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ROR observatory concrete slab versus wooden deck questions

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#26 Tom K

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Posted 15 April 2021 - 02:32 PM

I got a tip from fellow CN member OldManSky to put decorative "lava rock" under my observatory to discourage burrowing rodents.   We have ground squirrels and gophers around here that apparently don't like the rough surface of this type of rock.   My observatory has only been up for about a year but so far so good on keeping them away!



#27 wotalota

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Posted 15 April 2021 - 04:14 PM

No mice in the observatory, I lure them away with a nearby house. More cozy in here during the winter.


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#28 John Fitzgerald

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Posted 15 April 2021 - 04:38 PM

Concrete slab, the thicker, the better. 

 

When I built my 16 foot x 16 foot roll-off, I got a full truckload of concrete (9 cubic yards).  I had to pay a trip charge anyway, since it was so far, and the extra concrete wasn't that much. 

 

The slab is a monolith, with 16 inch deep footings, and the rest of the slab averages nine inches thick.  That thick slab retains the coolness of the ground, and helps keep the inside cool in summer.  I have never seen it get above ambient in the heat of summer.

 

The slab has plenty of reinforcement mesh and rebar.  It has not cracked in the 13 years it's been there.

 

Plenty of screened eave and soffit ventilation is also important.



#29 BKBrown

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Posted 15 April 2021 - 05:08 PM

Will the walls of your observatory be standard wood construction?

If so, do you realize that building on a ground level slab will cause the bottom of the wood walls to rot and give easy access to your critters.

 

dan k.

Yes they will be wood, and I have seen plenty of similar structures that don't have such rot issues when properly constructed. But thanks...

 

Clear Skies,

Brian snoopy2.gif



#30 John Fitzgerald

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Posted 15 April 2021 - 05:10 PM

If you build up the slab, and make it thick, it will be far enough above ground level.



#31 Pleiadescgs

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Posted 18 April 2021 - 02:40 PM

Brian,

 

When is your observatory scheduled for construction with BYO.  Mine should be sometime in June I think.  It sounds like you have a pretty good plan for yours.  Chris

 

 

Bruce,

 Your structure sounds very impressive with what you did with both a slab and deck  and your pier. That is a lot more than I was planning to do but I suspect yours will be around forever.  Thanks for the tips.

 

On all the critter comments, my ROR will be built at a local nature center so critters  come with the territory unfortunately.  I am not overly concerned as the structure can be constantly monitored and being on a slab it will be somewhat better at keeping things unwanted out.  However that being said,  I am just anticipating that I will have to do on-going rodent and insect control as part of the maintenance and upkeep.  But I definitely do not want skunks and groundhogs underneath a raised structure on piers.  I have lived with skunks taking up residence under structures and it can become quite toxic.     Chris



#32 Pleiadescgs

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Posted 18 April 2021 - 05:26 PM

John,

 

On your observatory and the 9 inch slab depth.  Mine will only be 4 inches thick although it will have corner and mid footers and an expanded perimeter all around that's deeper (12" x 12").  I thought about a thicker slab but wasn't sure that was necessary.  But it sounds like you have insured no thermal issues having it so deep.  Perhaps you are right, I could make my slab a little thicker since the additional concrete wouldn't be an issue too much as far as extra labor.  Do you have isolated piers or are you just using the slab directly?    Chris



#33 DeanS

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Posted 18 April 2021 - 05:39 PM

The thicker the better wink.gif

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#34 John Fitzgerald

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Posted 18 April 2021 - 06:52 PM

John,

 

<snip> Do you have isolated piers or are you just using the slab directly?    Chris

I am using the slab directly.  We made it thick to minimize any vibration, and to have a large thermal mass.  Underneath lies almost a foot of coarse crushed sandstone.  I crushed and compacted it myself with a sledgehammer over a period of weeks.



