Portable building foundation question

[David]

I'd like to build a portable building behind my garage. Per city code it has to 120 sqft or smaller and portable. I wanted to put it on a concrete slab, but it'll be over a utility easement and after getting all the underground stuff marked, I found there's a gas line right under where the building will go.

I thought about a flat (no beams) concrete slab with a couple of tow eyes at the ends so I could call it portable, but I think a 4000 lb slab would be hard to define as portable.

It looks like the most common way to go is 4x4 or 4x6 treated skids placed on concrete pads with 2x6 floor joists on top. I can go this way, but I really don't like having what'll be close to a 12" foundation.

Does anyone out there in Pelican Land have any better ideas?

[MT930]

How about a 8X15 /7X18 ft enclosed trailer you could use it for moving stuff around and storage. Very portable you could keep a car in it, no permits and you can always take it with you. ?

[70SATMan]

Originally Posted by 125shifter

I'd like to build a portable building behind my garage. Per city code it has to 120 sqft or smaller and portable. I wanted to put it on a concrete slab,
Does anyone out there in Pelican Land have any better ideas?

In Cal. a concrete slab puts it out of the realm of a portable or "temporary" structure. I built a full wood frame shed 9x11 on 2x6 joists spanning 4x4 beams. the 4x4s and whole floor is supported at the corners with trapazoidal concrete footings above ground. This qualified as portable or "temporary". More than adequate. I used two sheets of ply for the floor with tar paper sandwiched between for a vapor barrier. Then I framed the structure normally.

Your other option would be to do a beam and rock foundation. Essentially, bury 6x6 beams in a bed of gravel so that you get good drainage and your beams end up at ground level. Then a 2x6 floor frame sitting on that. In Cal. portable means a non permanent foundation.

[Moneyguy1]

A less than permanent foundation can be made with 8 x 16 or 16 x 16 2" thick concrete blocks (available at most garden stores like Lowes or Home Despot) over a bed of sand. If the base of the building is to be wood on concrete, make the sill plate out of pressure treated lumber and insulate it from the concrete with a product that is made for the purpose; a 3 1/2" wide plastic "tape" or at the least, srips of tar paper.

With the blocks, if access is needed to the gas line, all you have to do is pull up the blocks.

I hope that helps.

[URY914]

Use 4x4 skids and 2 x 4 floor framing on 12" centers. That will be less than 8" off the ground.

[David]

Thanks guys. I'm leaning toward the 4x4 skids with 2x4 joists 12"OC. I called the gas company and they said I absolutely couldn't build on the gas line. So a new issue is building it at a cost that I'm will to eat if they tell me to tear it down. Unfortunately I don't have room to actually move it if asked to since my garage extension goes right up to the easement.

Here's some pics showing the situation (the red line is where the shed would go):

[911Rob]

Dig out the shrubs and topsoil. Place a compacted gravel lift with poly on top and put down the 2'x2' conc patio blocks for a foundation. Build your shed on top of that. You can use the conc patio blocks for the floor or just the perimeter foundation bearing areas.

For a 120 SqFt shed, you can make it totally portable and simple.

The last one I did had a plywood floor on 2x6" joists resting on treated 4x4" foundation. total cost was about $2500 and its built solid enough to skid onto a tow truck bed:

[WI wide body]

ists

Originally Posted by 125shifter

I'd like to build a portable building behind my garage. Per city code it has to 120 sqft or smaller and portable. I wanted to put it on a concrete slab, but it'll be over a utility easement and after getting all the underground stuff marked, I found there's a gas line right under where the building will go.

I thought about a flat (no beams) concrete slab with a couple of tow eyes at the ends so I could call it portable, but I think a 4000 lb slab would be hard to define as portable.

It looks like the most common way to go is 4x4 or 4x6 treated skids placed on concrete pads with 2x6 floor joists on top. I can go this way, but I really don't like having what'll be close to a 12" foundation.

Does anyone out there in Pelican Land have any better ideas?

A buddy of mine had the same "code" issue. Since we had access to a steel yard we got about 3 or 4 huge 16 or 18 inch tall I-beams and he imbedded them side by side partially in concrete. Then he built his "temporary" building and mounted it with the floor joists diagonally to the I-beams.

Looked goofy as all hell...until the city started to give him grief about it being a permanant structure. So he tells them to stop by the house. When they arrived he jumped into his BobCat...positions the lift forks between the steel beams and picks up the building and sets it along side his drive and says "See, I told you it was only temporary!"

[David]

I've given more thought to a foundation that's strong and portable:

I'm thinking two 16' long 4x6 skids notched 1" deep along their length to accept 8' long 2x6 floor joists, 12" OC, that are also notched 1" to fit the skids. Once finished out, I think it would take being dragged around by a small tractor or fork lifted to a trailer if need be. It would take some extra router work, but it'll keep it almost as low as a 4x4 skid and 2x4 joist foundation.

[MotoSook]

David,

I know a company in Texas that can provide you a 120 sf (10X12) "skid mounted" pre-assembled or knocked-down building. They can hook you up with a firm (or they will) to come out and erect it for you. They will fabricate the skid and send it out with the metal sheets and all the material needed to erect it if you don't have a yard that will allow big equipment to come in and drop it in your yard. Erecting it is best left to the pros.

They made steel buildings for me when I was buying them for utility service. I would guess that a basic 10X12 knock down building will run about $12000 + maybe $3000K to erect. You can option for insultion on the walls and ceiling. I've even had the floor insulated. I don't know what your budget is, but if you are interested e-mail me for the contact info. There are lots of companies out there that will build you a skid mounted steel building with varying prices. I mentioned Texas, because shipping cost would be less for you.

No gas company will let you build on their easement. So forget about concrete slabs or sonotubes. When they do a right of way/easement survey you will be asked to tear it down. A skid mounted building (steel skid) might be pulled/towed on/off the easement if you have the means.

Of course building it on a treated lumber skid works too. Don't know what you other building requirements/codes are.

[David]

Souk, thanks for the tip. Many of my friends tell me I'd be way ahead if I hired out more of my projects. Unfortunately I have that Tim Taylor gene that forces me to do it myself. I may not create engineering masterpieces, but they're at least engineering case studies .

[MotoSook]

You can frame the building such that the walls and roof are "modular". Exposed studs make for good bolting point between adjacent wall sections, as are trusses. Then you just have to unbolt it when you are asked to move it. Gas companys really don't like folks putting anything on their easement.