finding those rafters... [RE-wrenches]

[Graham Owen]

All,

To me finding rafters is probably the most critical part of properly
installing solar arrays on rooftops and when done right will allow a
leak free roof for decades.  Many roofs in L.A. have rafters visible
under the eaves, find these and put a crayon marking at the roof edge.
On the roof put a square on the crayon mark and snap a chalk line up the
roof.  Then I hold my hammer handle very loosely and drop the head on
the roof, while making sure to keep a knee directly below the hammer
strike.  When you hit the rafter the hollow sound goes away, the hammer
bounces solidly and I can feel the vibration in my knee.  I use a 1/8”
pilot drill bit.  I typically drill deep into the rafter and if the bit
is too close to the side of the rafter the bit will very often get
pulled to that side.  Then I move the drill over about 1” and re-drill.

When the rafters are not visible from below I will take measurements to
approximate where the rafters should be and lay down a piece of
cardboard on the roof and run a deep-scan stud finder over the area.
After hopefully finding the rafter I still bank around with my hammer.

There will almost always be missed rafter holes on every job, which need
to be properly sealed.  I like to use roof mounts, which are at least 6”
long and with a hole in the center.  When bolted to the roof in a
horizontal manner the misses will be covered with a secure obstruction
for the path of water. Lately I have been using aluminum Tile-Trak feet
for this purpose, back in the 80’s we used redwood blocks, pre-drilled
and counter sunk and used 5 ½” x 3/8” cadmium plated bolts.  Now I use
stainless bolts.  

To make sure the roof is sealed properly, all missed holes need to be
filled.  I often use the wood debris from the drilling process to pack
missed holes and make sure the back of the mount is buttered
sufficiently to allow mastic to ooze out of all four sides when the
mount compresses down during the tightening of the bolt into rafter.
Henry’s 204 is messy stuff, especially on extension cords, but it works
well.  Another piece of advice is to always work on a DRY roof and not
speed up the install by trying to use Henry’s 208, which seals wet
surfaces.  208 is really for roofers to fix leaks in the rain.  208 will
turn gray and crack in a few years if not coated with a either 204 once
dry or a reflective roofing paint.

I have systems on roofs for over 20 years with no leaks and Henry’s
works well, I have done a number of installs in the past couple of years
on comp shingle using Sikaflex, which comes in a variety of colors to
match the roof.  I find the roof work and finding rafters to be a very
labor-intensive part of installing solar, but taking the time to do it
right as opposed to a speed philosophy is better.

The most labor-intensive roofs are the flat hot-mop with gravel on top.
On these roofs I find the general rafter location,  lightly scrape the
gravel away with the claw end of my hammer, then do the banging and
drilling.  Once the rafter hole is located I stick my lag bolt into the
hole a give it a couple of quick turns.  Then I scrape away the area
where the mount will go, making sure to remove every single piece of
gravel, then take a stiff wire brush and scuff the visible hot-mopped
tar.  Next I pull out the can off Office Depot compressed air and make
sure all dust is removed (very critical).  Then I get out the propane
torch, and carefully heat up the tar and make it black, shiny and
sticky.  The bolt in the rafter hole will prevent flame from getting to
the wood as well as from keeping the hole from filling with gravel
during the final scraping process.  (keep a water spray bottle hand,
just in case) I also flame the Henry’s in the can, ignite it for about
5-10 seconds, put the can lid on to extinguish the flame, and butter the
mount.  Then I put the lag bolt through the mount and butter the threads
on the bolt before tightening into the roof (also very important).  If
the bolt does not tighten and spins, pull it out and re-drill.  I always
use “shouldered” lag bolts which will never feel tight if only screwed
into the sheathing. If everything is dry, free of dust and tacky you can
drill and properly seal many dozens of holes in flat roofs with
confidence.  I also put gravel onto the mastic that oozes out from the
sides of the mount.

Finding rafters is critical but having exact dead center to me is not as
critical.  If you are tightening a bolt into a rafter and it pulls to
the side instead of going straight down then I pull it out and re-drill.
I try and drill as few holes into a rafter as possible so as not to
weaken it.  BTW if you don’t drill deep enough with a pilot bit the
rafter can split.

Graham Owen
GO Solar

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