Go to How Not to Build an Addition Index! This page includes notes on roofs and ceilings.


Ceiling joists are attached in much the same way as the floor joists. The rafters come next. You can buy a preconstructed combination rafter/ceiling joist. However, this is more expensive than cutting the rafter beams on site.

Rafters rise from the top plate at a certain angle and meet at the ridge-board or peak. Typically, the length of a rafter is determined by the slope of the roof and the desired height of the peak. This is one of the few cases in which you will need more than mental arithmetic to do a job. It is complicated; so follow closely:

1. Measure the distance from one support wall to the other.

2. Divide by two. (This is called the run.)

3. Determine the desired slope of the roof. (The most common slope is 5/12, meaning 5 feet of height for every 12 feet of run.) If you have to make your roof conform to an existing roof, you can use your old roof as an example. If not, I suggest using the least slope possible as it will make your roofing job less frightening.

4. Multiply the number found in step two by the number determined in step 3. This will give you the height of your ridge-board over your ceiling joist.

5. Now take the number in step two and square it. Add this to the square of the number determined in step four. Then take the square root of all that.

6. The result will be the age of your eldest child in dog years.

OK, now that I have completely muddled your brain, I will tell you how to do this the easy way. Lay your carpenters square at the 12-inch mark on the rafter where you want it to rest on the top plate. Adjust the square so that the 5-inch mark on the other side of the square is against the edge of the board. Make a mark with your pencil. Now move the 12-inch mark on the square to your pencil mark and progress the 5-inch mark up the rafter beam.

This is called stepping off. Do not take this term literally, as stepping off from a rafter, especially one that is in place could be hazardous. When you have taken as many steps as the number of feet of half the length of your ceiling joist, draw a vertical line. Make your cut here (This is called a plumb cut). At the bottom, where you started, you already have half of your cut line where the rafter will sit on the top plate. The other line will intersect approximately 3 1/2 inches in and will run toward the lower side of the board (known as a bird's mouth). Finally, you will make a cut parallel to your plumb cut at the base of the rafter beam at the point you want your overhang to end (the tail cut).

Do not step off every rafter beam. It would take forever, and (if you are like me) you are likely to screw up such a tedious job. Cut one, and use it as a template for the rest. Before cutting them, however, test one to make sure it fits properly.

Amazingly enough, I had no difficulties putting up my rafters. I was smart enough to time this job for when my parents visited on their vacation. It sounds like a nasty trick to play on your parents. Honestly, they love this kind of stuff. Whenever they come out for a few days, I try to have some construction job for them to do, preferably one that requires heavy lifting.

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Ceilings and Roofs
Good Carpenters

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About the Author:
W.J. Rayment built an addition on his house, and in the course of the project learned from his many mistakes. This on-line compendium is his effort to help you learn from his experience. The advice and stories are often humorous, sometimes silly, but always informative. for yourself or as a gift for family or friends.

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