on 10/15/02 10:07 PM, Jason Hissong at jhisson1@columbus.rr.com wrote:
> My question is, how much stress would a roof top tower put on the
> house? The models I am considering is 26' or 17.5'. I would of course be
> following all instructions on installation (and maybe putting more supports
> in the attic than the instructions require). Is this pushing it for a
> typical roof?
Jason,
No stupid questions.
To answer your first question - I am not sure. It depends on how much you
have up on your tower. The forces on a tower due to a particular amount of
windload can be calculated. Then you'd have to translate that down the
tower to your roof. The bigger the tower, the more force.
Now, consider the amount of windloading for each tower. The Glen Martin 26'
can only handle 9 sq feet at 83 mph and 7 sq ft at 100. That's not a lot of
area. The 17.5' unit is rated at 12 and 9 sq ft respectively. That's a
little more margin.
These towers are designed to be mounted on standard roofs with no problems.
If you follow the K7LXC Prime Directive and follow the manufacturer's
recommendations, you will be fine. If your roof is in good shape, you'll be
OK.
One thing you may want to consider is that some of the details though of the
Glen Martin installation instructions aren't completely accurate. For
example they show in their diagram for mounting adding blocking between the
2x6 joists in your roof. Great idea. They show 3 2x6's side by side as the
blocking to be installed under each leg. They show the tower being mounted
by lag bolts in the 1st and 3rd 2x6. The hole spacing on the foot plate on
each leg of the tower is about 5 inches. Consider a 2x6 is actually 1.5"
thick and you'll see their drawing isn't quite accurate. We found that out
when mounting my friend's quad pod last Saturday.
On my friends set up we also ended up drilling THROUGH the blocking 2x6's
and through the 2x6 that runs underneath the joints. We then passed a long
carriage bolt though all that and tied everything together that way - it's a
little stronger than lagging in like they show. Lags do have a possibility
(albeit slim) of ripping out. The tough part was drill through the side of
the 2x6. It helped that we had a drill press to make an accurate hole. But
it could be done w/o one.
Remember that when reinforcing your attic think about the concept of
spreading the load. If you can spread the forces of the tower among many of
the joists, it helps distribute those forces and is much easier on your
overall structure. Additionally, adding extra tie-ins between the joists
and your rafters below also strengthens things. I added a guy point for my
53 feet of R25 to my garage roof. Go back into the archives and read what I
wrote and how I reinforced the roof there. You can translate that into what
you do for the quad pod. If you do, you'll have no problems and will be
doing more than what the manufacturer recommends.
I would take the 17.5' tower and put it on my house roof. To me, the extra
benefit of the 26' one isn't worth the cost, effort to get it up on the roof
(don't forget that) and lower windloading.
73,
Jon
NA9D
-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)
Life Member: ARRL, NRA
Member: AMSAT, DXCC
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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