Flat Roof Rafter Layout

A common rafter comprises two beams that extend upwards from the top plate of either side of a shed wall.

Flat roof rafter layout.

Here the number is 941 86 inches. Click the button to calculate the stock size needed for this job. Roof rise this should be calculated from the base of the roof to the highest point of the roof gable the peak. If your project had a rafter length of 8 or less a safe and strong method is to use 2x8 framed 16 on center.

This calculator is to be used as an estimating tool only. The reduced roof rafter span shall be taken as the larger of the distances from the roof rafter support brace to the ridge or to the heel measured horizontally. Flat roofs are always built at a slight incline at least 1 8 inch per foot. Once you ve marked the lines use your pencil to draw the diagonal length on the rafter.

Rafter span tables use these tables to determine lengths sizes and spacing of rafters based on a variety of factors such as species load grade spacing and pitch. This will help you create a flat roof framing plan. For the purpose of determining roof rafter sizes in table 804 3 2 1 1 ultimate design wind speeds shall be converted to equivalent ground snow loads in accordance with table 804 3 2 1 2. Rafter run outer wall this should be measured starting from the center of the roof span and then extending to the very edge of the exterior wall.

Visit the roof framing page for more information on cutting roof rafters and visit the roof pitch calculator for determining rafter lengths based on rise and run. Calculating roof geometry was made much more simple for me after reading a different approach to rafter layout by john carroll. Rafter stock size calculator. The article is in taunton s book framing roofs.

To lay out a common rafter you need the pitch of the roof expressed in units of rise per foot of run. To cut roof rafters start by fixing the stair gauges on your framing square to the dimensions of your pitch. Each beam is notched at the bottom called a birdsmouth so that it rests flat on top of each wall. When framing a flat roof you will need to create a slight incline.

Next set the framing square on top of the rafter beam and use a pencil to trace the outside edge of the framing square s tongue. At the top where the rafters meet they are either angled to meet at a point or attached to a horizontal ridge beam. Run is the horizontal distance that the rafter covers. Some roofs actually slope in several directions ensuring optimal water runoff.

Input the rafter span on the flat eave overhang and pitch of the roof. You can do this with a foam layer that exactly fits your roof. In the gable roof example shown here a rafter with 6 in 12 pitch rises 6 in. As a general rule roofs that have a high human occupancy rate should be framed with rafters every 16 structures such as sheds or barns can be frames with rafters every 24 again make sure to consult local codes.