How to Build a Roof Joist for a Flat Roof


Flat roofs, seen in Spanish, Tuscan and Southwestern architectural styles, offer simple horizontal lines that visually complement most landscapes. When setting the joists for a flat roof, it helps to remember that the roof won’t really be flat. A gentle slope is necessary for adequate drainage. To achieve the correct grade, you’ll set the joists and then construct the slope. Despite the need for a slope, flat roofs are among the simplest to build. Once the exterior load-bearing walls are in place, you can set the joists.

Things You'll Need

  • Large-dimension lumber
  • Tape measure
  • Circular saw
  • Chalk line
  • Nail gun
  • 16d nails
  • 8d nails
  • Install a rim joist on top of the exterior walls. A rim joist forms a box around the top edge of the roof, like a picture frame, and it must contain the same size dimensional lumber as the joists. Flat roof joists are typically larger than standard roof rafters and the engineer or architect will specify the size of the joists, depending on the distance they must span and on the type of roof covering desired.

  • Set rim joists on edge, flush with the outer edge of the top wall plate, called the “tie-plate.” The standard exterior wall contains 2-by-4 boards, which make the tie-plate 3 1/2 inches wide. A 2-by-10, which is typical in flat roof construction, is 1 1/2 inches thick, and when installed as a rim joist, will cover the outer 1 1/2 inches of the tie plate.

  • Toenail the rim joists in place by inserting a 16d nail, with a nail gun, every three feet along the outside of the rim joist. Toenailing involves shooting the nails through the bottom edge of the rim joist into the tie-plate at a 45-degree angle.

  • Insert four 16d nails at the corner of the rim joist, evenly spaced, through the side of the joist that overlaps the end of the next joist. Do the same on all four corners of the rim joist

  • Set the roof joists, starting from one end and spacing them 16 inches apart. The edges of the joists will sit on the 2-inch ledge of the tie-plate that lies on the inside of the rim joist. To attach each roof joist, toenail it on each side with one 16d nail into the tie-plate. In addition, insert four 16d nails, evenly spaced, through the outside of the rim joist and into the ends of each roof joist.

  • Create a slope of at least 1/8 inch per lineal foot. The simplest way to do this is to cut a dimensional board, lengthwise, from corner to corner. For example, if your roof joists were 14 feet long, you would lay a 14-foot 2-by-4 flat and snap a chalk line from one corner to the opposite corner at the far end of the board.

  • Cut along the line with a circular saw to make two long wedges. In this example, when the wedges are installed on top of the joists, with the thickest part of the boards all on one side, the roof will slope at the rate of 1/8 inch per foot. Individual roof lengths vary, so figure and cut accordingly.

  • Install the wedge boards directly on top of the joists, toenailing with 8d nails every 12 inches, on alternating sides of the roof joists. The high end of the wedges should be flush with the end of the roof joist, but they should not extend onto the rim joist. Install all the sloped wedges in the same manner.

  • Build a mini rim joist behind the high edge of the wedge boards. In this example, you’d need a standard 2-by-4, set on edge, directly on top of the rim joist.

  • Toenail the mini rim joist in place by shooting one 16d nail through the outer edge and into the main rim joist below, every three feet. Secure the mini rim joist to the wedge boards by inserting two 16d nails through the outside of the mini rim, evenly spaced, into the ends of the wedge boards. Now you’re ready to install the roof decking and coating.

Tips & Warnings

  • You can increase the pitch of a flat roof, but do not decrease it to less than 1/8 inch per foot or the roof could hold water.
  • Follow manufacturer’s safety measures when operating power tools, including circular saws and nail guns.

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