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# what is the formula for a center line

#1
12-01-08, 07:22 PM
Member
Join Date: Dec 2008
Posts: 1
what is the formula for a center line

who can help me figure out the exact square center of a roof area from eve 2 ridge

#2
12-01-08, 08:32 PM
Group Moderator
Join Date: Dec 2004
Posts: 26,231
Not sure what you want...

If you are trying to make a reference line that is perpendicular to the bottom edge of the roof, you would first want to chalk a straight line all the way across the bottom of the roof that would represent- for example, the top edge of your starter row of shingles, or your first full row of shingles. You'd want to make sure this line is perfectly straight. Then pick a spot somewhere near the middle of the roof and partially drive a 7d nail in on that chalk line.

Then use 3:4:5 to find a right triangle from that spot.

So if your ridge is, say, 17 ft away from that chalk line, you want to make as big of a triangle on the roof as you can, so use the number 12 in place of the number 3 and use the number 16 in place of the number 4. (12:16:20) You'd hook your tape measure on that first nail and measure out 12' horizontally (along that chalk line). You would mark that spot by partially driving a second 7d nail into the roof.

This ratio works as long as you keep the ratio the same. 6:8:10, 9:12:15, 12:16:20, 15:20:25, etc. The last number is always the diagonal (hypotenuse).

You would then need to mark an arc on the roof by measuring 16' up from the first nail. You would then go to the second nail and mark an arc that is 20' long (this is the hypotenuse of the right triangle). The point where those arcs cross will be the top point of your right triangle.

You would then chalk a line up the roof, from your first nail, to the point at which those arcs intersect. Double check all your measurements before you snap any lines.

Many roofers find that it's easier to measure from an existing edge, assuming that it is straight. If you're using 3-tabs you usually want to plan things so that you don't end up with a thin sliver of shingle on either end. You also don't want to start out with a full shingle, and then find out that as you get higher, the layout is running away from the edge. So check all that out before you begin.