Trusses and rafters are different products
A roof truss is an engineered component. It is designed for a specific span, pitch and load, manufactured in a plant, delivered as a unit and lifted into place. It arrives with a design that somebody has sealed.
A site-built rafter roof is framed on site from individual members: rafters, ridge, ceiling joists or collar ties, with the geometry cut to fit. It is slower and it needs more skill, and in exchange it gives a roof space that is not full of webs.
That is the real trade-off and it is a planning decision rather than a structural one. Trusses are faster, cheaper and more consistent, and they fill the attic with structure. A rafter roof, or an attic truss designed for the purpose, leaves usable space. Deciding which you want after the roof is up is not a decision, it is a rebuild.
A truss is a system, and cutting one ends it
Every member of a truss is in tension or compression as part of a whole, and the connections between them are engineered. Remove or cut any part and the design that was sealed no longer describes the object.
This is the single most commonly ignored fact about roof structure, usually by somebody who wants a duct, a flue or a storage platform to pass through. A truss may be altered only under a design produced by a qualified engineer, and the alteration will usually involve added members or plates specified for that exact case.
Say the same thing positively, because it is the useful version: if you know you want the attic later, tell the designer before the trusses are ordered. Attic trusses and raised-heel trusses exist, they cost a little more, and they are ordinary items rather than special requests. Deciding at order time is free. Deciding afterwards is engineering.
Note
Any modification to a roof truss, however minor it looks, is a structural alteration and requires a qualified engineer. It is also generally permit work. This page describes what governs the decision and does not describe how to perform any of it.
Snow is a local number and it does not sit evenly
Design snow load is established from the adopted code and the standards it references, and it is specific to a location. There is no national figure and this page does not publish one, because a snow load copied from a web page is precisely the kind of number that reads authoritative and is wrong where you are building. [2]
What generalises is the behaviour. Snow does not lie uniformly on a roof. It drifts against anything that interrupts the surface: a wall where a lower roof meets a higher one, a parapet, a dormer, mechanical equipment. Those drift loads can be several times the balanced load nearby, and they are a common cause of localised failure on roofs that were otherwise adequate.
The design consequence is that roof geometry is a structural decision. Every step in a roof plane creates a place where snow accumulates, and a complicated roof is a more expensive structure before it is a more expensive roof covering.
Wind pulls the roof off rather than pushing it down
Wind flowing over a roof produces suction on the leeward slope and on the edges, which is why storm damage so often begins at a corner or an eave rather than in the middle of a roof plane.
Resisting it is entirely a matter of continuity. The roof covering has to be attached to the sheathing, the sheathing to the rafters or trusses, those to the walls, the walls to the floor structure, and the whole assembly to the foundation. The load path that resists uplift is the vertical path run backwards, and it fails at whichever connection is weakest.
This is why hardware matters more than member size in wind regions, and why the design provisions for wind and seismic are a separate standard in their own right. [1] It is also why an inspector spends time looking at connectors that appear trivial: they are the ones carrying the load nobody can see.
Ventilated or not, decided with the structure
A roof is either vented, with an airflow path from the eave to the ridge above the insulation, or it is unvented, with the insulation in contact with the underside of the roof deck and the assembly detailed to control moisture without airflow.
Both work and both are code-recognised. What does not work is choosing halfway, or choosing late, because the decision changes the framing. A vented assembly needs a continuous clear path from eave to ridge, which needs depth at the eave, which is why the raised-heel truss exists. An unvented assembly changes what insulation may be used against the deck.
The moisture reasoning behind the choice belongs to the envelope rather than to the structure, and it is set out under insulation and air sealing on the front sheet. What belongs here is the timing: the structure has to know which one it is serving before it is ordered.
Questions
- Can I store things in a truss attic?
- Only if the trusses were designed for it. A standard roof truss is designed to carry the roof and a ceiling, not a floor load, and adding one is a structural change. Attic trusses designed for storage or for a room are available and are specified at order.
- Are trusses stronger than rafters?
- They are engineered, which is not the same thing. A truss comes with a sealed design for a stated span and load. A site-built rafter roof is either inside a prescriptive path or engineered as well. The difference is where the responsibility sits, not which is inherently stronger.
Sources
- 1American Wood Council. Codes and Standards publications: NDS, WFCM, SDPWS, PWF and FDS. Accessed 2026-08-29
- 2International Code Council. International Code Adoptions. Accessed 2026-08-29