#35 John Fitzgerald

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Posted 18 April 2021 - 07:13 PM

This pic is a few days before the pour.  After this, to get to as perfect a level as I could, I fastened 1/8 inch by 1.5 inch steel banding  all around the inside of the form boards, since the steel is very straight, and boards tend to be irregular.  I got it as level as the builders level would read. Some places the steel would stick up about a quarter inch or a bit more, other places it was level with the boards.  Second pic shows the steel banding.  I lightly oiled all the forms on the inside before the pour.

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Edited by John Fitzgerald, 18 April 2021 - 07:21 PM.


#36 ghilios

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Posted 23 April 2021 - 02:22 PM

For my ROR observatory, I went with a raised wood deck for the floor. It is supported by 12 concrete piers, with a 5/8"-11 threaded bar in each, to which the stud mounts for the floor beams are attached. Details can be seen under the "construction" page of my website which appears in my signature. My observatory has been in operation since early 2003, and I have had no issues with the floor. Still solid as can be! The telescope pier is mounted to a concrete base which extends 4 feet below grade and contains sixteen 80lb bags of concrete. The hole for this pier was hand dug in the site, which is a pasture that has never been tilled. The pier hasn't moved an iota in the past 17 plus years (according to the bubble level on my mount). I went with the wood flooring to assure quick thermal equilibrium. Just my 2-cents!

 

Ron Abbott

Ron, I live in a rural area and am interested in taking the approach you have with a wood deck instead of concrete. I'd use concrete piers that start below the frost line though. What sort of pointers do you have and lessons learned? Anything that's a must consider? I don't have construction experience, but I am generally handy and engineer of a different sort.



#37 greenstars3

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Posted 04 May 2021 - 03:07 AM

I poured a thick slab  ( about 9 inches) for my RoR and at first had heat retention problems with the slab.

The slab was holding about 7 degrees above ambient for about 1 to 2 hrs after opening up the roof and would slowly reach ambient later in the night

I solved the problem by putting down single layer sheet of those cheap space blankets (the thin throw away ones at the outdoor sporting shops) 

I covered the mylar sheets with some outdoor carpet (that has the added benefit of saving eyepieces when dropped)

The delta T is now 1/2 a degree above ambient when the roof is opened.

 

Robert 



#38 kpem

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Posted 04 May 2021 - 12:39 PM

I built my 16'x10' ROR on a wood deck for three reasons. FIrstly, I'm 35 miles from the nearest transit-mix plant, so the concrete cost went up significantly. Secondly, I knew that I would be doing nearly all of the work myself. A deck would be easy, but finishing a slab solo would be difficult. Third, I wanted to try an athermal design that would follow the outside ambient temp. as closely as possible, which meant a thermally"light" structure. I live out on the desert SW of Tucson, so heat is the main concern. The building has gable and soffit vents and two fans for forced ventilation, no insulation and a steel roof. The deck is supported by eight concrete footings and is from 8 to ten inches above grade, so air can circulate freely under the deck. The design has worked well over the last seven years. The interior temp is usually no more than two to three degrees above the outside air, even in the summer. The fans are run for an hour or two before opening up to help equalize the temps. I still have two concrete piers; can't do much about them. They are finished in porcelain coin tile, which may retard the heat transfer from the concrete, but that's speculation. I've been very happy with how everything has worked, with no second thoughts on slab vs. deck. I think it comes down to personal choice and circumstance, and if you do a good job, you'll be happy with either one. Good luck!


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#39 John Fitzgerald

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Posted 05 May 2021 - 09:08 AM

I poured a thick slab  ( about 9 inches) for my RoR and at first had heat retention problems with the slab.

The slab was holding about 7 degrees above ambient for about 1 to 2 hrs after opening up the roof and would slowly reach ambient later in the night

I solved the problem by putting down single layer sheet of those cheap space blankets (the thin throw away ones at the outdoor sporting shops) 

I covered the mylar sheets with some outdoor carpet (that has the added benefit of saving eyepieces when dropped)

The delta T is now 1/2 a degree above ambient when the roof is opened.

 

Robert 

My thick slab seems to be the opposite. It's almost always cooler than ambient air temp.


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