Roof Meets Chimney: Building A Roof Saddle Part 1 - Framing

By Bruce W. Maki, Editor

	This roof leaked around the chimney. The previous owner had built an addition and installed a new chimney without making the proper roof alterations.
	They had piled on lots of asphalt "goop" in an attempt to stop the leaks.

This is actually an elementary problem: water needs a slope to properly drain.  Mr. Previous Owner either did not know what to do (ignorance is no excuse) or was too lazy to go through the extra effort of building a saddle between the chimney and the roof.

	The first step in planning was to determine the roof slope. 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.

Some Demolition:

	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.
	I re-installed the marker nail, and re-snapped the chalk lines that extend to the chimney corners.

The Fun Begins:

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. The red arrows point to the red layout lines on the chimney, barely visible in this shot. The red lines and the blue lines are 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 recut 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, then cut the board to length with the simpler, miter cut.

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 is 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.

Then, 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 pays 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 roof plywood and the chimney. Otherwise, the ends of the rafters would not have adequate support.
	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 joint.

The completed saddle framing, sturdy enough to walk on.

Notes:

A larger saddle might require:

• More rafters and heavier framing lumber, possible 2x6 or 2x8. 
• Venting along the ridge. 
• A hole (or series of holes) in the old roof to let damp attic air pass through to the new attic area and it's ridge vent.
• Consulting a carpentry book, or a building inspector, would help. 

Follow the story as shingles are applied.

Copyright © 1999, 2005  HammerZone.com

Written December 27, 1999
Revised January 15, 2005