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Wood Deck or Concrete Slab for SkyShed POD?

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#1 GraySkies

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Posted 11 April 2019 - 09:23 PM

Hi,

 

Looks like I'm getting a SkyShed POD next weekend and now exploring options for placement.

 

The Pier will be suitably deep poured concrete tube with footing, and there will be spacing between the pier and the floor.

From reading I'm seeing two options:

 

1. Build a "Deck" of Wood on concrete blocks

2. Pour a Concrete Slab

 

3. Gravel base with concrete stone slabs (from a previous walkway which was replaced) with mortar as required.

All three options are possible at my location, I'll be doing the work myself (with the help of a friend) but I'm wondering people's experiences and pro/cons of each option I might want to consider.

 

I'm located in Canada so we get snow etc and I plan to use my observatory during the winter months; there will be power & internet routed to the dome as I do mainly astrophotography so there will be a connection hole through the floor.

 

Thanks



#2 MikeTahtib

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Posted 11 April 2019 - 09:46 PM

Concrete will be more durable and maintenance free, no splinters, no bugs or weeds crawling up between the boards.  Also, dropped things (focuser screws, eyepieces, any other little bits and pieces you might have on your rig or use while observing) will be much easier to find on a poured concrete slab than if it falls between the deck boards or onto the gravel between concrete stones.  On the other hand, if you ever kneel to observe, the wood will be warmer and less abrasive (although you could get kneepads for the concrete).  If you go with wood, make sure it is thoroughly insect-proof.  I thnk poured concrete is the more practical option.

Having said all that, when at home, I enjoy observing from a low wood deck, and haven't lost anything yet.


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#3 TOMDEY

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Posted 11 April 2019 - 09:50 PM

Well... I've always provided an ample "breathing space" under my domes and scope sheds. And grassy surround with bushes and then trees farther out. A lot of that has to do with the way the terrain is here, regardless... but I've become convinced that it also fosters better seeing. I've noticed that many CN experts insist that half-are-sec seeing is essentially nonexistent. Well, it is just common enough here to really benefit large good scopes. Not typically 0.5 sec... but frequently. Anyway, here's how I keep my scopes and domes above ground with natural breathing air wafting underneath...    Tom

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#4 OleCuss

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Posted 11 April 2019 - 09:59 PM

From my perspective it depends on your site and cost.

 

I sort of ended up doing both.

 

The door to the SkyShed Pod is pretty low and I'm rather tall - not a great combination IMHO.  We also didn't want to work at maintaining the decking over the years to come - and really wanted some other concrete work done.

 

So I put a concrete block in the ground.  It's a big concrete block oriented North-South with three sets of bolts.  So I can position the pier a little to the North, centered, or positioned a little to the South (I like options).

 

We got an elevated concrete slab poured but left a pit slightly smaller than the diameter of the Pod.  Decking inside the pit which is at the same level as the sidewalk outside.

 

The net effect is that when I enter the observatory I step over a relatively high threshold to go from the sidewalk outside onto the decking inside the observatory.  Under the decking I have about 18" of pit where I can run cables and such (I put in several sets of conduit in before the concrete was poured).

 

This means I have 6" less bending over that I have to do when I enter the Pod.  I'd have gone to 12" high elevation for the Pod except the Pod would then have been high enough to be obnoxious to neighbors.

 

Because I have decking inside the Pod/pit I maintain flexibility of positioning of the pier and of fiddling around within the pit.

 

Upshot is that for me the answer was a combination of concrete and decking.  Oh, and since the decking is protected from the elements it should last for a very long time. . .

 

More complex to get set up but for where I am I'd do it again.


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#5 macdonjh

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Posted 12 April 2019 - 09:01 AM

Either way you go, make your foundation good. 

 

Some club members set up a couple of SkyShed PODs on decks, but they only used those "deck pier" precast concrete blocks, directly on the ground, for the foundation.  It was really quick to build, and looked good for a few weeks.  However, over the past four years (nearly five now), the ground has shifted under those decks and everything is crooked.  One of the PODs has leaks, too.  I can't say whether the leaks are caused by weather sealing materials, or if the panels of the POD are out of place because of the crooked deck it's on.  

