Cutting and Setting Rafters
Here come the rafters...
STEP 1: Go to the gable end and pull your tape from the edge of the ridge board to the far corner of the wall plate. Record the measurement.
STEP 2: Pull the measurement on the opposite side and record it. Do not influence the position of the ridge while you do this, allow it to stand just as it is. The two measurements should be the same, but they very rarely are. Just find the average point between the two. That is, if one is 21 feet 7 inches, and the other is 21 feet 9 inches, then your proper length is 21 feet 8 inches.
STEP 3: Load a heap of rafter boards on your saw horses, crowning and marking them as you go.
STEP 4: Cut the pitch angle on the top end of two boards.
STEP 5: Hook your tape on the point and measure down the length to the dimension you arrived at. Holding your pencil at that point of the tape, scribe an arc across the face of the board.
STEP 6: Place your speed square on the board at the pitch angle as though you were cutting another top end angle cut here. Position the square so that the 4.5 inch mark intersects the arc. When you have it right, draw the pitch line. (Remember this is the 4.5 inches we added to the total brace height earlier).
STEP 7: Perpendicular to that line, at the 4.5 inch intersection point, draw another line out to the edge of the board.
STEP 8: Now extend this line past the pitch line a little. Measure 1/2 inch along the extended line and make a mark. Draw another pitch line parallel to the first one, 1/2 inch farther down the board. Doing this will allow your bird mouth to clear the sheathing and also give a little adjustment room.
STEP 9: Cut out the small triangle piece to leave the ”bird mouth” in the rafter tail. Leave the rafter tail uncut for now. Cut the two boards complete and then take them over to the roof.
STEP 10: With one person up on the catwalk and another at the lower wall, have the person at the lower wall nail a block on the outside end of the wall, sticking up a few inches above the plate. This is to prevent the rafter from sliding off the end of the wall and causing a mess.
STEP 11: Now hand the rafter up and position it in place against the ridge board and on the wall. The point of the raf-ter should meet the edge of the ridge board, and the edge of the bird mouth should have a ¾ inch gap from the side of the wall top plate. Tack the rafter in place with one nail on top. Don't drive the nail up.
STEP 12: Hand up the other raf-ter and put it in place opposite the first. With both hung, they should both have a 3/4” gap to seat correctly. You may need to push the ridge over slightly to make them both fit right. That is fine. The raf-ters will center the ridge for you. The goal is to make sure they both fit correctly. If they don't fit correctly, find the reason and correct it. Keep checking the fit of the pair until you are satisfied they are right.
STEP 13: When the first pair are right, label one of them as “pattern” and use it to trace the cut lines onto the remaining raf-ter boards.
STEP 14: Cut all the raf-ters and place them where they can be handed up easily.
Comon Rafters IMPORTANT:Before you start hanging raf-ters, install 2 long diagonal braces from the ridge board down to the base of the uprights. If this diagonal passes another upright, even better. Nail it to both. These two long braces need to be solid because there will be a considerable amount of weight once all the raf-ters are installed, and you do not want the assembly to rack and collapse.
STEP 15: From the end you started at, hang the first pair and after checking center, nail them in place.
STEP 16: Skip 5 sets of raf-ters along the length of the ridge board and set the sixth pair. Get the two gaps even and nail the raf-ters in place.
STEP 17: Continue along the entire length, skipping five sets and setting the sixth.
STEP 18: Finally, set the two end two raf-ters and nail them.
STEP 19: Now have a look at your center line string. It should be in the center of the ridge board along the full length. If it's not, find out why and correct it.
STEP 20: When the ridge is straight, you can fill in the gaps. Nail the center rafter pair in the middle of each 5 raf-ter section first, while checking for gap in the bird mouth seat and the string line straight.
STEP 21: When these middle ones are set, you can just nail the remainder of the raf-ters where they sit, without regard to gaps and the center line string. They should all be close at any rate.
STEP 22: When the rafters are all in place, cut a “collar” board for each pair. This board will cross from one raf-ter, under the ridge to the other raf-ter, directly under the ridge board. You want these collars long enough to nail solidly, but not so long as to stick out above the raf-ter tops. Use boards about 3 feet long and cut the angle on each end. Place three face nails in each end of the collar.
IMPORTANT: The collar ties create a static load across the top of the raf-ters and help everything hold itself together. You will place some longer collar ties lower, later. For now, the short collars are enough to stabilize the roof.
STEP 23: Add diagonal braces on each end that cross each way and nail them together where they do. A little overkill is fine with this. You do not want to have the roof lying on the ground when you arrive in the morning.
STEP 24: After all the roof framing is in place, install metal raf-ter ties (also called hurricane straps) on all corner raf-ters and every other interior rafter.
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