Rafter Drilling Guide


One of the challenges to mounting a solar frame (rack) onto a roof is getting the bolts screwed into the center of the rafters.  The "pros" do this by tapping on the roof with a hammer, and make an educated guess where it is.  Sometimes they get lucky and hit the rafter in the center.  Other times they catch an edge of the rafter.  And yes, sometimes they completely miss. 

I don't want unnecessary holes in my roof, and being that I live in hurricane prone FL, it's important to attach to the rafters without splitting them.

One clever solution I found online was to clamp a magnet to the side of the rafter, inside the attic.  Then go on the roof with a piece of ferrous metal, and it will stick to the roof due to the magnet.  This is pretty good, but the author sometimes missed too.

My solution is simple.  I have good access to the rafters in my attic.  By clamping a drilling guide to the rafter in the attic, the hole will be centered in the rafter each time.

Scrap hardware was used to build the drilling guide.  Even the De-Stac-O clamps were a junkyard find.

Hopefully in a week or two, I'll be using this for real for mounting the solar frames to the rafters.  I'll post pictures when I do.


Drilling a hole into the side of a scrap piece of 2x4 is the most critical part of this project.  This hole needs to be straight, or you'll end up drilling sideways through the rafter.

The 1/4" bit on my drill press was too short to drill all the way through.  I finished the hole with a long 1/4" bit, and a cordless drill.

This is the exit hole.  Pretty close to center.

I put a center line on the 2x4 to make it easy to align with a chalk line I'll strike across the rafters in the attic.

Next step was to attach a piece of plywood to the back of the 2x4.  Just be careful not to put a screw into middle of the board or it'll end up in the 1/4" hole the bit will travel in.

Last step was to add a couple clamps to the 2x4.  You could use wood clamps to hold the jig to the rafter, but these are quick.

Now to test it.  The 2x4 in my hand represents the rafter.   The rafter could be 2x4, 2x6, or even 2x10.  The jig doesn't care.  As long as the drill bit is long enough, it'll work.

The marks are lined up.

Clamps engaged.

A long 1/4" bit was used to drill through the simulated rafter.

Viola, a hole all the way through the center of a 2x4.

From the roof side, sealant will be applied to a 5/16" stainless bolt, then an L-bracket will be fastened using the bolt and a washer.

A 1/4" hole is the correct pilot hole for a 5/16" bolt.  Threads in tightly without splitting the 2x4.

I'm using 3.5" long bolts.  With the thickness of the tar paper, shingles, and bracket, the bolt will be just shy of poking out the bottom of the rafter.  I didn't want any of the bolt protruding, making something sharp to hit my head on!

The edges were sanded, and an ID (so many jigs, so little memory) was added.

Drilled a hole to hang it up on the side of a garage cabinet.  Now ready for use.


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Last updated 09/02/13    All rights reserved.