Use of Wood Roof Trusses in Canada

The project site involves a barn that employs the use of wood trusses for its roofing needs.  A roof truss is a structural framework that reinforces the roof as it bridges the space between the room and the roof (Menges, Schwinn and Krieg 2016, 108). The truss has chorded and webbed triangular arrangements that that transfer weights to reaction points. The structural design provides them with more strength to weight ratios as well as greater flexibility in the roof plans.  The Truss design date to medieval times and was a constant architectural component in the past centuries. The shape of the truss rarely changes, only the material composition, spans and safety requirements change.

At the onset of the modern era, onsite construction of truces was common. The site building involved the use of plywood plates or nailed boards for the truss joints.  The evolution of truss construction was witnessed in Pompano Beach Florida in 1952 when offsite construction of trusses was patented. Carrol Stanford established Sanford industries that led to a worldwide truss industry.  The common design trend in the truth manufacture is the use of metal plate connected wood trusses.

History and Use

In Canada and the larger North America, wood frame construction techniques such as truces have dominated the market for the past 200 years. The roof trusses were initially built onsite before the 1950’s. At the moment nearly 95% of Canadian roof trusses are manufactured offsite. Initially, Truss joints were designed using plywood gusset plates and enjoined using nails. The 1950s and 1960 are shifted the joint designs to use metal connector plates (Roth and Clark 2016, 32).

In Canada, more houses are built using wood roof trusses and not metal roof trusses.  60% of residential homes in North America use roof trusses, and 95% of new houses in Canada use wooden trusses (Sabatino and Liscombe, 24). The reason for these is the strength that emanates from the structural wood system. It is also economical to incorporate the use of wood as a framing solution. To some extent, wood trusses are versatile and will accommodate most design requirements. Wood has also been favored for use in Canada since it is renewable and sustainable. 

Conservation of Wooden Roof Trusses

Wooden Roof Trusses are very durable when properly prepared and installed. Most roof trusses are designed and developed in factories away from construction sites. The factories that design them and constrict them are expected to use a conservational approach in their construction. Onsite design and construction of Trusses are costly and wasteful. The factories can minimize wastage and observe conservational efforts when sourcing materials.

Penetrating Damp

Water is a major damaging agent to wooden roof trusses. The water leaks often reach and impact the wood by causing rot and allowing insect attacks. Water often impacts wood even when it is treated. The treatment fails to penetrate through the thickness of the wood to prevent penetration from external elements.  The water penetration might compromise the structural integrity of the truss. The wood upon being impacted by the water might warp, bow or rot (Cotton 2016, 47). The warping and bowing of the wood disrupt the joints of the connecting plates in the wooden trusses. The chord and webs are compromised from the loss of connecting bolts and nails. Even when water does not lead to wood rot or insect infestation, it might lead to warping and bowing. Warping and bowing might also emanate from extreme weather conditions such as the winter cold and the hot summers. As the wood keeps moving due to water damages its bolts, nails, and joints are weakened and eventually fall.

The wood trusses have to be protected from water. The water is not supposed to change or damage the wood trusses.  The treatment is done chemically to help the wood less absorbing water. The water deformities on wood include warping, dampening and bowing.  These actions lead to joint weaknesses and might lead to the loss of joints and bolts. 

Other ways to prevent weather damage on roof trusses is through covering and sealing the roof edges to avoid water penetration and the direct effects of weather elements. The water gutters are also useful in collecting and directing water away from the vulnerable wooden trusses. Regularly coating the wood with paint also eliminates the adverse effects of water by minimizing contact.

Joint Damage

Most trusses get a careful and purposed design at the factory with complex software. One ought to have a clear understanding of the truce design before considering any repair attempt.  The design of the truce has to be available to avoid additional work and possible weakening of the structure. The components that are designed at the factory are mostly specific in purpose. Any attempt to repair the truces that might be contrary to the original factory design measurements might compromise the integrity of the roof.

Axial forces, shear forces, and moment forces within the truss can affect the repair work. The axial forces act parallel to the truss members (Molloy 2016, 88). The forces create tension or compression to the members and emanate from the resistance of gravity, wind or design loads.  The shear forces are perpendicular and are concentrated on load and bearing locations. Moment forces cause rotation and are focused at the joint.

The Modern roof structure uses nails, screws, and glue on the joints. The timber installed in the roof trusses at times shrink and expand as a consequence of weather changes. The timber might at times shrink and result in loose joints.  So as to eliminate instances of shrinking, it is important to kiln dry the wood. Kiln dried wood experiences minimal instances of shrinking (Branco, Descamps, and Tsakanika 2018, 138).

