Expert: Dan Griffin - 11/16/2009

Question
QUESTION: I want to build a 2 car garage with attached shop in my backyard. The footprint will be around 20 x 30, but I need more space than that will allow. I cannot go with a second floor as it would not fit with the house or the neighborhood.
I think I want to do this with a full basement to get the look and the square feet. Possibly with a hole in the floor like a quick-oil-cange place under one of the garage bays.
Am I begging for trouble? Would the increased cost be prohibitive? I was thinking of one of those precast basement panel systems set up for a 9' basement wall, and 12" barjoist with 4" of concrete on top.

ANSWER: Karl, a shop is a great asset.  I do have to say that having one in a basement or an upstairs sure takes a lot of the fun out of it for any large or bulky hobbies like wood working/furniture making.  Basement footage is the least expensive square foot under roof. Pay extra attention to water issues - provide French drain, waterproofing, sump, capillary break under slab and you should end up with a great space.

Most insurance and inspectors really frown at the work pits.  Notice the extensive grillage required at an oil changing establishment.  Check with your agent and inspector before taking that approach.  You do want to make some definite plans to make the basement space as accessible as possible.  You must also be aware that you lose a fair amount of floor space both top and bottom to stairs and railings.

YOu might consider going to 5 inch concrete on coroform decking.  Look into core slab if it is available in your area - thinner decks and clear span.

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QUESTION: Is "coroform" regular corrugated galvanzed decking that we por concrete on?
Core Slab, if it is the precast concret planks with the prestressed reinforcing wires and 5-6 inch round hollows are available I think. Would they be strong enough in a 20 foot span to support a pickup truck?

Answer
Core slab is a fairly common way to deal with garage floors.  You would need to contact them for details.  

Coroform aka. corruform is corrugated sheet, usually galvanized that spans the bar joists to hold the concrete and reinforcing.  Gauge of sheet would be subject to span between joists and depth of concrete.  Remember there will be men and wet concrete on this stuff - lots of weight.