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Inching towards the build...need some slab advice

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#1 BKBrown

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Posted 24 October 2020 - 11:32 AM

So the grounds crew finally cleared and graded the two northern acres, roughly 100 yards by 100 yards, for the observatory build site this week. They still have to drag for roots and smooth things out before seeding next week, but I can now lay out the observatory pad...

 

NW corner.jpg

 

NE corner.jpg

 

I have further simplified the building by doing away with an interior wall and door, forgoing the foyer/hallway to save some money. Instead, I will build an exterior "mud room" entrance to help support insulated warm room climate control...

 

PG01.jpg

 

My last observatory was built on a wood deck supported by beams set in concrete, this time I plan to build on a slab. I have a really good contractor who will build the frame and oversee the pouring and finishing of the slab, but I have to prepare the holes for the isolated piers. Frankly I have never done anything like this before, and neither has my contractor. So my question is "What is the best way to proceed with the preparation of the pier hole/forms? What should I consider doing during the building stage that I might not be able to do later? I do not need to trench and install conduit since I plan to run the power to the warm room, and from there to the piers under a raised wood floor. I am probably overthinking this problem but would appreciate any further recommendations as a follow on the a previous discussion we had here. Thanks!

 

Clear Skies,

Brian snoopy2.gif


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#2 Garyth64

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Posted 24 October 2020 - 12:04 PM

Here's a link to a friend of mine's build:

 

https://www.cloudyni...rs-observatory/

 

I like the idea where the roof rolls off onto the warm room.  It just seems like a good idea to utilize that empty area that would otherwise be there.


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#3 Couder

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Posted 24 October 2020 - 12:16 PM

I like your design, and am happy for you your plans are proceeding. Mine are on hold. I assume you have the plans done for how tall the pier/concrete will be? You can obviously have that part done before, and you will need to leave room around the form. I am thinking a couple layers of the good foam rubber. 

As for the pad itself: a few years ago I bought a farm from my dad. He kept 2 acres. (Looking back = really bad idea) We both put up pole barns at the same time. I had the 6x6 uprights put in before we poured the pad. I put down thick plastic between the gravel and the concrete. After the concrete was poured, I let it sit for maybe 6 weeks. Then I had the contractor come back and put the barn up. Dad was in more of a hurry, so he had the barn put up, put down the same plastic I used, then had the pad poured.

My barn floor never once got moisture on it, dad's looked like it rained in there frequently. 

So when the pandemic is over and we can look for a place, I know how my new observatory will be built. 


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#4 BKBrown

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Posted 24 October 2020 - 08:03 PM

Here's a link to a friend of mine's build:

 

https://www.cloudyni...rs-observatory/

 

I like the idea where the roof rolls off onto the warm room.  It just seems like a good idea to utilize that empty area that would otherwise be there.

I like that concept too, Scott sent me some pix of that build early in our discussions. It might, however, be substantially more expensive than a standard club model. The only real downside to the configuration is that it appears to add some considerable height to the roof in the direction it rolls off; not sure I would want that blockage. I may re-engage Scott on that idea...

 

Clear Skies,

Brian snoopy2.gif


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#5 BKBrown

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Posted 24 October 2020 - 08:08 PM

I like your design, and am happy for you your plans are proceeding. Mine are on hold. I assume you have the plans done for how tall the pier/concrete will be? You can obviously have that part done before, and you will need to leave room around the form. I am thinking a couple layers of the good foam rubber. 

As for the pad itself: a few years ago I bought a farm from my dad. He kept 2 acres. (Looking back = really bad idea) We both put up pole barns at the same time. I had the 6x6 uprights put in before we poured the pad. I put down thick plastic between the gravel and the concrete. After the concrete was poured, I let it sit for maybe 6 weeks. Then I had the contractor come back and put the barn up. Dad was in more of a hurry, so he had the barn put up, put down the same plastic I used, then had the pad poured.

My barn floor never once got moisture on it, dad's looked like it rained in there frequently. 

So when the pandemic is over and we can look for a place, I know how my new observatory will be built. 

Why do you think this happened? If you both had the same floor construction, was it the curing and settling of the pad prior to erecting the structure that prevented the interior moisture? Not sure what the dynamic is here, but I would certainly like to avoid interior rain if possible smile.png

 

Clear Skies,

Brian snoopy2.gif



#6 Couder

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Posted 24 October 2020 - 08:17 PM

Being as the 2 barns were on the same dirt, and we used the same chat, same plastic, same rebar, and same concrete, the only difference was mine curing.
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#7 BKBrown

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Posted 25 October 2020 - 11:24 AM

One thing I did not mention in my first post is that I am planning to bring in a cement truck to pour the slab, and I want them to pour the pier footers at the same time. So I have to juggle both set ups simultaneously. The footers will be 24" across and 6' deep with rebar reinforcement, poured directly into the holes with no Sonotubes (I want the cement in direct contact with the hole walls). I think this will be adequate for the loads they will bear. They will have to clear the slab by 6" to be level with the raised floor, a fabricated metal pier will be mounted to each footer. Hopefully this paints a more complete picture. With this information in mind, what are the best practice approaches to isolating the piers during the one time cement pour-fest? Thanks!