 

And, as OleCuss alludes to, the wood hasn't fared well in the Third Coast weather either.  That can be mitigated with some maintenance, though (sealing every two years).



#6 kathyastro

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Posted 12 April 2019 - 09:29 AM

My dome is on a ten foot square building that just sits on a flat gravel pad.  The "foundation" of the building consists of four 6x6 pressure treated beams lying on the ground.  The flooring is conventional wood framing sitting on the beams.  There is a 1cm gap between the floor and the pier, which has a concrete foundation below the frost line.

 

As long as the pier is isolated from the floor, go with whatever construction is going to be easiest for you to build.

 

Pay attention to the transition of your cables from the room to the pier, so that they don't transmit vibrations.  Mine run in conduits that rest on the ground, so that any vibrations from the building are damped out long before they get to the pier.


Edited by kathyastro, 12 April 2019 - 10:36 AM.


#7 cmaier

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Posted 12 April 2019 - 09:43 AM

I think a concrete pad will be a better long term solution. I went with a wood deck, then a 3/4" outdoor rated  plywood cut in a circle to set the dome on to try and ensure a seal to the bottom of the dome and keep the water from leaking under it.  This has worked nicely for about 4 years and still keeps water from coming in but the exposed part of the plywood absorbs moisture no matter how often I try and seal it. I think the plywood will last another 3-5 years , then I'll replace it with   a thick rubber pad instead. The cedar deck itself has held up well. As you can see, my dome is a TI PD-10...

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#8 GraySkies

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Posted 12 April 2019 - 11:36 AM

Okay so the census is towards a concrete pad for a zero maintenance solid foundation; which works as I'll be pouring an isolated concrete pier anyways.

 

My thought is excavate to level (like 3-4" down from top grade) and add in about 1-2" of gravel as the base (I might poke a few 4" holes a bit deeper to "hold" that wouldn't have gravel).

Pour the concrete slab 4" in depth as a hexagon shape so that the inner circumference of the hexagon is bigger than the dome.
Put Eye-bolts (or threaded achors) at each of the 6 corners, this is so I can do a tarp tie-down over the dome if anything ever happens and I need to cover it.

There will be 1" spacer between the pier and the slab, this will be filled in later with something that won't pass vibrations (Thoughts?)

 

Going with a hexagon shape because:

1. Flat edges are easier to board then round.

2. Uses less concrete than square.
3. Minimizes exposed pad which could cause water ingress.

4. If I ever want to add a Bay to the POD I can just add concrete to one side of the structure as the hexagon will be oriented to match the walls.

 

Any concrete tutorials / videos / tips for observatories are appreciated.


Edited by GraySkies, 12 April 2019 - 11:37 AM.

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#9 macdonjh

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Posted 12 April 2019 - 12:42 PM

Okay so the census is towards a concrete pad for a zero maintenance solid foundation; which works as I'll be pouring an isolated concrete pier anyways.

 

My thought is excavate to level (like 3-4" down from top grade) and add in about 1-2" of gravel as the base (I might poke a few 4" holes a bit deeper to "hold" that wouldn't have gravel).

Lots of work but not a bad idea.  Excavating the surface soil will also get rid of any organic material, which is standard practice in construction.  You don't want dead plants rotting under your slab, causing voids.

 

Pour the concrete slab 4" in depth as a hexagon shape so that the inner circumference of the hexagon is bigger than the dome.

4" is plenty thick for strength, but consider thicker to get the top of concrete a few inches above your soil level.  That way you'll be above any water that might collect on your property, you'll keep any wood in your observatory dry, and you'll be able to see the tunnels subterranean termites build if they decide to eat your building.

Put Eye-bolts (or threaded achors) at each of the 6 corners, this is so I can do a tarp tie-down over the dome if anything ever happens and I need to cover it.

Anything that protrudes above the top of concrete will be a tripping hazard and you may find yourself cursing them if you trip in the dark.

 

There will be 1" spacer between the pier and the slab, this will be filled in later with something that won't pass vibrations (Thoughts?)

I simply left a gap between my slab and concrete pier.  Friends of mine have filled the space between their piers and slabs with various foams and caulks.

