Bob,
Some factors to consider:
* Rohn rates 25g for 2 sq ft. 30' above a house bracket.
at least, that's what I recall. It was published in
their commercial catalog, some years ago.
* I would recommend using 45G, if you can find it, although it
is heavy. An 18" face is lots easier to climb on.
* I had 50' of rohn 45g house bracketed @ 5' and 15', in Vermont,
for several years. It had a 10' mast out the top, and a 10
element lpda on top, plus an r7000 vertical on top of that.
It was solid as a rock. (and it was sitting on a rock...about
2' down. No concrete.)
* My house bracket system used wood. pressure treat 2x8,
which spans at least 3 joists. details follow:
A 5' 2x8 plate is either lag-screwed into joists, or thru-
bolted to a backing plate inside the building. (preferred)
The plate needs to be longer than 48", so you can tie into
the three joists.
If you lag-screw in, you must drill pilot
holes sized to provide maximum thread depth, and you must
get them centered in the joists if you're going in end-on.
(no small task!) I used 12" lag screws. Undersize the holes
and split the joists...so be careful.
The tower is centered on the backing plate, and held laterally by
two 2x8 pieces, one on either side, sized to provide a tight fit
against the tower legs.
The lateral plates are held by carriage bolts which come from
the back side of the backing plate...lateral plates are drilled
for clear fit, and bolted down. I usually countersink these
bolts, so there's no interference with the next layer, the TOPPING
plate.
A topping plate goes through the tower, and is held in
place on either side by longer carriage bolts, also coming from
the back of the sandwich. This plate must extend as far as possible
along the length of the backing plate.
The result of this system is that the tower is held close to the
house, with no moment arm. It can't wiggle.
This obviously won't work if you have to stand off from an eave.
In that event, you have to use Rohn's bracket...even though it's
much less sturdy, and doesn't spread the load enough.
Those brackets can wiggle and produce creaking noises.
All bolts are 1/2" zinc plate, by the way. Stainless is hard to
come by. Use flat washers on ALL surfaces, and split washers under
all nuts.
* If you use 25g, I would anticipate guying it, in some fashion. Even
if it's non-optimal, given the load you're anticipating.
* If you need to get a building permit for this project, forget about
what you're thinking. You won't pass, and Rohn isn't here to help
you square away your engineering submission.
I got away with mine because I was in Vermont, and simply didn't
ask.
N2EA
jimjarvis@ieee.org
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