Decking Over A Slab – On the House
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Decking Over A Slab

By on November 9, 2015

Concrete makes a great patio. It takes irregularity out of the grade, produces an interesting and easy to maintain surface, it can be tooled as a surface-drain to distribute water away from the home in any fashion desired, it is relatively inexpensive and can be formed and poured in a weekend. Better yet, a concrete slab can last a lifetime. Unfortunately, concrete is brittle and sometimes cracks, becoming a trip hazard and an eyesore. The good side is that even a cracked, ugly concrete slab can still 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 may not want to consider. Breaking out a large concrete patio can take days and is grueling work at best. Not to mention the cost of the jackhammer, the concrete saw and the dump fees. Oh, and don’t forget replacement cost. But there is an alternative that you may not have considered. Why not leave that ugly old patio right where it is and use it as a foundation for a beautiful new wood deck?  Who was it that said, “Don’t throw the baby out with the bathwater?”

This is a really cool one and it is easy to do. All you have to do is attach strips of wood (support strips, nailing strips, furr 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 six 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 – and away from your fine carpet, vinyl, hardwood or other flooring finish.

No, 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. Keep in mind that 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 specie, 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: 1) powder actuated pins, and 2) 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. A lag shield is driven into of 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 task you will need a high quality carbide tipped concrete drill, a good drill motor and lots of elbow grease. Drilling holes in concrete isn’t rocket science, but it is physically tasking. Be prepared to work hard here.

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. The process is not physically tasking, is relatively inexpensive and can be done in a short period of time. In fact, for a medium size project all of the pins necessary can be driven in less time than it takes to drill 2 or 3 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 – as you would handle any gun. You will want the person at the rental store to provide a through briefing on how the tool is used. You should also experience actually using the tool to drive one or two pins before leaving the rental establishment. If the store proprietor won’t agree to this condition go to a different store.

Placement of the support strips is important. They should be parallel and the space between them should be equidistant. 2×4 deck boards can span a distance of about 2-feet where 2×6 deck boards work well up to as much as 4-feet. The choice is yours. We like the 2×6 size because fewer boards are required to cover the surface. This equates to easier logistics, fewer attachments per a given area – and best of all – fewer support strips are required.

The support strips should be placed as not to impede drainage. Basically, you don’t want to build a dam. Ponding water can prematurely damage the support system not to mention the fact the still ponds breed mosquitos. 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 keep in mind that it is important to stagger the joints at every course. Staggering joints adds strength and looks better.

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