Expert: Dan Griffin - 12/8/2007

Question
I am in West Texas and am converting a carport to an office: approx 24' x 27'. I am getting conflicting advice on the cement.  We will use cement with fiber additive. Do we need rebar or metal wiring?  It will be office space so no vehicles on it.  We have built it up 5 inches and plan to pour 4 inches of concrete with footings.  Please let me know if I need rebar, and what you would recommend for footing size, if any. Thank you for your time. Don

Answer
Lubbock?

Cement is one of several ingredients in concrete.  I'm sure your question is about concrete.

Footings have to do with the structure that is pushing on them and the bearing capacity of the soil they are on; their depth is regulated by the frost depth in your location.  Your local building department should be more than able to give you the local frost depth (mine in Oklahoma City is 18" and I suspect yours will be the same), rebar requirement, and minimum acceptable footing width.  The footing will require at least 2 #5 rebars tied through the corners.  All laps will require 32 bar diameters (20 ") or more.  They will probably expect 12" wide footings, but again this depends a bit on what loads are being carried.  Brick walls can weigh more than siding.  Depending on your roof design, you may need footings under a load bearing wall at mid point.

I would be happy with the 4" floor.  Remesh does NOT reinforce concrete, but is quite normal in residential floors.  Fiber is purported to be an equal to WWF (remesh).  A 4" floor gains little or no help from rebar, the time and money are much better spent on subgrade selection and compaction.  Plan on jointing or sawing the slab to sections less than 12', so once down the middle and twice the other way.  Try to cut on the same day as the pour.  Cure appropriately (I would use Conspec 21).

Consider Visqueen or other moisture barrier under the slab .