premium price and reduced span.
Scissor truss
Bottom chords sloped upward, building a vaulted ceiling into the engineering, cathedral looks without stick-framing, common over great rooms.
Hip, girder & specialty
Hip truss sets step down to frame hip roofs; girder trusses carry other trusses at openings; mono, gambrel and studio profiles frame their namesake shapes, the catalog covers nearly any roofline.
Truss costs and spans
Trusses price by span, pitch, profile and lumber market, but the shape of the math is stable: common trusses for ordinary homes run modest per-unit costs that undercut the labor of equivalent stick framing decisively, attic and scissor profiles carry meaningful premiums, and delivery-plus-crane is its own line. A typical home needs one truss every two feet of building length. The practical homeowner encounters truss pricing in three places: additions and garages (where a truss package quote beats stick framing on speed), storm or fire damage replacement (matched trusses re-ordered from the plant), and the attic-conversion conversation, where the answer is usually that the trusses were not designed for rooms and the retrofit is an engineering project, not a Saturday, per the framing guide.
The never-cut rule
Here is the rule that pays for this page: never cut, drill, notch or remove any part of a truss, not a web to fit a furnace, not a chord to drop a stair, not a member that “looks extra.” Every piece is load path; severing one redistributes forces the design never anticipated, and trussed roofs sag, crack ceilings, and in snow loads fail from exactly this amateur surgery, usually discovered years later during a roof inspection or sale. HVAC installers, plumbers and past owners are the usual culprits; attic remodels the usual motive. Anything that genuinely must pass through or modify a truss goes through a structural engineer and often the truss manufacturer, who design reinforcements and issue stamped fixes, the only path that keeps the structure, the insurance and the resale paperwork whole.
Damage and repair
Trusses do get hurt honestly, cracked chords from impact or overload, gang plates backing out with humidity cycling and age, rot at eave ends from the leaks and ventilation failures that plague all wood, fire and storm damage, and the sagging signals our sagging roof guide reads. Repair follows the engineering chain: assessment, then engineer-specified fixes, plywood or steel gussets sistered across breaks, plates re-secured or supplemented, members scabbed per detail drawings, or damaged trusses replaced outright with plant-matched units. What repair never looks like is improvised lumber nailed where it seems helpful; on an engineered system, undocumented fixes are damage with better intentions. We coordinate truss assessment, engineering and repair as part of structural roof work across our service areas, the below-the-deck half of the trade our decking guide begins.
Roof truss FAQs
What is a roof truss?
A factory-engineered structural triangle, sloped top chords, horizontal bottom chord, and a web of interior members, connected with pressed steel gang plates and set at intervals to frame a roof. Software designs each truss for its exact span and loads, which is why trussed roofs frame fast, use smaller lumber, and must never be field-modified.
Can I cut a roof truss to make room?
No, never without an engineer. Every member is a calculated load path, and cutting webs or chords (for furnaces, storage, stairways) redistributes forces the design never anticipated, causing sagging, cracked ceilings and genuine collapse risk under snow. Required modifications go through a structural engineer or the truss manufacturer for stamped, documented reinforcement.
How much do roof trusses cost?
By span, pitch and profile: common trusses for ordinary homes are economical per unit and decisively cheaper than stick-framing labor; attic and scissor trusses carry premiums; delivery and crane time add their own lines. Homes use one truss per two feet of length. For additions and garages, a truss package usually wins on speed and total cost.
Can attic trusses create a room in my existing roof?
Attic trusses must be designed in from the start, their room is engineered into the web layout. Existing common trusses cannot be cut open into rooms; converting a trussed attic is a structural re-engineering project (and often impractical), which is the honest answer behind many disappointed bonus-room plans. New builds and additions can specify attic trusses upfront.
How are damaged trusses repaired?
Through the engineering chain: assessment, engineer-specified details, then gussets sistered across breaks, plates re-secured, members scabbed per drawings, or plant-matched replacement trusses installed. Improvised lumber nailed across damage is not repair on an engineered system. Common damage sources: impact, overload, eave-end rot from leaks, and past amateur cutting.
Truss damage, sagging, or modification plans?
Assessment, engineering coordination and documented repair, the only way structure should be touched. Get a free assessment.
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