Jon Montelin, Locust Grove, GA
I plan to build a porch with a 14-ft. by 18-ft. concrete floor. The porch will have a concrete-block foundation supporting corrugated-steel decking covered with 4 in. of concrete. What gauge should the corrugated decking be, and should there be an intermediate support for the steel deck?
John La Torre Jr., a general building contractor in Tuolumne, California, replies:
Steel decking (also called pan decking) has deep ribs to provide rigidity to support concrete during the pour. These ribs also strengthen the cured concrete. The steel panels, typically 3 ft. wide, must be spot-welded together. I buy decking from Verco Manufacturing (602-272-1347; www.vercodeck.com
), and their catalog includes span and load charts.
To span 14 ft., you can use 16-ga. galvanized decking with 3-in.-tall ribs and 3 in. of concrete above the ribs, for a total slab depth of 6 in. Temporary shoring would be required during the pour. If you include a permanent center support, the span will be only 7 ft., and you can use lighter 22-ga. decking with 2-in.-tall ribs and 2 in. of concrete above the ribs.
The concrete must be reinforced with a minimum 6-6-10 welded wire mesh. I prefer to use 1/2-in. rebar because it is easier to suspend in the center of the concrete above the ribs. The rebar can be bent into the supporting block wall to reinforce the joint.
If you want a waterproof porch floor, you can spray the steel deck with Liquid Rubber (EPDM Coatings LLC; 866- 311-3736; www.liquid-roof.com) before pouring the concrete.
Although you can design a concrete deck using the charts in a manufacturer’s catalog, I recommend that you hire an engineer. If you’re not familiar with this kind of construction, a simple mistake (such as an insufficient connection to the house) can lead to disaster.