Can my roof hold the weight ?

[Bob G.]

I have a 24x24 foot building (barn roof) in my backyard which serves as a garage...

I am into astronomy and because of trees etc the only good place to "construct" an dome shaped observatory is to place it on top of the barn...

The weight of the Fiber glass dome with a 6 foot 6 inch headroom in the center is only 350 pounds (the walls dome itself)... which leads me to think that I really could build a 8x8 platform on top of the existing roof and erect the fiberglass dome...

Existing rafters are 2x6;s spaced 24 inches on center gable shaped . I would not be removing any...just cutting a hole in the roof under the dome so I can climb up a ladder to enter...

Seems to me that adding 350 pounds PLUS "me" and the Telescope and the laptop on the roof would be possible..

What I do not know yet is how to mount a pier up there so that it would be stable enough to hold the telescope stable ... First things first... can the existing roof hold the weight....?

Bob G..

[Goedjn]

Insufficient info: We need the roof-span, as well as the rafter size and spacing, and the weight of the scope assembly. On the other hand, you're talking about a concentrated load of around 500#, plus possibly another person. I'd sister up the rafters in question anyway.

[calhoun]

I would think it would "hold" the weight. the question is could you use a telescope on such a platform. There will be a certain deflection to any such arrangement. (jump up and down on the roof and you will notice it moves). Any telescope I have used needs a solid non moving base for any decent observations.

[jhill]

also you did not say anything about snow loads in addition to the dome.

[DT]

Exactly. any roof is WAY too bouncy for a telescope mount, and your roof is very lightly built by roof standards, with only 2x6's on 24" centers. Look around the web for home built scope articles, they all use large concrete foundations.

If the roof is the only suitable spot for viewing, you will need to add something like vertical posts embedded in concrete and mount the platform on top of them to isolate it from the building. Of course, then you give up space in the garage due to the posts.

Dennis

[Michael Daly]

You need to isolate the platform from the building so that the building doesn't have to carry the high loads. It is also a good idea to isolate the telescope pier from the platform. I know folks that have built a thick pier for the telescope and separate columns to hold up the platform. The platform, dome etc are not connected to the pier except through the foundation/soil. The telescope pier should be well damped to avoid vibrations.

If you're going to put up a fiberglass dome, you will eventually keep adding telescopes and other gear. The weight adds up fast.

If you are going to go to that much trouble, I would caution against actually building the dome on the existing building. Thermal effects will mess up your seeing - better to build a stand-alone telescope dome if you have the land.

Mike

[LM]

Too much bounce. Any movement on the floor will transmit to the telescope. Vibrations will make the scope almost unusable. Forget photography.

It has been done by making a "Very Solid" pier from the ground up to the second floor. The pier would have to be something as solid as a 16" concrete pile 15-20 ft into the ground. The entire structure has to float free of the pier, not touching it at all. The only thing on the pier is the telescope.

I used to be a member of an astro. group that had an observatory on the second floor. The telescope, a C 14 was mounted as I described, but as I recall the pier was at least 20" in diameter.

By the way, there can be no heat in the building below. Warm air rising will interfere with seeing.

You may want to try the newsgroup sci.astro.amateur

[3rd eye]

I'd suggest you post this to alt.building.construction.

[HeyBub]

calhoun"

[Mark D]

Hello Bob, Others have brought up good points, which I myself wouldv'e brought up to you also.

You make no mention of scope, mount, and if you've already purchased the dome yet?

I'm an Amateur Astronomer myself in Southern NM, and a few years back, when living in Chicago, I bought a 2 month old Celestron C-14 OTA from a fella living about 100 miles south of Terre Haute, IN.

He sold the C-14 to upgrade to a more expensive 12" RCOS Cass.

He was using an AP1200GTO Mount, which I myself also used to own (just sold mine) and it was on the second floor of his house. The pier went through the observatory floor, through the first floor, through the basement floor, and down to bedrock. No matter what he did, (Bracing, etc on the lower floors) he still would get shake/vibrations, and had to be extremely careful when imaging with his SBIG CCD Cam.

With his arrangement, he had constructed a rool off roof on his house, instead of using a dome, as you wish to do. A roll off Section could possibly be built lighter than the 350 lb Fiberglass Dome you propose, and no doubt much much cheaper also.

As another mentioned, thermals will most likely torture you also. I can recall this fella I mention needing to resort to AC in his observatory to bring temps down.

I'm sorry to hear that you don't have too many options with the surrounding trees in your area. Hope you can solve your problems. Mark D.

[Frank Warner]

[Bob G.]

====================================== Thanks.... I honestly do think I could avoid most of the thermal effects since the garage is unheated unkless I am out their playing with one of the cars (another hobby ..)...but I think that I would have major problems stablizing the scope .... I have to use vibration supression pads under the tripod feet now...plus I use a electronic focus motor because no matter how gentle I try to adjust the focus the scope rocks and rolls so to speak...(using a Celestron Heavy Duty Tripod..which is not up to the task...)

Just got the idea of mounting a dome on the roof to avoid the trees that I planted 30-40 years ago...I can construct a small shed with a roll off roof for a lot less money then purchasing a dome anyway...and install a pier anchored solidily in concrete to mount the scope on . Still will give up most of the Northern Sky but what the heck..

Thanks everyone... Bob G.

[Michael Daly]

Even if it is unheated, the roof alone will pick up a lot of heat from the sun. That heat will take quite a while to dissipate and will ruin your seeing the whole time. This problem will occur winter or summer, though in winter you might see the roof cool off quicker. The skies are more turbulant in winter anyway, so you just can't win!

Learn from others - you do not want to have a large roof just below the level of your observatory.

Mike

[maradcliff]

I'm a farmer. If you are referring to an actual barn (some people call anything except their house a barn), then I would not even worry about putting 500 lbs up there. Heck, snow adds several tons to roofs at times. I would however, run a few vertical posts under your dome to the supports under the floor below just to add stability.

As far as using a telescope, I dont know anything about them. Listen to the others on here.

Mark