Using concrete makes for a great patio. Concrete takes irregularity out of the grade and produces an interesting and easy-to-maintain surface. It can be tooled as a surface drain to distribute water away from the home. It is relatively inexpensive and can be formed and poured in a weekend. And, a concrete slab can last a lifetime.
Unfortunately, concrete is brittle and sometimes cracks, becoming a tripping hazard and an eyesore. Even a cracked, ugly concrete slab can be useful. Filling cracks with a high-quality polyurethane caulking compound won't necessarily make it look better, but it can help to prevent further heaving and will definitely aid in proper watershed. Replacement is an alternative you might not want to consider. Breaking out a large concrete patio can take days and is grueling work. Also consider the cost of the jackhammer, concrete saw, dump fees and replacement cost. There is an alternative you might not have thought of leave that ugly old patio right where it is and use it as a foundation for a beautiful new wood deck.
All you have to do is attach strips of wood (support strips, nailing strips, furring strips call them what you like) to the concrete and apply the decking to the strips of wood. There is a condition regarding this procedure. A step down must exist between the floor of your home and your patio. A distance of 6 inches or more is plenty of room to do what we suggest. It is important for the top of the finished deck to be slightly lower than the floor inside your home. An inch or so is all that is needed. This precaution will do much to keep leaves and wash-water spray outside; away from your fine carpet, vinyl, hardwood or other flooring finish.
The heaving concrete patio probably won't stop shifting, but chances are the problem won't telescope through to the surface of the new wood deck. Wood is substantially more flexible than concrete. Where even the slightest ground movement can sometimes show as a radical variation in concrete, wood simply stretches, bends and flexes.
The strips of wood that are attached to the concrete should be either a pressure-treated species, or cedar or redwood. In our experience, pressure-treated material seems to last the longest when in regular contact with moisture. There are a couple of alternatives when it comes to affixing the strips to the concrete: powder-actuated pins, and lag bolts and shields.
In the case of the latter, holes are drilled through the wood strips and into the concrete at 2 to 4-foot intervals along each board. Lag shields are driven into the holes in the concrete. The lag bolts traverse through the holes in the wood strips and into the lag shields in the concrete. As a lag bolt is screwed in, the shield spreads, holding both the bolt and the wood strip tightly in place. For this you will need a high-quality carbide-tipped concrete drill, a good drill motor and elbow grease.
Using powder-actuated pins is easier, but does require some special skills. Here, a stud gun is used to fire special nails through wood and into concrete. For a medium-size project, all the pins necessary can be driven in less time than it takes to drill two or three holes for lag shields. The problem with powder-actuated pins lies within the "gun" part. Stud guns are dangerous and need to be handled with caution. Have the person at the rental store provide a thorough briefing on how the tool is used. You should also experience using the tool to drive one or two pins before leaving the rental store. If the proprietor won't agree to this condition, go elsewhere.
Placement of the support strips is important. They should be parallel and the space between them equidistant. Two-by-four deck boards can span a distance of about 2 feet, whereas 2 by 6 deck boards work well up to as much as 4 feet. The choice is yours. We like the 2 by 6 size because fewer boards are required to cover the surface. This means easier logistics, fewer attachments per given area, and best of all, fewer support strips are required.
The support strips should be placed so as not to impede drainage. You don't want to build a dam. Ponding water can prematurely damage the support system, not to mention that still ponds breed mosquitoes. The deck boards are laid perpendicular to the support strips and can be attached with nails or the deckmaster-style fastener that provides an invisible connection. Finally, when attaching the deck boards, it is important to stagger the joints at every course. Staggering joints adds strength and looks better.
James Carey and Morris Carey are nationally recognized experts on home building and renovation.