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What is a roof truss?

Roof trusses are engineered structural products that when considered as a group make up a building's roof system. In most cases roof trusses comprise the part of a building above the walls. When walls for a home or commercial structure are built they are topped off with a board often referred to as the top plate. Roof trusses are placed on this top plate. The area of the truss that actually rests on the plate is "bearing" on the wall.

The actual shape of a roof truss varies with each application. Before the advancements in truss designs available today, trusses were mostly built in a triangular shape. Multiple configurations are now available to enable the consumer to incorporate variations in roof and ceiling design.

Why use roof trusses?

Practical alternative

Let's assume you are building a 40 feet by 40 feet structure without any interior walls, similar to the layout of a small grocery store. The foundation is formed so that the outside perimeter of the building can accept load-bearing walls. Now if you considered how you would form the ceiling joists and roof rafters with dimensional framing lumber, you would have a difficult task in developing feasible alternatives. First, lumber is not readily available in 40 feet lengths. Even if it were, it would not span the distance without unacceptable sagging to say the least.

The previous example shows one of the basic reasons for the development and feasibility of roof trusses. Incidentally, assuming you are going to apply plywood decking, felt, and shingles, trusses could be fabricated out of 2x4 lumber and be placed every two feet (requiring 21 trusses). These 21 trusses would provide the triangular shaped roof system for our hypothetical grocery store.

Economical

By using roof trusses, costs of a construction project can be reduced in several ways. Because most residential and many commercial jobs can use trusses fabricated from 2x4's, the need for larger and considerably more expensive dimensional lumber can be eliminated.

The delivery of trusses can be timed so they arrive at the job site as the project's walls are erected. Often, a crane in used to assist in the erection of larger trusses. An experienced crew can erect the roof trusses of a typical residential project in a matter of hours, and feasibly have the project dried-in the next day. Had the roof been conventionally framed, depending on complexity, this process could take as long as two weeks.

Diversity

Trusses can be fabricated into numerous shapes allowing the user to achieve incredible variations in roof lines and ceiling configurations. Ceiling heights can be raised within the design of the truss. Often times, this eliminates the need for costly wall framing with longer studs. Cathedral ceiling, trays, coffers and vaults can be built into the truss system adding volume and character to the project.

Specific Information

Span and Run

If you have the gist of what trusses are, let's learn some specifics. Say you have been assigned a "honey do" of magnificent proportions where you are expected to add a room onto your home. You are to add a room 20 feet wide and extending out from your home 16 feet. You want the addition to appear as though it was part of the original design of the house.

You already have some of the essential information to successfully order the roof trusses for your project - span and run. The width of the addition determines the span of 20 feet. This would agree with a measurement taken from outside of wall to outside of wall. The run determines the quantity of 20' trusses you will need. Trusses are generally spaced two feet on center - two feet apart. Your room is twenty feet wide and you need trusses to cover 16 feet out from your house. If you divide 16 by 2 and add 1 truss to start with, you have determined that you need nine trusses that are 20 feet wide. Easy, and nearly half the battle.

The last truss (or outermost truss) can be built as a gable with uprights 16 inches on center. This will facilitate attaching your siding or material of choice in the gable end.

Pitch

Roof pitch is simply the number of inches of rise in a 12 inch run of your roof. Determining your roof's pitch will match the slope of your roofs.

Overhang

Most rooflines extend past the exterior walls providing a protective overhang for windows, doors and the house itself. If you want to keep the same overhang for your addition, measure your existing overhang on a horizontal plane. Because you will more than likely cover the outside edge of your overhang with a board or two (fascia), you will need to remove a section of existing fascia to view the existing overhang. This is generally done in an area where the addition meets the existing house. With the fascia removed, the overhang can be measured accurately and the material needed to duplicate the existing fascia can be determined.

Heel Height

You now have the ability to check your slope and overhang. One more variable is required for a perfect match between your addition and existing house - heel height. The overall heel height is the vertical distance of rafter material (trusses or common framed) above the outside edge of your top plate.

Cantilevered Trusses

Previously, we discussed truss overhangs. Typically, the overhang of a truss is merely an extension of the top chord as illustrated in the drawing to the right labeled 'overhang.' There are two other alternatives for overhangs. One extends the bottom chord out over the bearing, commonly referred to as a cantilevered overhang. Another overhang option is provided by the an energy truss.

One of the advantages of utilizing a cantilevered overhang is the resulting rise of facia height. If a project requires circle tops over windows or a transom above a door, an overhang extending from the top chord reduces the amount of wall area that can be viewed "from the street." If you have 9 feet high walls, a 6 on 12 roof pitch, and an overhang of two feet, the overhang will drop 12" below the plate line. This will allow only eight feet to place window and door treatments that can be viewed without overhang obstruction. Using the same specifications as above, a cantilevered overhang allows all nine feet of wall to be seen.

An energy truss offers the same advantages as the cantilevered truss (raising the facia); however, the soffit is not returned. By the way, a soffit return is the lumber filler placed horizontally from the end of the top chord to the wall. The energy truss allows the project designer an option of raising the overhang several feet above the plate line. This provides the illusion of higher walls and the additional space for brick or stone work over tall windows and doors.

Along with esthetic reasons, the cantilevered and energy trusses prove to be beneficial with respect to installation of attic insulation. By raising the roof line considerably higher than the outside walls, insulation can be applied in a more effective manner to the plate line.

To be continued . . . . (check back with us soon!)

6 on 12 roof pitch

overhang

total heel height

cantilevered

energy