 

Going with a hexagon shape because:

1. Flat edges are easier to board then round.

Granted.  In industrial construction most contractors pour octagonal foundations for round structures because 45o angles are easier to form than 60o angles.  

 

2. Uses less concrete than square.

+1

3. Minimizes exposed pad which could cause water ingress.

If you put a slight slope in your pad (1/8" per foot), with the high point in the center, low point at the edges, you won't have to worry about that.

 

4. If I ever want to add a Bay to the POD I can just add concrete to one side of the structure as the hexagon will be oriented to match the walls.

The idea that makes my octagon suggestion a poorer choice.

 

Any concrete tutorials / videos / tips for observatories are appreciated.

This isn't apropos to your project, but if you were building a roll-off roof observatory, I'd suggest putting piers at the corners and at the ends of the gantry.  When I dug for my pier, I also used the auger to dig/ drill 12" diameter holes about three feet deep (that's when I hit clay, much more stable than the sand that makes up the surface soil at my site) at those six locations.  That way, when my roof is closed or open, its weight is transferred through the corners of my building or the gantry posts to concrete piers rather than my relatively thin slab.  

My comments in red...

 

Only two more thoughts:

 

Be careful when selecting the diameter of your concrete pier: if you go too big, your mount's counter weights may hit the concrete.  Ask me how I know...  Also, consider running conduit through your pier.  I ran 1" (for 12V power) and 2" (for data cables and their big connectors) through my pier and really like that I have no cables crossing the floor of my observatory.  I also ran a 1" conduit under the slab to my pier for 120V AC power for a hair dryer in case of dew.  


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#10 SkyShed

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Posted 12 April 2019 - 02:06 PM

Grayskies,

 

Lots of good answers from members here for you. I find them informative too.

 

Call me as well if you like. I'd be happy to chat about it with you. We spent time almost every day discussing bases with owners. My cell # is on our websites.

 

CS!
Wayne


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#11 greenstars3

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Posted 12 April 2019 - 04:41 PM

I poured a 8" slab for my ROR and it stands about 3" higher than the soil around it to prevent water from coming in and it works great - the only issue I found is the thermal mass was a problem until I put in space blankets on the floor and covered the thin mylar sheets with an outdoor carpet. Before I did this I had a delta T of about 7-9 degrees F higher temp coming off the concrete - now the delta T is about 1-2 degrees F above the outside air all night. Another benefit is that if I drop something it will fall on carpet.

 

Robert    



#12 PastorBillV

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Posted 12 April 2019 - 04:45 PM

I'd suggest concrete for all the reasons people have mentioned above - especially with your winters. I'd also put a rubber mat floor or great carpeting! If you are going to use a deck, consider Trex.  I just had a deck built with it for my observatory build and it really got tested with our brutal winter.  Looks like new.  

 

Post pictures as you build!

 

Bill



#13 GraySkies

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Posted 12 April 2019 - 11:58 PM

My comments in red...

 

Only two more thoughts:

 

Be careful when selecting the diameter of your concrete pier: if you go too big, your mount's counter weights may hit the concrete.  Ask me how I know...  Also, consider running conduit through your pier.  I ran 1" (for 12V power) and 2" (for data cables and their big connectors) through my pier and really like that I have no cables crossing the floor of my observatory.  I also ran a 1" conduit under the slab to my pier for 120V AC power for a hair dryer in case of dew.  

I'll double check the clearance on the GH8, the pier plate will likely be slightly elevated to allow for alternative attachment in the future as my next "big" astro purchase after this Observatory is an upgraded mount... or a larger refractor to replace my 80mm... see how the wind blows.

‚Äč
Yes I'm looking at conduit to go from the garage into the ground and then into the pier through the concrete slab, likely 2-2.5" as I have some left over from an earlier project I can reuse a segment with confidence, it will need a drain on the far side though as the garage is slightly uphill from the proposed POD location.

Grayskies,

 

Lots of good answers from members here for you. I find them informative too.

 

Call me as well if you like. I'd be happy to chat about it with you. We spent time almost every day discussing bases with owners. My cell # is on our websites.

