Precision in Construction: Verifying Dimensions for Trusses
Ensuring accurate truss measurements is crucial for a smooth and efficient building process, preventing costly delays and modifications. A technician will first verify site access, ensuring that large delivery trucks can reach the job site without obstructions. Once accessibility is confirmed, they will take precise measurements, focusing on dimensions like roof pitch, heel height, and overhang length, which are especially critical when matching existing structures. Proper preparation and understanding of the process can save time and prevent unexpected issues during installation.
Site Access Requirements
The first step is ensuring the truck delivering your trusses will succeed in accessing the job site. The technician will look for overhead obstructions such as powerlines and tree branches. The driveway must be wide enough and straight enough for the truck to maneuver in. If the jobsite does not meet these requirements, the shipping department will work with you to find a solution to get the trusses on site. The technician will move on to measuring for the structure after determining the accessibility of the job site.
For a brand new build, the technician often only needs access to the foundation or the framing in cases where that process has begun. They are looking to ensure that the dimensions in the design match the dimensions of the job site. In remodels on existing structures, they need to measure for roof pitches, heel heights, overhangs and other aspects depending on the shape of the trusses and building.
When adding to the gable end of the building, where the siding goes all the way to the top of the peak and you can see the slope on both sides of the truss, it is critical to get exact measurements for a seamless transition between the old and the new. With a T-style addition, where they add a space perpendicular to the roof of the existing building, there is a “valley” where the old and new join. In this style, the dimensions are less critical, but are still an important part of the build.
Preparing for Measurements
It is helpful to provide a ladder for your technician and be prepared to assist with measurements. The siding and soffit material needs to be removed when verifying for remodels on existing structures. Soffit material is the siding that is on the underside of the overhang. Exposing the framing allows the technician to precisely measure from the peak to the bottom of the truss. Both sides of the truss need to be exposed in case the framing configuration varies between sides. It is wise to have plastic sheeting available to create a vapor barrier after the measure. Trusses are generally measured a week before delivery, so there is little danger of long term exposure.
Key Measurements
A cantilevered overhang with span and length.
Span and Length
The span of the truss is the measurement from the outside of the bearing on one side of the truss to the outside of the bearing on the opposite side. The measurement excludes any sheathing that may be on the framing.
The length is the measurement of the whole truss, including any overhang past the bearing.
Heel Height
The heel is the portion of the truss that sits over the outside of the wall. Trusses can be built with a standard heel or a raised heel. The bottom chord sits on the bearing in both of these heel configurations.
A standard heel has a 2x4, 2x6, or 2x8 top chord that sits on the bottom chord, while a raised heel is built with a member between the top and bottom chords. The member allows for more insulation space near the end of the truss. If the technician finds that the framing has a raised heel, they will want to measure how tall the heel is. The height is measured from the top of the bearing wall to the top of the truss in the same vertical space.
A third possibility when the heel is exposed is to find a heel framed onsite from rafters, resulting in a Bird’s Mouth. A Bird’s Mouth is a way to connect the truss to the bearing by cutting the truss around the bearing to achieve an overhang. A problem may arise if the cut at the bearing wall shortens the width of the top chord to less than 3.5 inches. In this instance, the truss manufacturer may build the truss to match the height of that heel and the framer can add the overhang. The truss manufacturer may also lower the bearing wall for the rest of the addition so the 2x4 can pass over the wall.
Roof Pitch
The heel is also where the roof slope begins. Roof slope and roof pitch are interchangeable terms in the truss components industry. The pitch of the roof is referred to as how many inches of vertical rise there is in 12 inches of horizontal run. Standard terminology may be a 4 in 12, 5 in 12, etc.
The optimal way to measure the roof pitch is to use a string line or laser level from the peak of the truss to the end of the truss on each side and from the peak to the very bottom in the center. Often, it can be difficult to get this measurement with bumps in the roof or other obstacles. A technician will more commonly use a level and mark where the end of it lands horizontally. Based on that point, they will take vertical measurement to determine the rise. It is important to get this measurement to the 16th of the inch.
Now that the technician has taken their measurements, they can use a calculator such as a Construction Master to determine the pitch of the truss.
Although it may be convenient to use a speed square to measure the slope of the roof, there is not enough length on the square to measure with precision. It is more beneficial to use a 2 foot or longer level for the measurement, even if it feels tedious. With a length projected over multiple feet, a 16th of an inch off can result in inches of misalignment down the road.
Overhang
There are two commonly used methods of creating an overhang. The first method is doing an open rafter, where the overhang is a single piece of lumber. This piece may be a 2x4, 2x6 or a 2x8. Depending on the thickness of the lumber, there are length restrictions for the overhang. Most of low lying Western Washington builds overhangs for a 25 psi snow load. In this area, 2x4 overhangs are limited to three feet. This restriction prevents creep, which is when the overhang starts to sag. For all lumber width restrictions, it is advised that you consult with a truss manufacturing professional, such as Truss Components of WA.
The second commonly used method is an enclosed soffit, which is when the whole truss extends past the bearing wall in a cantilever. Trusses can be built with one-third of the truss length extending past the bearing wall or one half of the truss span. Twice as much of the truss needs to be inside the bearing wall as what is outside it.
Measuring for Various Truss Types
When matching to a common or scissor truss, the technician will expose the framing and find out where the bearings are. They will measure the span and from each bearing to the peak. If the original structure was built with stick framing, it is possible that the peak is a few inches off from the center of the truss. It is critical to align the new trusses to the existing peak. They will measure the overall height from the ceiling or level of bearing to the peak. Along with the heel height, the technician will determine the thickness of the top chord and measure the length of the overhang.
When measuring a mono slope truss, the technician will expose the heel of the truss to determine the height and the overhang. They will also expose the taller end of the truss to measure that height. The measurement of the bottom chord and the height of the taller end of the truss, the technician will be able to determine the roof pitch.
A porch truss is a truss with a shallow slope on the outside that then becomes steeper as it nears the peak. It is often used to cover a porch without drastically extending the height of the truss. Along with the measurements that would be taken with a common truss, the technician will also measure the pitch breaks on a porch truss. A pitch break is a point where the slope changes on the roof. They need to know where this occurs horizontally on the truss and what the various pitches are. This is also an important measurement on a gambrel truss, which is commonly used on barns and has multiple pitch breaks.
Conclusion
Accurate measurements ensure a perfect fit for trusses, reducing the risk of structural issues and costly modifications. Providing site access, exposing necessary framing, and assisting with measurements can help streamline the process while minimizing delays. Although there may be a fee for a site visit, this cost is far less than the expense of replacing or modifying trusses that don’t fit correctly. Taking these steps ensures a seamless installation, saving time, money, and effort in the long run.
