What’s the difference between Rafters & Trusses

Recently a client asked about how their roof was constructed.

It was explained that his Etobicoke home was originally built over eighty years ago; and that the method involved stick framing the original roof, and that covering the framing were roof boards about one inch thick.

The home had an additional flat roof at the rear, which had been constructed using a boxed truss system and that it had been sheeted over with three-quarter-inch plywood. The addition was only ten years old he noted.

As a recent owner of the home, he was curious to understand what he had.  His next questions essentially involved asking me what the merits and details of each construction method were.

I explained that his original house roof was a combination hip and gable design having different pitches and as a result, carpenters of the day would have had to hand-cut and frame the roofline to achieve the complex roof structure.

Trusses were used on the addition in modern times because the supporting structure would be repeated (a series of rectangular frames each identical to the other); which could be raised on the roof saving considerable time over building each “roof frame” by hand.

I further explained that a truss design was used for the flat roof because the designer had intended to create somewhat of an attic beneath the flat roof to accommodate a high degree of thermal insulation in the ceiling, and to permit an air space above.

Although not uncommon to use trusses in residential flat roof construction, most flat roofs are framed using a series of roof joists made of dimensional lumber, and are also typically framed by hand.

The client felt that the explanation was quite interesting. Me, not so much, but later I realized that in our job (as roofers), it is important to be able to identify the structural elements of the roof for such issues as achieving proper ventilation and insulation methods, roof drainage and when having to deal with structural roof repairs.

As a result, perhaps our clients would benefit from understanding some basic information on roof rafters versus roof trusses in order that they could achieve a greater understanding of what they ultimately would be paying for when undertaking major roof work.

What is a rafter?

source: nations home inspections

In the context of roof framing construction a rafter is a structural member made of dimensional lumber such as a (2”x4”),(2”x6”),(2”x8”), etc. Most residential roof framing on sloped roofs today utilize (2”x6”) spruce lumber.

On flat roofs which are hand framed,(2”x8”), (2”x10”) and (2”x12”) roof joists are the most commonly used and the size used is based on the span (width) of the roof, and the distance specified between each individual rafter.

Rafter spacing is usually completed on 16” centers, however roofs which must sustain a heavy load may be spaced at 12” centres. Both roof rafters and roof joists rest on the perimeter framed house walls, on what are known as a doubled top plate. Modern residential exterior walls are typically constructed using (2”x6”) lumber. Many older homes were built with (2”x4”) walls.

Larger wall framing members (known as wall studs) not only provides greater strength, but permits higher degrees of wall insulation to be installed,  in order to meet today’s building code requirements.

When framing a roof, there are a few common types of rafters which are used. At the top of the roof there is a member called a ridge beam. The ridge beam is the uppermost point of the roof where rafters are attached.

King rafters are used on gable roofs and the field surface of hip roof designs. They are characterized by having the same degree of pitch cut at the ridge as at the eaves area.

All rafters on a sloping roof have a “birds mouth or bird beak” as it is termed, cut toward the bottom of the rafter; which in effect is a notch cut which allows the rafter to rest on the top plate of the wall. The cut is made using the rise and run (number of pitch degrees), in order to achieve a perfect fit.

On a hip roof design (a four-sided roof characterized as having at least two triangular opposing slopes); hip rafters must be cut to form the sloping corners of the roof, running from the ridge beam to the intersecting outside corners of the top of the exterior walls.

The hip rafters require two complex angular cuts at the top of the rafter, in order that it may be nailed both into the ridge beam and also the king rafter on the adjacent slope. Rafters which must be attached to the hip rafter at their top-most point are referred to as jack rafters, and also require a complex miter cut to enable attachment to the hip rafter which is on a descending corner angle.

To complete such cuts requires the expertise of a skilled framing carpenter. Hip rafters are also constructed with a bird’s beak to permit attachment to the outer corners of the tops of the walls.

Essentially any roof which involves a complex roof design or multiple pitches requires stick framing methods, performed by a carpenter.

What is a truss?

Roof trusses are pre-engineered framing structures that can provide a number of advantages. In keeping with modern building methods; most roof trusses are computer engineered and factory built to a specific project’s requirements, or on mass; which makes standard pitches and spans available for builders.

Some advantages of using roof trusses include the following:

  • Can be pre-engineered to accommodate snow load and wind requirements
  • Can save labour and time in relationship to installation
  • Lower manufacturing costs versus conventional framing lumber
  • Can be shipped to site and craned onto building
  • Can lessen construction time
  • Can incorporate complex interior ceiling design such as vaults and barrels

Most typically roof trusses are ordered by a builder who supplies drawings to the truss manufacturer. The truss manufacturer then replicates design specifications into their computer which can program material cuts. Shop staff can then factory assemble the trusses and prepare for shipping to site.

Trusses utilize galvanized metal plates which are placed strategically where intersecting frame members adjoin one another.

Unless standard truss designs and sizes can be utilized, the builder must be prepared to wait several weeks for the manufacturing of custom orders to be completed.

Once delivered to the jobsite, trusses can be lifted to the roof where typically two or three framers can stand up the truss sections and secure them to the top plates of the walls. Dimensional lumber is then used to tie/brace the newly erected trusses to one another.

As there are specific names for rafters designed for a particular purpose; trusses also have specific names based on the particular design characteristics of that truss. As an example, certain trusses may have an integral framing member in the shape of a “W” which is termed a “King” truss.  Those who are familiar with roof trusses can then specify a particular design of truss by name when describing what they may require.

With modern roof building methods, many homes and buildings are constructed using a combination of roof trusses and conventional framing. As an example; if building a structure is designed with a gable roof; the builder could erect triangular trusses to comprise the main building structure, and then utilize the carpenters to hand frame a series of dormers having a different design and pitch.

In the roofing industry; there are always requirements to repair and replace structural roof and wall damage which can occur as a result of prolonged leaking, mould, fire, and original design flaws. As a result, AVENUE ROAD ROOFING® has experienced, skilled carpenters and team members versed in the requirements of restorative roof work.

The carpentry division at ARR® also constructs beautiful and strong roof decks, ground decks, can slope flat roofs and rebuild porches and structural columns related. Heritage homes that have exterior wood siding that needs repair are another service provided among other building envelope maintenance items.

To learn more about roof-related carpentry, carpentry, roofing, repairs and maintenance contact: www.avenueroadroofing.com or visit us on YOUTUBE™.