The Practical Observatory #2.5
by Dennis Allen
It's been twenty years since we built the 16' by 20' roll-off (see "The Practical Observatory #2"). Served us well, but started to show it's age. The wood floor got too soft to hold a big step-ladder steady. The north wall had begun to bow out. Even with a chain to pull the north/south walls together, took two people to roll the roof open. Became just too difficult to use the telescopes. For the last five years the building has been used only for storage. The roof remained in great shape, however, so we decided to rebuild. With a couple of big donations, we were ready to begin.
The first thing we did was to roll the roof off and secure it. Not trusting the condition of the outside posts, we added bracing. Then we began the tear down. We cut the 40' sections of channel in half. With the channel gone, we removed the OSB plywood, the door, then the wood floor. Removed several yards of dirt that sat between the floor joists. We left the wall posts in, to help set the forms. Under the OSB was a 16" section of treated plywood, half in the ground. This plywood was driven down and staked to create our forms. Once we had our forms, we removed the wall posts. Half the posts were rotten. We saved what material we could, to be recycled into the new building.
It was a long a fourteen hour day to get the forms down, had to dig the edges out and add rebar. Only took couple hours to float the concrete, but a another whole day to put down one layer of foundation block. Craig did a great job on the blocks, so said the inspector. Straight and level, made putting up the walls almost easy. Just remember, when setting blocks have two anchor bolts in every corner.
We decided to make the walls 50" high. Allowing for an 8" block, that's 42" to the top of the header. By laying a full sheet of plywood on top of the sill plate, we had 7.5" inches of plywood above the header. Just the right height to cover the channel and wheels from the elements. We secured three 4" by 4" braces to the north, south, and west walls, to keep them stiff. Each brace lays at a forty-five degree angle, secured to cement pads with 4" by 4" brackets and cement screws. Oh, we noticed the north braces came close to a bench. So we added three 12' pieces of treated wood to the north braces for a nice viewing bench. So nice, in fact, we fabricated another bench for the south wall.
To stiffen the east wall, where the door is located, we cut a 4' square section of plywood diagonally and made two corner tables. Once the braces and corner tables were in, we rolled the roof back on the new walls. Now the new walls are 3" higher than the old ones, so we had to jack and block up the front wheels. Once we got the first sets of wheels on, the rest went on easier. With only the front and back wheels for support, didn't notice any warping of the roof. Very surprised on the strength of the roof. Also surprised it took just three of us to pick it up.
Once the roof was on, we cut a full door to fit the new height, about 50" high. It's a full panel door that swings outward and has pin-less hinges. To cover the cut, we used a replacement threshold door sill.
To keep the roof from blowing off, we installed six chain binders (also called load binders). One for each corner of the building, plus two in the middle. 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 wall. I taped the chain binders to their chains, so people will always know how to lock the chain binders back down.
We reused most of the old vinyl siding and some leftover material from another garage. For trim, we used 4" 5/4 corner and 6" 4/4 PVC white trim. No worry about rot, no need to paint. With new eye-bolts, door locks, vapor barrier wrap, and vinyl siding the building is now secure. Already passed final inspection. We'll finish the building in the spring.
My only regret thus far is the north/south walls. The new north/south walls must be an inch or so closer than the old walls. Despite a lot of shimming, a couple sheets of OSB were still grabbing the roof. In the end, we had to shave the OSB down and use vinyl siding trim to cover the resulting gaps. Not pretty, but works. Just goes to show, it's easier to build the walls first then fabricate the roof to fit the walls.
To complete the rebuild, we put down 14 sono tubes, 12" wide and 2’ deep. Five posts for each channel, plus anchors for the side braces. Came out to a yard of concrete. Then we spent a weekend putting up the outside posts. We used short pieces of steel channel and stainless steel bolts to reattach the outside channel to the inside channel. Used heavy gauge post bases with 3/8” anchor bolts to mount the posts to the concrete. Nice and strong! No need for extra cross braces.
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 roof is a bit hard for one person to push, but no worse than before. We ended up installing two AC electric winches, one to pull the roof open and one to pull the roof close. No need for pulleys or tow-straps. Nice, more than enough power.
When we made the original roof, we forgot to start with a one foot overhang. So we used a 10' sheet of roofing and made a one foot overhang. We also rebuilt the east gable, bringing it out 1.5 inches. Now the east gable overhangs the entire east wall.
Anyway, the roll-off roof is once again operational! As a finishing touch, we put down two yards of gravel around the building. Nice. Still got to finish the electrical outlets, but that can be done anytime!
Our original plan was to move the club 15" telescope into the big roll-off. It became clear, however, the 15" telescope was going to need a lot of work: The equatorial platform needs to be rebuilt, the mirrors needs a recoat, etc. So the club decided to mothball the 15" telescope for now and let me to put my 24" telescope in the big roll-off. As it turned out, well worth the move. The big roll-off is ideal for this telescope. The walls are far enough away that the telescope never touches them. No need for the telescope to sit on two layers of bricks, so even looking straight up is comfortable on an eight foot ladder. With my big telescope closer to home, I've already used it more times this summer than the last couple years up north. Images are fantastic, at least that's what the club members tell me (might have to install a coin operated device so I can get a turn)<g>
So far everything is working fine. I added a second layer of cap weather stripping. Caulked every cap nail I could find, but we still have a small leak above Ron's scope. Annoying, but we can live with it. I also greased all the caster wheels. Had to fix a couple grease fittings, but it was well worth it. Unless I'm very tired, I can open and close that 800lbs. roof without the electric winches. We got the electrical outlets installed, but the corner tables still need lights.
Building looks good. Added more red and white lights. Got a couple leaky roof nails, but not bad. Next summer I plan to spend time replacing the roof nails with screws. The trick will be removing the nails without hurting the sheet metal. I plan to use a block of wood made from a deck hand rail. Wish me luck.
Last summer I replaced several rows of roof nails with screws. No sign of any leaks. The only problem right now is the roof. The roof can't be rolled past the west wall, so it blocks any object that drops too far west. We have another observatory going up next year. When that happens, we plan to put down a couple sono tubes of concrete for posts and add another 4' of channel.
I'd like to thank everyone who played their part in this project. Without donations from Ron Holst and Steve Salisbury, we couldn't have gotten started. Without drawings from Dale Johnson, we couldn't have gotten the building permit. Without Craig Lehan, we couldn't have floated the concrete or laid the blocks. Without Gary Gangwer, we couldn't have got the walls framed or the roof back on. And don't forget Ron Northman, Mike Galvin, Christine, Norm, and the nephew Robbie who also helped out. Again, many thanks.
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