The Practical Observatory #4
by Dennis Allen

Back in the summer of 1985, the Muskegon Astronomical Society built a 12' club dome. In 2002, we had to replace the dome skin. This building served us well, but after thirty years the dome was showing it's age. The dome was difficult to open, became hard to move, and every panel had leaks. Rather than spending a crazy amount of time and effort laying down a new skin, we decided to replace the dome with a roll-off roof. Roll-off roofs are simple to build, easy to use, and require little maintenance. We already had 4-1/2' high cement walls, just had to put down the outside posts and fabricate the roof itself.

First, we tore down the old dome. Didn't have to worry about saving anything, so the tear-down went quickly. The next step was to make a new top wall sill. We used two bags of redi-mix to put down a new set of anchor bolts. FYI: Anchor bolts hold better when you drill and hammer them down. Once the sill was down, we dug holes for eight 12" sono tubes, two foot down. They serve as pedestals for the six outside channel posts and two side braces.

After the sono tubes were filled with cement, Menards delivered the 12' trusses, one regular and three 6/12 (internal 3/12) scissor trusses. The delivery also included ten sheets of 8' White McElroy corrugated sheet metal, six 14'x2"x4" for purlins and five sheets 7/16" paneling (looks like 8" rough cut with a primer coat). Harbor Steel then delivered two 20' and two 7' pieces of 3" hot-rolled steel channel.

To prep for the big work party weekend, I bolted eight 5" caster wheels to the two 12'x2"x6" truss sills. Kept the outside wheels 8" from the end trusses, the four inside wheels would sit under the internal trusses at 4' centers. Drilled and counter-sunk screw holes in the steel channel and rough cut the 4"x4" outside posts. As in our big roll-off roof rebuild, we're used heavy gauge post bases that mount with 3/8 anchor bolts. We screwed the channel to the posts, but left the anchor bolts off until we could get the channel squared.

It was a long three day weekend. On Friday we got the trusses up, with three 2"x4" purlins on each roof side. We went with a 9.5" overhang, the length of a rib-to-rib section on a sheet metal panel. We did a two truss test push of the roof, to make sure the wheels were sitting center in the channel.

To keep the roof from blowing off, we installed four chain binders (also called load binders). One for each corner of the building. Bolts hold a short section of chain to a roof truss. Each chain binder then secures the chain to a big eye-bolt in the top wall sill. I taped the chain binders to their chains, so people will always know how to lock the chain binders back down.

 I spent Saturday putting down the outside post anchor bolts. I managed to get all the bolts down, but the cement was so full of gravel I ruined several drill bits doing it. Gary spent Saturday framing out the gables. He used the upper door leftover from rebuilding the big roll-off roof. Since the lower door wasn't actually centered, he had to mount the upper door a bit north of center. Also had to recede the lower door, so that both doors would match. On Sunday we hung the sheet metal roofing and the paneling for the gables. Oh, we found that 8" white PVC trim covers the 5" caster wheels perfect, hanging off the 2"x6" truss sills.

I spent a couple of days fixing mistakes and making adjustments. Fortunately, we used screws instead of nails so adjustments were relatively simple. Took time to caulk, paint, cut/fit trim, weather-stripping, and fill sill gaps with storm door bottoms. Used half of a 50' roll of 24" flashing to cover the soffit. Still got to put in our red lights, but that can be done anytime.

After the last heavy rain, we had water coming into the building from the south channel. Must be a slight dip on the south wall. I drilled several drain holes in both channel and the building has remained dry.

Note: After a couple weeks, the outside posts will tend to shrink. Make sure you check, and if necessary, tighten down your post bases.


The new roll-off roof is now operational! I'd like to thank Ron Northman, Dennis Stroh, and Rocky Frees who helped in this project. Especially want to thank Gary Gangwer, for getting the roof itself up and covered.




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This web page was last updated 02/04/19