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Hack 69 Cheap but Effective Roof Mounts

Install wireless gear on your flat roof without
piercing it with mounting hardware.

If you intend to make a long distance
wireless link, you will likely need to get your antennas up as high
as they can comfortably go. For many people, adding a dedicated tower
is out of the question (for aesthetic and local ordinance reasons),
so the next logical place for gear is the rooftop.

Ideally, the equipment should be installed in such a way that it
doesn't pierce the roof of the building, lest the
rainy season come and bring expensive roof repair bills with it. If
you are working with a flat roof, you may find it useful to build a
small "sled" on which to mount your
gear. Figure 4-33 shows a typical plywood sled with
a 24 dBi dish mounted on it.

It consists of a piece of plywood that is a few feet square around
with sections of 2 x 4 screwed to it from above. This gets
the wood slightly up and off of the roof's surface
to allow rainwater to flow past as it normally would. The sled is
weighed down with cinder blocks, and has a cheap aluminum tripod
mount (found at Radio Shack) bolted to it. Figure 4-34 shows a detail of the
"experimental" equipment housing.

In this installation, cost was one of the primary concerns. The owner
had a Stylistic 1000 that needed housing, and the cheapest deal we
could find at the time was a rubberized mailbox (just a couple of
dollars at the hardware store). When closed, the mailbox is
practically water tight, but unfortunately, it has a matte black
finish, which would likely soak up the sun and overheat the
electronics inside. This was mitigated by wrapping the box in a cheap
mylar windshield reflector, which keeps the inside of the box
surprisingly cool, even on hot summer days. Note that the Stylistic
has no ventilation requirements, so an airtight box was an ideal
choice. The cables were run through a hole cut in the side of the
mailbox that was then filled in with Silicone compound.

This choice of mounting hardware may sound ridiculous, but you
can't argue with this node's
uptime: one year and counting! Granted, this is in Northern
California, where the winters are quite mild, and the summers
aren't usually too hot. But then again, this node
isn't even running on a Compact Flash card, but
instead uses an 8-year-old PCMCIA hard drive (found installed in the
Stylistic when purchased from the local surplus electronics store).
We originally installed it just to see how long such a setup would
last, and were pleasantly surprised at the results. The only downtime
this node has suffered has been to upgrade the radio card.

The other end of the link
isn't on a flat roof, so we
couldn't use a sled. Fortunately, there was plenty
of signal available at the pitched edge of the roof, allowing us to
use a recycled DSS dish mount. You can see it in action
in Figure 4-35.

By adding a short piece of pipe, the DSS dish mount was extended
enough to accommodate another 24 dBi dish. The metal box beneath the
eaves is a $10 metal sprinkler box with an Orinoco RG-1100 installed
in it. By bolting directly to wooden studs, we avoided piercing the
tar paper on the roof. The two white lines running to the box are
Ethernet and power for the RG-1100. The owner had a considerable
amount of Ethernet cable on hand, and decided to run the data
separate from power, rather than bother with full-blown PoE [Hack #68].
These cables were later tacked back and run under the eaves, and were
virtually invisible from the ground.

Another approach is to
avoid the use of plywood altogether, and simply make a base out of
wide wooden planks (as in Figure 4-36). These can be
weighed down with sandbags, and the pole steadied with guy wires if
necessary. These materials can be easier to get up on a roof than a
sheet of plywood, and won't have as much potential
wind
load. If you're curious about the white antenna feed
line in the photo, yes, it is in fact Heliax. The omni and yagi
terminate in a metal sprinkler box (screwed to one of the boards),
which contains a Soekris net4511 [Hack #51]. The Soekris is fed
Ethernet and power over a piece of outdoor CAT5, which enters the
building through a skylight. The 12-dBi yagi feeds a link to another
part of town a couple of miles away, and the 9-dBi omni provides
local service.

Roof mounts don't
necessarily need to be elaborate. Antennas for 2.4-GHz gear tend to
be small and have little wind load, allowing you to get away with
surprisingly little for structural support. Whenever working on
roofs, remember to take your time, bring a friend, and work only in
good weather when you have plenty of daylight. Building your own
rooftop node can be very exciting, but remember that roofs can be
dangerous places, no matter how many times you have been up on

them.