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How to build roll-off foundation??

Started by Roo, 07/29/2008 09:08AM
Posted 07/29/2008 09:08AM Opening Post
Hello Everyone!!

I am about to begin construction on a roll-off roof in south eastern Idaho. I need some technical specifics if you got them:

1. Where I want to place the observatory is about 20 feet or so from a power transformer. Will that affect tracking, astrophotography, etc?

2. When putting conduit into the observatory is it important to have separate conduits for data transfer and power?

3. What is the best way to pour a permanent concrete pier? We need to go 4' deep to get below the frost line, but how should I design it? Should the base be a big rectangular slab with the 12" pier coming up out of that for stability, etc.?

Any pictures and instructions on how you've built your roll-off roof would be greatly appreciated!!


Posted 07/29/2008 10:44AM #1
1. Should not be a problem but to be sure use a battery backup as a filter. They are cheap at $75.00 and will protect your electronics.

2.I have seen both cables ran close to each other without problems but all the networking books say parallel cables should be separated by 18". I would separate them or go wireless.

3.Below frost line is important. I would make it 4ft square and 6-12 inches thick with re-bar tying the pier into the base. Personally, hughe cubes of concrete are over kill.

Just my two cents from what I have learned over the last 6 years.

Magic Man
Posted 07/29/2008 01:11PM #2
Conduit is cheap....DIgging is difficult. Once you have dug the hole, put in several runs of large conduit for in case things change later.


Posted 07/29/2008 03:01PM #3
Good luck on your project. I'm finishing a small roll off that was started in March 2007 and made my share of mistakes along the way. For what it's worth, here is some advice.


Take the advice others gave and use PVC conduit for the electrical runs. It costs a bit more than just burying UF cable but IMO the advantages outweigh the extra expense. Electrical code requires separation between the feeder lines and the data and low voltage wires. They cannot be run in the same conduit or raceway, and inside the structure AC wires running in walls need a 2" minimum separation from the others. A good source for NEC related info is the search feature at:

If you have elaborate ($$$) gear in your plans, I'd recommend you consult with an experienced electrician to make sure your structure's grounds and bonding meet the code and will protect the equipment if lightning strikes nearby. Consider buying a UPS for the observatory with a built-in isolation transformer.


The pier should be designed so that freezing earth can't heave and push it upwards. That means the hole for the pier needs to taper outwards as you go deeper. Keep the reinforcing steel rebar inside the cement -- anything sticking into the ground will rust away in a few years. Remember to isolate the pier from the slab if you decide to go that route for the floor.

I used a wood-deck approach to my foundation and floor system and here are a couple of pictures that show the initial work. The concrete portion of the pier is mostly below grade and consists of a reinforced block about 3'x 4'x 6', with a small tapered section poking out above ground level. This is the base for a steel pier that my mount sits on. The observatory structure is build like a deck with 4x4 legs sitting on six concrete footers poured at grade. In theory, the whole building (minus the pier) could be moved at some later date.

Attached Image:

JJM's attachment for post 123901

Jim McSheehy