 

Clear Skies,

Brian snoopy2.gif


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#8 John Fitzgerald

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Posted 25 October 2020 - 11:42 AM

So the grounds crew finally cleared and graded the two northern acres, roughly 100 yards by 100 yards, for the observatory build site this week. They still have to drag for roots and smooth things out before seeding next week, but I can now lay out the observatory pad...

 

attachicon.gifNW corner.jpg

 

attachicon.gifNE corner.jpg

 

I have further simplified the building by doing away with an interior wall and door, forgoing the foyer/hallway to save some money. Instead, I will build an exterior "mud room" entrance to help support insulated warm room climate control...

 

attachicon.gifPG01.jpg

 

My last observatory was built on a wood deck supported by beams set in concrete, this time I plan to build on a slab. I have a really good contractor who will build the frame and oversee the pouring and finishing of the slab, but I have to prepare the holes for the isolated piers. Frankly I have never done anything like this before, and neither has my contractor. So my question is "What is the best way to proceed with the preparation of the pier hole/forms? What should I consider doing during the building stage that I might not be able to do later? I do not need to trench and install conduit since I plan to run the power to the warm room, and from there to the piers under a raised wood floor. I am probably overthinking this problem but would appreciate any further recommendations as a follow on the a previous discussion we had here. Thanks!

 

Clear Skies,

Brian snoopy2.gif

Raised wood floors over slabs can become a haven for insects, unless totally sealed..  Sealing can trap moisture on the slab,  I strongly advise against raised wooden floors over slabs.



#9 BKBrown

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Posted 27 October 2020 - 11:05 AM

Hi John. I am prepared to deal with any unwanted critters of 4, 6, or 8 legs and plan to vent the floor to permit air circulation. I am curious about one thing: if the slab is allowed to cure and then air dry for several weeks, why should I anticipate significant moisture issues? Would a vapor barrier help or hurt? I want to use a raised floor to simplify upgrades and maintenance on my gear, burying everything in conduit under concrete limits my options. Thoughts?

 

Clear Skies,

Brian snoopy2.gif



#10 John Fitzgerald

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Posted 27 October 2020 - 11:19 AM

I have a roll off 16x16 feet with a concrete slab floor 9 inches thick, and 16 inches thick around the edges, built in 2008.  The floor still sweats badly when it's been cold at night and the next day is warm and humid.  A vapor barrier lying on the floor would sweat on top.  If you vent the floor with fine screening, and use treated wood for the floor base, you should be OK.  Alternatively,  and more difficult, you could lay foam sheeting on the floor between the floor joists, but I would still use treated wood. 


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#11 HunterofPhotons

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Posted 27 October 2020 - 12:43 PM

You haven't given us any details on your slab construction.

If you're not going below the frost line with a perimeter wall you realize the slab will move in the winter freeze and if your observatory is supported on that slab it will move too?

 

dan k.



#12 John Fitzgerald

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Posted 27 October 2020 - 02:15 PM

Thats why I have a perimeter wall down 16 inches, sitting on rock underneath.  Our usual frost line is only a few inches.



#13 kathyastro

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Posted 27 October 2020 - 02:44 PM

Would a vapor barrier help or hurt?

Around here, normal practice is to lay vapour barrier on the ground under a slab that will have a building over it.  It prevents moisture wicking up into the concrete from the ground below.  Concrete is somewhat porous, and any moisture it absorbs will be released eventually into the building.  In your case, into the space under the floor.  Vapour barrier below and venting above should prevent moisture problems.


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#14 John Fitzgerald

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Posted 27 October 2020 - 02:57 PM

Moisture problem in my roll off is caused by the enormous mass of concrete cooling at night, and remaining below the dewpoint during most of the following day. Vapor barrier under is no help in this sort of case.  


Edited by John Fitzgerald, 27 October 2020 - 06:24 PM.

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#15 BKBrown

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Posted 29 October 2020 - 08:20 PM

This whole discussion is making me start to second guess myself and start seriously reconsidering a frame built wood floor on posts. I had no issues with moisture inside my wood floor observatory, and what moisture there was was handled by a dehumidifier.

 

Clear Skies,

Brian snoopy2.gif



#16 Garyth64

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Posted 29 October 2020 - 08:52 PM

Some people may have the problem, and some don't.  I have an unattached, unheated garage, 16x20, the concrete does not sweat.  I have never had a moisture issue with it.  There wasn't a vapor barrier under the concrete, but I do have 2' footings around the perimeter. A "rat" wall.  It was all poured at the same time. 

Do you currently have a garage? Does it sweat?

