[TowerTalk] Will my roof stay on?

[K7LXC@aol.com]

In a message dated 8/22/2005 9:08:21 AM Pacific Daylight Time, 
towertalk-request at contesting.com writes:

>  I'm about to install a roof tower with a 2.5 square feet winload antenna.
I'm contemplating mast length and have come to realize that I don't really
know what computations I should be making to avoid damage to my roof.

>  I was thinking of a 10 foot mast, 6 feet would stick out of the tower. But
why not a 20 foot mast?   How about 13?  How would I know?  

>  I'm looking for references for the computations involved and the forces 
that
a roof can be expected to endure.    I need to decide what the mast is going
to be made of and how long it can be.  At 80 mph is the roof going to lift
off?

    Okay, I'm not an engineer but I've installed 5-6 roof-mounted towers with 
varying loads (one of them has a C4 and a pair of 2M CC plus a pair of 440M 
antennas). Yes, with a mast sticking out the top you've got more lever arm and 
bending moment but consider your typical roof construction. You've got roof 
joists with a layer of good-sized plywood over it. The footprint of the GME 
9-footer is 32" by 32" or almost 9 sq.ft. Using thru-bolts instead of lag bolts 
and 2x4 backing plates in the attic across 3-4 joists, your load will be spread 
over a larger portion of the roof structure. 

    Using an aluminum mast will help to dissipate wind forces by bending 
before it sends the forces down the tower to the base. Your combined tower and 
antenna windload is probably less than 5-6 square feet. If you're really 
concerned about it, you can guy the tower and spread the load over a MUCH LARGER area 
- then you can probably go up as high as you want. 

    If this were my installation, having 6 or 8 feet of mast sticking out of 
the top wouldn't bother me. If the antenna was an HF tribander I'd be more 
conservative.

Cheers,
Steve    K7LXC
TOWER TECH -
Professional tower services for commercial and amateur
Cell: 206-890-4188