wood trusses

Environmental Impact Rating

Harvesting — Negligible to High

Wood sustainably harvested from the local region can have negligible impacts. Unmanaged forestry can have impacts that include significant habitat destruction, soil erosion and ground water contamination. Third-party certification can help to ensure that impacts are minimized.

Metal fasteners may seem like a small component, but dozens of kilograms can go into a frame wall. Ore for steel production is mined in an intensive process with impacts including habitat destruction and soil and water contamination.

Manufacturing — Negligible to Moderate

The sawing and planing of structural lumber is a relatively low–impact mechanical process.

Processing raw ore into nails and screws is a multi-stage and intensive process requiring multiple infusions of high heat and fossil fuel use with impacts including air and water pollution.

Transportation — Negligible to High

Sample building uses 2,610 kg of wooden trusses:

3.9 MJ per km by 15 ton truck

2.5 MJ per km by 35 ton truck

Timber is a heavy and high-volume material and accrues significant impacts proportional to distance traveled. In most regions it is possible to source lumber a reasonable distance from the building site.

Installation — Negligible

The number of offcuts is minimized by truss companies, and no cuts are typically required on-site.

Waste: Negligible

Trusses are built to size, so no offcuts should be generated on-site.

Chart of Embodied energy & carbon

Wood truss floor joists:

wood truss roof:

Energy Efficiency

Truss roofs and floor joists are typically deeper than their solid lumber counterparts, and this gives more room for insulation. If required, truss roofs can be designed with “raised heels” to ensure that full depth attic insulation can be installed over the walls.

The open spaces in the webbing of a truss will cause less thermal bridging than a solid wood member, but will still contribute to some bridging unless additional steps are taken.

Material costs: Low

Trusses are typically the lowest cost option for roof and floor framing. The use of small dimension lumber and the efficiencies of factory assembly result in low cost assemblies. Simple roofs and floors, as in the sample building, will see the most cost benefits from truss systems.

Labour Input: low to moderate

Trusses can lower the amount of labor required to build a floor and especially a roof. The trusses will come sized according to the plans and will require little or no additional cutting or framing, other than temporary and/or permanent bracing to add rigidity to the system. With all the geometry done by the truss manufacturer, the layout and fastening of the trusses is very straightforward and can be done in a relatively short time.

Skill level required for homeowners: Low to moderate

Beginners can do the installation of trusses, especially with some study of resource material in advance. One skilled carpenter can often direct one or two less-skilled laborers in the installation of trusses. Simple designs are very straightforward to complete using trusses, with skill level rising as complexity rises.

Sourcing & availability: Easy

In any region where wood-framed houses are common, there will be regional or local truss manufacturers in operation. Trusses made with third-party certified lumber may be more difficult to source.

Durability: moderate to High

Trusses have been in widespread use for over forty years, with no signs of common durability issues. The system is completely reliant on the metal fastening plates. In normal use, these plates are durable. However, constant exposure to moisture can cause plates to corrode and wood to soften or rot behind the plates and cause failure sooner than with solid lumber. However, with well-maintained sheathing there should be no issues.

The failure of truss roofs in fires is well documented. Modern fire-fighting practices assume the use of truss roofs and the quick failure of roof structures as the metal plates heat up and twist or melt. While it is difficult to plan for durability in fires, trusses do have a poor record in this regard.

Code compliance

Truss manufacturers will engineer floor and roof trusses to comply with local codes. Building departments will typically require a set of the truss drawings stamped by the in-house engineer as part of the building permit submission, and will use these drawings to perform their inspection of the roof and floor framing.

Indoor air quality

Roof trusses exist outside the building enclosure, and so will have little effect on IAQ. Floor trusses will be contained within the living space (although typically covered), but being composed of timber and metal plates, do not create any IAQ issues.

Resources for further research

Metal Plate Connected Wood Truss Handbook. Madison, WI: Wood Truss Council of America, 2002. Print.

Future development

Truss design and construction has been streamlined over the past decades, and are unlikely to change much in the future.