There are three main types of curved rafters in roof construction. A tower or conical concave rafter or roof is a modified tall A-frame type rafter system with the curve being towards the inside of the “A”.
Elliptical convex is a type of roof with a slightly smaller angle, having the curve towards the outside. Ogee is a decorative combination of concave and convex curves.
This article will describe the steps required to fabricate an elliptical convex rafter joist.
Step 1 - Design the Desired Curve
The first thing you’re going to need to do is to design the curve you want. You’ll need to figure out what size boards you’ll need to buy in order to maintain structural integrity and still get the curve you want.
Step 2 - Transfer Design to the Rafter Lumber
Once you know what the rafter curve will look like and what you need to maintain structural strength, you need to draw the desired curve out on the boards you’ll use for your rafters. Once you have the exact curvature laid out, draw a second curve on the board, allowing room for cutting and sanding.
Step 3 - Make the Needed Cuts in the Boards
Now that you have the desired curves laid out on the boards you’ll be using, it’s time to start making your cuts. Remember to cut a little on the big side, so that you can sand out the roughness in the wood and make minor corrections to the cut with the sandpaper. You may have to cut from both sides of the jigsaw blade you have doesn’t go all the way through. Transfer your cutting guidelines to the opposite side of the wood and cut accordingly.
Step 4 - Sand Out Roughness in the Wood
Now that you have your basic curves laid out and cut, it’s time to do the finishing work. Start with the 60 grit and go over the whole length of the ceiling rafter that you’ve just cut. Use the 60 grit to make sure the curve dimensions are exactly what you want. If not, the 60 grit will remove wood more quickly than the 150 will to bring out the desired curves. Once you have the desired curves, switch to the 150 grit and sand out the roughness left behind by the 60 grit.
Step 5 - Treat the Wood
Once you have all the sanding done, it’s time to treat the wood before assembly. If the curved rafter is going to be decorative and visible, you might want a stain instead of a plain treatment. If the rafter won’t be visible, then a plain treatment such as Thompson’s will suffice.
Step 6 - Assemble
Now it’s time to put the rafter together. If the length is more than a single board in length, you’ll need to use Simpson Pocket Strong-Ties to join them. For added strength, it’s also recommended to place a 2-foot section of 2x4 centered over every point where 2 boards come together. Secure these with the wood screws. Once this is completed, it’s time to frame them up into your ceiling.