Roof Rafters Construction Algorithm — Pro Barn Plans

Roof Rafters Construction Algorithm - Pro Barn Plans

Barn Roof rafters are sloping beams that support the weight of a roof. Detailed instructions and images for laying out, sawing, and installing. Trust the pros.

Whether you intend on ever finishing the ceiling or not, you will need at lest some ceiling joist. You might as well go ahead and put a joist beside each of the roof rafters. Nail down a 2 x 4 rat run down the center of the joist. It will make walking around much easier. It will also give you a chance to straighten the joist. Joist give you more to nail the rafters to and add strength to your building. You are now ready for rafters. If you are not familiar with laying out roof rafters. then there is an alternate approach you can take.

  • Find the center on one of the gable walls. Erect a 2 x 4 roughly the height you wish your roof to be and brace it.
  • Tack a 2 x 6 on the outside edge of the gable wall and the 2 x 4 you just erected. Allow just enough of the board to hang past the 2 x 4 to make the cut. Make sure the level cut on your rafter notch will be from 2 3 1/2 inches long.
  • Mark the 2 x 6 rafter pattern at the top next to the erected 2 x 4. Mark it on the bottom around the top of the wall.
  • Roof Rafters Construction Algorithm - Pro Barn Plans
  • If you want an overhang, then measure out horizontally from the band and mark it on the rafter pattern. It should be no less than 6 and no more than 12. Use a level to scribe a plumb mark. You now have a rafter pattern. Write pattern on it.
  • Use your pattern to mark 2 rafters. Saw them and check their fits against the erected 2 x 4. Remove the 2 x 4.
  • Cut the rest of the rafters.
  • Nail the bottoms of the rafters for each corner. Make sure these are select boards so will not have to wrestle with them later. Lean the rafters against each other at the top.
  • The ridge board needs to be the length of the building if at all possible. Move it into position below where you have the rafters tacked together. Remove the tacks and slide the ridge into position. Nail one side at a time.
  • Put another 2 x 4 back where you had removed the one you were marking beside. It should fit nicely from the bottom of the ridge to the top of the gable wall. Put a brace on it down to the rat run and plumb the gable.
  • Nail on the other rafters. Use two toe nails down into the top plate and three into the joist. This is minimum. Make sure there are no gaps.

Barn Roof Rafters Laying Out

If you are familiar with the process of laying out a rafter then you should go to  for the best online construction calculator available.

In a nutshell, the roof rafter will start by determining the run. To do this measure the span where the rafters will go. This is most likely the same as the width of the building. If the building is 20 wide, then the roof rafter will have a run of 9 11 1/4. This is based on the formula Building Width minus 1 1/2 (ridge thickness) divided by 2.

This run will determine the length of the rafter.  The more traditional method of using a framing square would involve stepping off 9 full feet with whatever pitch the roof rafters will be on and then adding the 11 1/4 as the last step

No matter the method used to determine the roof rafters. The three parts of the them are all the same. What people will see when they look at your work is the fit of the framing members. You do not want gaps.

  • Ridge cut: A plumb cut at the top of the board. This cut can be determined precisely by the pitch.
  • A birdsmouth or notch: This is a plumb and a level cut and is where the board sets on another framing member.
  • Tail: A cut just like the ridge cut in orientation. It will determine the overhang.

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