How to Measure for a Roof Truss

Installing a roof truss is a serious project that must be performed with great care according to exact specifications and measurements. The trusses are important structural units that bear the load of the roof. The size, spacing, and span of the roof trusses must be carefully calculated to hold the weight of roofing materials and accumulated snow loads. Prefabricated trusses are available from hardware stores and these are appropriate for a wide range of common projects. Specialized applications will require the builder to frame their own trusses using 2x4 or 2x6 lumber.

Step 1 - Design Trusses

To design an effective roofing frame you will need to start with several measurements. These include the slope of the roof, the length of an overhang, the size and location of ceiling joists, the size of ridge beams and bearing beams, and the load weight. Trusses can be fabricated by a lumberyard based on construction drawings or you can cut them yourself. If you order customized trusses, inspect them immediately upon arrival at the job site. Take an inventory and notify the supplier if you received the wrong number of sizes.

Step 2 - Mark Cap Plates

Roof trusses are usually installed 24 inches on center, but check the manufacturer’s codes and local specifications. It’s possible that these may require 16 or 12-inch spacing. Starting at one end of the roof, mark the first truss flush with the outside edge of a cap plate. Use a level, square, and measuring tape. Hammer a nail in place to hook your measuring tape and space out the subsequent trusses every 2 feet or other designated spacing. Measure out 5 to 6 trusses at a time. Mark the landing point of the truss heels on one side and then the other. To make sure that the first pieces of 4 foot by 8-foot plywood roof sheathing sit flush with the first truss but land in the middle of other trusses, you may have to reduce the spacing between the first two or three trusses by ¾-inch.

Step 3 - Cut Lookouts

For overhangs, use 2x4 boards to cut lookouts as long as the combined length of the overhang and the distance between the end trusses. Nail these lookouts in place with one end to the side of the first full set of trusses and the other ends over the gable end trusses with the tops of the lookouts flush to the tops of the full trusses.

Step 4 - Plumb Rafter Tails

Plumb the rafter tails to the proper length before installing roof sheathing and sub-fascia. Use a level to measure to the bottom of the first rafter tail. Measure out the overhang length minus the length of the sub-fascia. Mark the first and last rafter tails, then snap a chalk line to mark between the ends. Mark a plumb line up the side of the rafter tail and cut along this line with a circular saw. You can nail the ends of the sub-fascia into the rafter tails.

By considering all height and pitch requirements you can precisely take all measurements for your roof truss.