 

CS!
Wayne

Once I have the observatory in hand I may give you a shout to discuss attaching the POD to the foundation, and if I need any parts/upgrades (I'm debating adding an alcove to the unit for my laptop and may need to replace a wheel or two if they are worn). I'll be in your neck of the woods for Starfest if not earlier (although my friend has already vetoed bringing the POD with us... no room in the truck).

 

I poured a 8" slab for my ROR and it stands about 3" higher than the soil around it to prevent water from coming in and it works great - the only issue I found is the thermal mass was a problem until I put in space blankets on the floor and covered the thin mylar sheets with an outdoor carpet. Before I did this I had a delta T of about 7-9 degrees F higher temp coming off the concrete - now the delta T is about 1-2 degrees F above the outside air all night. Another benefit is that if I drop something it will fall on carpet.

 

Robert    

Good advice, I'll look at doing something similar to avoid things hitting a hard floor.
I'm also going to be having a large shiny tarp over the POD to reflect sunlight during the day to help keep it cool.

 

I'd suggest concrete for all the reasons people have mentioned above - especially with your winters. I'd also put a rubber mat floor or great carpeting! If you are going to use a deck, consider Trex.  I just had a deck built with it for my observatory build and it really got tested with our brutal winter.  Looks like new.  

 

Post pictures as you build!

 

Bill

 

I'll look into flooring options, thanks!


Edited by GraySkies, 13 April 2019 - 12:02 AM.


#14 TOMDEY

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Posted 14 April 2019 - 09:03 PM

Returning to your opening question, regarding "Wood Deck or Concrete" >>>

 

Little more regarding seeing...

 

If your observatory and scope are sitting directly on concrete, that puts a huge thermal mass right there.

 

My situation tends to ~rinse~ the heat away at sunset. I can actually feel the heavy, cool air gliding down the hill (from deep woods uphill to the east) and washing under the dome, flushing out the hot/warm bubble that has collected there during the day. I open the shutters and door a couple hours before sunset and start two giant box fans blowing in at the doorway, aimed right at the (36-inch) scope, which has all the covers off. An hour after sunset things are good and an hour after that the seeing is often wonderful! I'm sure that the grass, bushes, trees and woods also help a lot. And the elevation 700 feet higher than the valley floor, a mile to the west. The scope never gets dew; sometimes the grass does.

 

Whatever the detals of that mechanism, half arc-sec seeing is often enough to benefit, a lot!

 

Every climate and local terrain will be different.

 

Just one more thing to consider... and maybe able to address, provide for, optimize... before finalizing the design!    Tom

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#15 GraySkies

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Posted 15 April 2019 - 11:13 AM

Returning to your opening question, regarding "Wood Deck or Concrete" >>>

 

Little more regarding seeing...

 

If your observatory and scope are sitting directly on concrete, that puts a huge thermal mass right there.

 

My situation tends to ~rinse~ the heat away at sunset. I can actually feel the heavy, cool air gliding down the hill (from deep woods uphill to the east) and washing under the dome, flushing out the hot/warm bubble that has collected there during the day. I open the shutters and door a couple hours before sunset and start two giant box fans blowing in at the doorway, aimed right at the (36-inch) scope, which has all the covers off. An hour after sunset things are good and an hour after that the seeing is often wonderful! I'm sure that the grass, bushes, trees and woods also help a lot. And the elevation 700 feet higher than the valley floor, a mile to the west. The scope never gets dew; sometimes the grass does.

 

Whatever the detals of that mechanism, half arc-sec seeing is often enough to benefit, a lot!

 

Every climate and local terrain will be different.

 

Just one more thing to consider... and maybe able to address, provide for, optimize... before finalizing the design!    Tom

Thanks for the insights Tom.

I'm on a hillside ~50' above lake Ontario.
I've been calling the observatory location "North Shore Observatory" for years... but now it will have walls and a ceiling!
I get dew, so dew heaters are a must all the time; fog isn't a huge issue for me though.
 

My largest Scope is 8" diameter, anything larger for this area and the number of seeing nights I can reasonably expect to see better (from a resolution standpoint) with them drops dramatically.
So thermal "mass" isn't as big an issue for me as I can cool the scope to ambient pretty quickly; main heat issue is tarping the observatory when not in use to reflect the sunlight during the summer.




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