[TowerTalk] Guying a Glen Martin Roof Tower

[K7LXC@aol.com]

In a message dated 5/2/2005 5:08:26 PM Pacific Daylight Time, 
towertalk-request at contesting.com writes:

>  I am currently preparing for a home addition.  My existing home uses
2x4's for the roof rafters instead of 2x6's.  The Glen Martin specs
call out for 2x6's.  Glen Martin (obviously) cannot recommend
installing one of their roof towers on a roof with 2x4 rafters.  I
asked the architect and structural engineer to spec reinforcement of
the roof rafters where I would like to install a roof tower.  I gave
him the specs for two of the Glen Martin towers: the 9ft tower and the
17ft tower.  The structural engineer had no problem with the 9ft
tower, but he was very concerned about the leverage generated by the
17ft tower.  In the end, he recommended "doubling up" the 2x4s but
still wouldn't recommend putting the 17 footer up there.  The
architect suggested guying the roof tower.  I know that there have
been numerous discussions on this board about NOT guying self
supporting towers.  Would that advice still apply to a 17ft. roof
tower (assuming I wanted to go that route, rather than the 9 footer). 
Any other suggestions, advice, etc. related to this?

    The 17-footer will exert quite a lever arm on your roof. I'm not an 
engineer so I'd go with what your engineer determined. 

    I've installed a bunch of the 9-footers and even had one pretty loaded in 
a bad wind spot. That one was even in a house so old that there wasn't any 
horizontal bracing for the rafters - they were all just 2x4 diagonals under the 
roof.  

    After I added horizontal bracing between the rafters, I installed the 
tower bolted with thru-bolts to backing plates inside the attic. Then I added 
some guys, or rather tether lines, to big screws into the rafters. There was 
barely any tension on them but if the wind blew it would spread the forces over a m
uch larger area of roof rather than just the tower leg bolts. That might work 
for your installation too since it sounds like the engineer was just 
concerned with the roof directly under the tower.

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