Rafter Calculator

A roof truss is a triangular wood structure that is engineered to hold up much of the weight of the rest of the roof. If you can’t get onto your roof to complete this measurement, it should be identical to the measurement of the width of your home. If a rafter is supported on to points only (single span) and an appropriate size cannot be found in the tables then an additional support (underpurlin) is needed. The truss top chords, bottom chords, and webs are of 2×4 that weighs about 1.2 lbs/ft.

Many of the major manufacturers also offer complimentary software which do sizing and load analysis type calculations. Whenever the lower span dimension is changed the other dimensions are automatically recalculated to maintain the same regular proportions. Alternately, there are now computer software programs that will both calculate your roof truss load and render a diagram of what the end result should be. This is based on the numbers of members and nodes you enter.

After you’ve braced and installed all the rafters, you’re ready to start installing a layer of roof sheathing-basically just plywood, over which the weather stripping and shingles will be installed-nailing it to the outside of the rafters accordingly. To locate the Rafters under the sheathing, snap a chalk line onto the sheathing marking the center of each Truss Rafter.

Determine the distance from the edge of the roof to a point on the same plane directly under the highest point. Vented roof air space: created by leaving a 51 mm (2”) space between the plane of the top of the joist and the top of the insulation. To lay on the next run of membrane or felt, start fitting more batten to the gauge you have worked out, continue this sequence all the way to the top of the roof and overlap the felt or membrane at the apex as 150mm or 6″.


It may also be important to install supplementary ridge boards along the walls, to provide extra surface on which to secure the trusses. Although the tables cover the majority of roof timbers you are likely to come across in most domestic or small commercial buildings, very small or large timbers may not be covered. If you are building an extension you will probably need to match it with the existing soffit size of the main building. I am working on a project where we are using wood trusses (2×6 for top and bottom chord, 2×4 for web). The labor to set the trusses is no different than had you not included the room.

Structural analysis of trusses of any type can readily be carried out using a matrix method such as the direct stiffness method , the flexibility method or the finite element method. If the width and height of the roof need to be determined by direct measurements of an existing structure, take appropriate precautions before accessing the roof area. Refer to Figure 7 to find the worst case in the roof structure (longest underpurlin span).

Nail size obviously depends on the size of the lath or batten you are using, but it’s recommended that the nails penetrate through the batten and a minimum of 40mm (11/2 ) into the rafters underneath. It uses data from span tables to work out the maximum permitted span of the roof timbers based on the imposed loads from the roof covering and the solar array.

The difference between the green span and the purple span below is the addition of the horizontal overhang shown above to allow a fixing for the fascia and soffit boards. By that I mean for estimation purposes I can stand looking at the face or faces of a roof and write down the Width and Height of each individual face of the roof on a piece of paper.

For example, if the height is 7.5 feet and the width is 15 feet, the tangent of the roof angle is 0.5 because 7.5 divided by 15 equals 0.5. You can’t move into a house without a roof , but we have seen people living in a house with no window or makeshifts windows covered with ordinary bags and cardboards. Input the hip end pitch in decimal format in inches per foot of run (i.e. 7¼ on 12 would be input as 7.25). However, this only applies to the two ends of the roof peak from the Gable Ends to the first Truss.

It is also handy when quoting a price for a roof that you haven’t been supplied a technical drawing for. The first tool on this page works out approximately the percentage increase in loading on the roof, assuming that the array weight is spread evenly over the area that the panels cover, and using typical figures for the dead weight of the existing covering.