In This Article:
Some shingles are torn off behind the chimney, and careful layout marks are made. 2x4's are cut to form the ridge and two legs of a saddle, and covered with OSB.
3-4 (Intermediate - Advanced)
About 2 Hours
Bruce W. Maki, Editor
When a masonry chimney is built in the center of a house, it often passes through the peak of the roof, and rain water is naturally diverted away from the joint between the chimney and roof shingles.
But sometimes a chimney goes through the lower edge of a roof. To divert water away from the chimney, a saddle or "cricket" is usually built into the roof structure, before the shingles are installed.
This roof leaked around the chimney.
The previous owner had built an addition and installed a new chimney without making the proper roof alterations.
Just imagine rain falling on the section of roof uphill from the chimney. In your mind, follow the water downhill. Where does the water go? The water hits the chimney... and then what?
They had applied lots of asphalt roofing tar (red arrows) in an attempt to stop the leaks... but during the spring snowmelt, water dripped from the ceiling just in front of the fireplace.
This is actually an elementary problem: water needs a slope to drain properly. Mr. Previous Owner either did not know what to do (ignorance is no excuse) or was too lazy to make the extra effort of building a saddle between the chimney and the roof.
My first step in planning was to determine the slope of the roof.
I used a 2' and a 4' level. Note how the rise is 12" for a run of 48", which is a 3:12 roof pitch.
Then I measured the width of the chimney. I called it 28 inches.
My initial plan was to make the saddle with the same 3:12 slope as the roof. But that low slope would have created a saddle just over 3" high, which just didn't seem like enough of a difference for the work involved. So I decided to double the saddle slope, to 6:12.
I drew marks on the chimney where the top of the plywood peak would be (upper + mark ) and also the top of the 2x4 framing (lower + mark).
Then I used the 2' level to figure out where that elevation would intersect the roof of the house.
I used a small level to extend the reach of the 2' level. Care must be taken to ensure that the level is perpendicular to the chimney (as viewed from above).
I drove a small nail into the roof at that point.
This point marks the intersection of three roof planes: The main house roof, and the two sides of the saddle. Another way of thinking: the two sides of the saddle intersect in a straight line (the ridge, or peak) and that ridge line intersects with the main roof at this point.
I snapped a line extending from the nail to each corner of the chimney. These 2 lines mark the location of the two valleys that will be formed.
I used a small crow bar to remove the tar and metal flashing. I did not remove the metal flashing on the narrow sides of the chimney, only on the side that will have the saddle.
I removed any shingles that got in the way.
Removing shingles is easy. First I held up the tab with a flat pry bar...
...Then I used a small bent pry bar to pry up the nail head.
I removed a large patch of shingles that would be in the way of the saddle.
The blue lines represent the chalk lines I snapped earlier, which were lost when I removed the shingles.
I re-installed the marker nail, and re-snapped the chalk lines that extend to the chimney corners.
This picture tells much of the story.
The 2x4 is cut on a 3:12 slope on the left side. It is aligned with the red mark I mentioned near the beginning. I used 3" deck screws to install this piece, and I pre-drilled the holes to prevent splitting.
I edited this photo to emphasize the red layout lines on the chimney. The red lines and the blue lines mark where the plywood will terminate, so the framing is just below by 1/2" or so.
Note how the red chimney lines and the blue roof lines meet at the points where the old roof intersects the uphill chimney corners.
Next I installed the tiny rafters. These little rascals are quite tricky to cut, because the lower end is a compound angle.
Anybody who makes a mistake cutting these rafters is automatically forgiven. They can be quite troubling for many people.
I have seen experienced carpenters cut and re-cut rafters like these. I have a mechanical engineering degree, and plenty of mind-bending, complex 3-dimensional drafting experience, and this type of rafter still trips me up once in a while. (But not on this occasion.)
In a nut shell, it's like this: First, cut the compound angle (the lower end of the rafter), then cut the board to length with the simpler, miter cut.
The Compound Angle: I set the circular saw to cut on an angle... the same angle as the main house roof slope. A 3:12 slope is very close to 14 degrees. (This involves... oh no... Trigonometry) Actually, the Speed Square has markings for roof pitch values and angles, so no Trig was necessary. Had you scared, eh?
In the middle of a long 2x4 I drew a line that was equal to the saddle pitch. A 6:12 slope equates to about a 26.5 degree angle. I used a Speed Square to lay out this angle on the board. But... 26.5 degrees from what?
The board will ultimately rest at a 26.5 degree angle above horizontal, so the line I just mentioned needs to be 26.5 degrees away from the long edge of the board.
Since this cut can't be done on a miter saw, I cut along the line with the circular saw. Both of the difficult cuts were done, because each side of the board can be used for one rafter (they are mirror images of each other) I know what you're thinking... I should've taken pictures. (This concept really requires another entire article.)
Then I cut each board to the right length, being careful to keep track of which edge of the board is the uphill side or downhill side. It helps to carefully lay out everything on the roof with a pencil or pen.
The upper end is a plain miter cut. The rafters were attached with 3" deck screws.
The rafter was set back from the chimney because of a big gap between the existing roof plywood and the chimney. Otherwise, the ends of the rafters would not have adequate support.
Before you hurt yourself, read our disclaimer.
The first pair (and only pair) of rafters. This small saddle did not need any more than this simple frame. The sheathing will have no span greater than 24" in this structure.
A piece of OSB sheathing was installed. A simple triangle.
I bevel-cut the edge that met the old roof. This made for a cleaner and tighter joint.
The completed saddle framing, sturdy enough to walk on.
A larger saddle might require: