If you’re building a new home or adding an addition to an existing house, one of the major decisions you’ll have to make is: How will you hold up the roof? Unless the plans call for a flat roof structure, you basically have to choose between rafters or trusses.
A visit to the attic of almost any older home will reveal the fact that rafters were the overwhelmingly popular method in the past. Today, however, about four out of five new houses built in the U.S. utilize truss construction. Obviously, things have changed. Still, however, both methods have unique advantages that may recommend one over the other and new construction featuring either method — or both — is still ongoing every day.
What’s a Rafter?
The method that utilizes rafter roof construction is generically known as “stick framing.” The rafters are lengthy planks, usually 2 x 10s or 2 x 12s that slope down from the central ridge beam at the peak of the roof, intersecting with the outside walls. These long planks support the roof sheathing and roofing.
Meanwhile, ceiling joists extending horizontally across the floor of the attic space between the outside walls secure the walls together and, along with the rafters above, produce a coherent roof and attic structure.
What’s a Truss?
A truss is a prefabricated wooden structure that integrates a triangular webbing of structural members to provide support for the roof above while tying the outside walls of the house together. The long sloping beams extending to the top of the truss and the beam that runs horizontally along the bottom are called chords. The beams and ties that compose the interior triangular webbing of trusses are designed to distribute the weight of the roof over a broad area.
A typical home roof construction is composed of multiple trusses spaced out evenly over a span extending from one end of the house to the opposite end. Unlike stick-built rafter construction that is done on-site, most trusses are made in a factory and delivered to the work site ready for installation.
Which Do You Have?
If you’re buying an existing house, of course, the choice of rafters versus truss construction has already been made for you. Per the above, if it’s an older house it will almost certainly incorporate rafters while newer homes tend toward trusses. If it’s brand new construction in a new development, truss construction can be considered a slam-dunk. However, some new, custom-designed houses may still use rafters in part or all of the structure.
How Do They Compare?
More Attic Space
If you think you might want to convert the attic into an extra room someday, or simply need a lot of unobstructed room up there for storage, rafter construction leaves the attic void wide open for such uses.
If you want to include a specialized ceiling design such as a vaulted or a cathedral-style ceiling, rafters leave room in the attic for the necessary structure. Trusses can be custom-configured to accommodate some vaulted ceilings, but it is basically a non-standard procedure that requires special considerations.
Installing attic insulation and other efficiency-related measures like a radiant barrier is generally quicker and more straightforward in the open space created by rafter construction.
Less Lead Time
Because trusses are prefabricated in a factory, accurate plans with exact dimensions must be submitted well ahead of the actual time when they will be required. Because rafters are stick-built on-site at the time of need, less advance planning is needed.
More Time-Consuming and More Costly
Stick-building rafters on the construction site is slower and takes more people. A large residential rafter roof might take a week to build. Depending on local labor costs, this can drive up the cost of rafter construction substantially.
Skilled, Experienced Workers Required
As the truss construction method becomes the standard in the residential construction industry, workers with the specific skills to build rafters fast and accurately are harder to find. In many cases, these craftsmen are getting older and, increasingly, many may have retired by now.
Factory Quality Control
Because they are manufactured in an indoor factory environment, often on computer-aided saws and fabrication equipment, quality control is very consistent. Trusses may also be inspected and certified by a third-party organization.
The building project can be choreographed with the truss manufacturer so that finished trusses arrive on the exact day when they will be required. On-site storage is thus not required and trusses are not subject to weather exposure.
When trusses are small enough to be manually lifted by two or three workers — which is the case for many residential projects — an entire roof can be trussed in just one or two days.
The beams and ties that compose the interior triangular webbing of trusses are designed to effectively distribute the weight of the roof over a wider area. Also, the triangle structure makes greater use of shorter, less expensive lengths of wood.
Reduced Cumulative Costs
A study by the National Association of Home Builders compared two identical homes. The house built using trusses resulted in a 16% reduction in labor and material costs versus a house that utilized traditional rafters.
Reduced Attic Space
The triangular webbing of evenly spaced trusses, however, segments the open attic space and makes conversion to an extra room impossible and even substantially diminishes available space for storage of boxes and other items.
Higher Up-Front Cost
The initial cost of factory fabricating trusses is higher than the cost of raw lumber to make rafters. (Less labor time for installation usually compensates for this.)
Delivery and Installation Considerations
Finished trusses are bulky and the amount required for a large home will require an 18-wheel truck to deliver. Therefore, the site must be readily accessible to a vehicle that large. In addition, trusses fabricated for larger square footage designs usually cannot be manually handled by a few workers. In these cases a crane will be required on-site to lift the trusses into place.
Little Flexibility for Design Changes
Once the blueprint and dimensions are submitted to the factory and the trusses are fabricated, any afterthoughts about the design are problematic. Last-minute roof or attic design changes will be either not doable or doable only at great cost.
A truss system is an interdependent structure. Once it is fabricated and installed, later modifications may cause issues and could potentially pose a danger due to diminished structural integrity. This could include measures as minor as simply drilling holes through truss members to accommodate cables passing through the attic. Therefore, an inspection and informed opinion by a qualified contractor is advised before making casual modifications to trusses.