 

Also, my driveway is cement too.  It is about 80 feet long, 10 feet wide.  They are just 6" slabs. They do not sweat either.

 

I think if the ground has a moisture problem to begin with it may be a concern.

 

I'm late in reply on this next answer, but I would not pour the footing of the pier with the floor.  I would do separate pours.


Edited by Garyth64, 29 October 2020 - 08:56 PM.


#17 John Fitzgerald

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Posted 29 October 2020 - 09:01 PM

This whole discussion is making me start to second guess myself and start seriously reconsidering a frame built wood floor on posts. I had no issues with moisture inside my wood floor observatory, and what moisture there was was handled by a dehumidifier.

 

Clear Skies,

Brian snoopy2.gif

There are potential problems with wood floors on posts also.  Unless they are well above the ground, animals will make homes in the dirt under the floor.  I believe any wood floor should be high enough for a person to crawl under and maintain things, if necessary.  It's a trade-off, either way.  Slabs in unheated buildings around here always sweat when a warm front comes through after a cold snap.  Put your wood floor in with treated wood, ventilate, and screen it, and you probably won't have much problem.

 

Edit: I don't have any problem with ground moisture under my thick slab.  There is rock fill under the slab, and the entire thing sits at the very top of a watershed.  It's all poured in one piece, and a bit thicker in the center.  I can jump up and down on the floor next to a telescope, and the observer sees no vibration in the image at over 200 power.  We actually tried this. 


Edited by John Fitzgerald, 29 October 2020 - 09:05 PM.


#18 kathyastro

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Posted 30 October 2020 - 06:59 AM

This whole discussion is making me start to second guess myself and start seriously reconsidering a frame built wood floor on posts. I had no issues with moisture inside my wood floor observatory, and what moisture there was was handled by a dehumidifier.

 

Clear Skies,

Brian snoopy2.gif

My observatory has a frame-built wood floor.  It is supported on 6x6 sleepers around the perimeter, so there is no access to critters.  The whole thing sits on vapour barrier on top of a bed of packed gravel.  In an area prone to a lot of hurricanes, you'd probably want to anchor it to the ground, but mine has weathered a couple of category 2 hurricanes and the building has not moved at all.  I did have it anchored with iron spikes at its first location, but, when I had it moved, there was no way to drive the spikes with the building on top, so I just cut them off.

 

I have had no problems with animals getting under the building.  My only unwanted guests have been ants.  I finally managed to keep them out with lots of caulking.  I had one mouse inside the building, but it got in under the dome skirt, not via the foundation.


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#19 John Fitzgerald

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Posted 30 October 2020 - 09:09 AM

Hi Kathy,

I guess it depends on location.   Here in the south US, rats, mice, opossums, etc will make homes under buildings that are low to the ground.  If the building is a bit higher, it doesn't afford protection from predators,  and animals won't tend to use it.



#20 Couder

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Posted 30 October 2020 - 09:50 AM

You don't have to be in the South; I'm in the Missouri Ozarks - I have armadillos, raccoons, and rabbits all scurrying under buildings when they see me coming.



#21 kathyastro

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Posted 30 October 2020 - 10:09 AM

Hi Kathy,

I guess it depends on location.   Here in the south US, rats, mice, opossums, etc will make homes under buildings that are low to the ground.  If the building is a bit higher, it doesn't afford protection from predators,  and animals won't tend to use it.

Mine isn't "low to the ground".  It is on the ground.  The only access to the under-floor area is by tunnelling, and the packed gravel base discourages that.  No one has tried to tunnel under it.


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#22 John Fitzgerald

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Posted 30 October 2020 - 10:17 AM

Mine isn't "low to the ground".  It is on the ground.  The only access to the under-floor area is by tunnelling, and the packed gravel base discourages that.  No one has tried to tunnel under it.

That's almost like being on a slab, as far as tunneling.   I have two outbuildings low to the ground, neither on packed gravel, both over plastic vapor barriers. Critters have badly damaged the barriers, piled up dirt under the buildings, and made a general mess.  Any future building will be on a slab or high off the ground. 



#23 KTAZ

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Posted 30 October 2020 - 10:29 AM

It's interesting discussion, but every case is different based on the users geographic location, weather patterns, and even their local critter population.

I have been in construction for 40 years. I can tell you that a plastic weather barrier over your crushed stone and below your concrete slab is a must in Michigan, but unnecessary in Arizona.

Posting opinions here without first posting your location and normal weather does not help anyone.
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#24 John Fitzgerald

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Posted 30 October 2020 - 10:34 AM

.

Posting opinions here without first posting your location and normal weather does not help anyone.

You can see from my profile that I am in N Arkansas.  Humid, very little ground freezing, lots of burrowing critters.


Edited by John Fitzgerald, 30 October 2020 - 10:36 AM.


#25 jcj380

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Posted 30 October 2020 - 11:33 AM

https://dricore.com/...icore-subfloor/ might be worth a look.


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