 Another approach is to treat the timber used for trusses with high pressure. The timber is chemically immersed in a room of high pressure. The pressurized chamber ensures that the chemicals get to the core of the wood fibers. It is considered to be more effective than simply immersing the wood in the chemical. The treatment prevents fungal decay and attack from termites.

Sustainable Approach

There are instances in which an entire roof truss or a portion of the wood truss might need replacement. In such instances, the replacement material selected is vital. The replacement situation provides the owner with the chance to select a new material that is suitable for the replacement and will last long (Cibecchini et al., 2016, 379). In that instance, it is advisable to select a material that is strong, durable and lasting. The best material that meets these standards and is sustainable is wood. It is prudent to perform the replacement with wooden trusses or parts of wooden trusses instead of steel.

Damaged portions of the roof truss that require replacement are first identified. The next step is to examine the details of the timber and the extent of the damage. When replacing a portion of the timber, the point that is stable to splice gets noted.  The suitable props and are places and the damaged section cut.  The targeted section is drilled with the number of holes intended for the bonding. The resin is used alongside nails to strengthen the new joint. The technique ensures that there is a partial replacement of the roof truss instead of a whole replacement.

The partial replacement of wooden truss is environmentally friendly and sustainable.  Wood as a material for construction is considered sustainable and renewable. Wood as a construction component gets favored by environmentalists over concrete and steel (Khatib 2016, 78).  The main reason is that wooden building materials become easily repurposed and renewed once they reach the end of their cycle. The wooden portions removed from the replaced portions can be purposed for uses that do not require similar strength as that of a roofing truss.  In essence, wood is favored since by using it; carbon dioxide is not released into the atmosphere.

Wood use might be considered to be a danger to the forests. The problem gets eliminated through reforestation (Cibecchini et al., 2015, 156). Companies are urged to regrow the portions of the forest they cut down to source the timber. The use of steel is not sustainable in the long run. Its continued uses exhaust the mineral desists. Wood, on the other hand, is renewable through forestation and poses little threat to the environment.

Conclusion

Wooden trusses have become an integral part of the Canadian construction industry. Their construction moved from onsite construction to complex offsite designs. The wood is susceptible to water damage and joint damage.  The water damage might result in the rotting and bowing of wood which compromise the structural integrity of the roof trusses.  The impact of the water is eliminated through constant painting and proper wood treatment. The joint repair has to consider the concentration of the shear, axial and moment forces. The repair sometimes requires the partial replacement of the wood segment. Wood is the appropriate replacement material since it is sustainable and renewable.  The use of wood trusses is likely to continue in the Canadian construction industry. The Government and environmental groups should impose strict lumbering regulations to ensure that wood retains its status as a renewable and sustainable construction material.

Bibliography

Branco, Jorge M., Thierry Descamps, and Eleftheria Tsakanika. "Repair and Strengthening of Traditional Timber Roof and Floor Structures." In Strengthening and Retrofitting of Existing Structures, pp. 113-138. Springer, Singapore, 2018.

Cibecchini, Daniele, Niclas Bjorngrim, Hélder S. Sousa, Marco Jorge, and Jorge M. Branco. "Repairing of a timber truss through two different techniques using timber elements and screwed metal plates." In Historical Earthquake-Resistant Timber Framing in the Mediterranean Area, pp. 371-381. Springer, Cham, 2016.

Cotton, Lyndsay. "The benefits of pre-fabricated timber roof trusses: environmental engineering." Civil Engineering= Siviele Ingenieurswese 24, no. 7 (2016): 47-48.

Cuadrado, Jesús, Mikel Zubizarreta, Belinda Pelaz, and Ignacio Marcos. "Methodology to assess the environmental sustainability of timber structures." Construction and Building Materials 86 (2015): 149-158.

Khatib, Jamal, ed. Sustainability of construction materials. Woodhead Publishing, 2016.

Menges, Achim, Tobias Schwinn, and Oliver David Krieg, eds. Advancing Wood Architecture: A Computational Approach. Routledge, 2016.

Molloy, Edward. Roof Construction and Repair. Routledge, 2015.

Roth, Leland M., and Amanda C. Roth Clark. American Architecture: A History. Hachette UK, 2016.

Sabatino, Michelangelo, and Rhodri Windsor Liscombe. Canada: Modern Architectures in History. Reaktion Books, 2016.