Lest the critical info be lost, there was a post which referred the
Uninitiated to a QST article on how to house-bracket correctly.
Fascia boards are trim, not structure. You must tie into the structure.
Using a 2x6 backing plate inside an attic sidewall is preferred.
But here's an alternative which has worked:
I did not have an attic, it was a flat-roofed house. All I could do
Is lag screw into the roof joists and the floor joists between the first
and second floors. That gave me two points of attachment.
My 'bracket' was constructed of 2x8's, 5' long, which spanned 3 bays of
Roof joists. I used 1/2" x 12" lag screws into the 2x12 roof joists,
Carefully locating their centers, and pre-drilling pilot holes, so nothing
split. There were 2 bolts at each of the 3 joists I intersected. Each bolt
was rated for 2700 lbs extraction force, and considerably more in shear.
The tower was captured by a sandwich of 2x6"s which were bolted to the 2x8
backing beams in question. So, I had the ability to withstand something
>12,000 lbs pullout, and 24,000 lbs shear failure at each of the two
brackets.
In turn, I calculated the windload with ice, from the antennas and tower,
and determined that at 90mph, it should be no more than around 5,000 lbs at
the point of the upper bracket. With a more than 2x safety margin, I felt
reasonably comfortable.
Let me point out that there were critical measurements involved, and that a
borescope was used to look at each pilot hole...and that each lag screw was
installed, removed, the hole inspected for structure, and reinstalled.
You can't be casual about whether you've hit the beam or not, in that
situation.
And NO, you can't rely on a double-wide at 10' to hold up a tower at 40',
with any kind of wind. You don't have the overbuilt structure that is
common with site-built houses.
n2ea
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