Framing dormers that don't leak is a mystery to some folks. There are two basic types of dormers: Full dormers that provide light and/or egress for living areas, and half dormers that are just for the exterior look of the house, and are in the attic on the inside.
You will be an expert at framing dormers if you follow the steps below.
To frame a real dormer, follow these steps:
STEP 1: Lay the sub-floor plywood.
STEP 2: Build the two dormer side walls and set in place.
STEP 3: Build the front dormer wall and set in place between the two side walls.
STEP 4: Brace the three walls to hold them in place plumb.
STEP 5: Install the top plate pieces when framing dormers.
STEP 6: Install the main rafters on either side of the dormer, immediately against the dormer sides, and nail them to the dormer.
STEP 7: If you have a support wall near, you can frame the dormer roof with valley rafters, but there is almost never a support wall there. So, set a header across the interior ends of the dormer walls. The header must be large enough to support the load across the opening.
STEP 8: Taper the ends of the header down to 4” so it does not protrude above the roof line.
STEP 9: Install the main rafters that will butt into and hang on the dormer header. These rafters must have a ledger board under them or be hung with joist hangers.
STEP 10: Determine the ridge height of the dormer roof. Find this by using the pitch and half of the total width. For example, say your pitch is 6/12 which means 6 inches of rise in 12 inches of run; and your total width is 60 inches.
That would be 30 inches of run at 6/12 or (6*30/12). This gives 15 inches of rise. Then you must add the amount of rafter board that is (or will be) above the wall plate. I usually use 4.5 inches. So, now you have 19.5 inches.
From this amount you must subtract the thickness of the ridge board. Say you are using a 2x8 ridge, you would subtract 7.5 inches for a result of 12 inches.
Framing dormers continues...
STEP 11: Cut a 2x4 ridge board support this length.
STEP 12: Install this ridge support board in the center of the dormer face wall. Scab a block on either side of the vertical board to make a pocket to set the ridge board in. Brace it with diagonals in two directions to hold it plumb and secure.
STEP 13: Cut a second ridge support board to rest on top of the rafter header. Subtract the height of the header from the 12”.
STEP 14: Install this ridge support board in the center of the rafter header. Scab a block on either side of the vertical board to make a pocket to set the ridge board in. Brace it with diagonals in two directions to hold it plumb and secure. If the support board is real short, you can just stack some material to the correct height.
STEP 15: Pull a string from the front ridge support board, across the second ridge support, over to the main roof when framing dormers.
STEP 16: Use the header support board to set the height of the string where it hits the main roof.
STEP 17: Cut and install a 2x12 block between the two rafters that the string hits between, with the bottom of the 2x12 block at the point where the string hits the main roof. (where the string hits the main roof indicates the bottom edge of the ridge board).
STEP 18: Stretch the string again and mark the 2x12 block where the string hits.
STEP 19: Measure this length and cut a ridge board to the dimension, with the heel of the pitch angle at the length of the measurement.
STEP 20: Set the ridge board in place, flushing the exterior end with the dormer face wall.
STEP 21: Check that the length is correct and the heel angle is right.
STEP 22: If all is good, nail the exterior end of the ridge board in place. Leave the other end loose.
STEP 23: When framing dormers, measure from the ridge to the wall plate, on the dormer face end, on each side. Be careful not to influence the position of the ridge while you do this.
STEP 24: Find the average of the two dimensions. For example. If one side reads 23 inches and the other side reads 25 inches, use a 24 inch length.
STEP 25: Cut two common rafters your average length, leaving the amount you allowed for in the bird mouth. (SEE MAKING COMMON RAFTERS)
STEP 26: Set the two rafters in place and check that everything is as it supposed to be.
STEP 27: Use one of the rafters as a pattern and cut as many common rafters as you need.
STEP 28: Fasten the exterior two rafters in place securely.
STEP 29: Layout the wall plates and the ridge board on whatever centers you are using. (SEE LAYING OUT WALL PLATES FOR RAFTERS AND JOISTS)
STEP 30: Install the interior most pair of common rafters, using the rafters to center the ridge board.
STEP 31: Fasten the ”tail” of the ridge board to the 2x12 block when framing dormers.
STEP 32: Build and install the barge rafters. (SEE BARGE RAFTERS)
STEP 33: Install the sub-fascia boards on each side of the dormer. Cut the “tails” of the sub-fascia boards enough shorter than the main roof so that the decking can slide under them.
STEP 34: Pull a chalk line from the center of the ridge board on the 2x12 block to the point where the dormer sub-fascia intersects (or would intersect) the main roof on each side and pop a line.
STEP 35: Install 2x12 blocks between the main rafters where the chalk line indicates a dormer rafter will land.
STEP 36: Cut the jack rafters one pair at a time. They should be the same length, and mirror images of each other. The “foot” or lower end will have to be beveled to match the slope of the main roof.
STEP 37: Nail the pair of jack rafters in place, maintaining your center layout on the ridge board, and matching the tail ends to the chalk line.
STEP 38: Repeat the process for however many jack rafters you have.
STEP 39: Cut boards for joists, as many as you need. The span dictates what size you use.
STEP 40: Crown the joists and mark them with an arrow.
STEP 41: Cut off the top corners of the joists so they do not protrude above the roof line when framing dormers.
STEP 42: Set the joists beside the rafters and nail to the rafters and the wall plate on each end.
STEP 43: Cut and install sheathing on the dormer face and sides.
STEP 44: Cut and install a board across the face of the dormer, between the two main rafters. Flush the corner of the board with the edges of the main rafters. Nail to the dormer face wall and the two main rafters.
STEP 45: Cut and install the main rafter sections that run from the dormer face to the main wall. Nail these pieces to the board across the dormer face. Maintain the original spacing you used for the main rafters.
STEP 46: Level back from the dormer sub-fascia to the dormer sides in two places, on the dormer sides.
STEP 47: Pop a line between these points on each side.
STEP 48: Cut a board for each side that includes the barge rafter overhang.
STEP 49: Nail these boards in place, using a level to flush the exterior end with the outside edge of the barge rafter. Use the chalk line to set the board height on the walls.
STEP 50: Finally, double check that the rafter header doesn’t protrude above the roof line.
Congratulations! You've just completed a full tutorial on framing dormers!
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