If you are a foundation contractor, one way to secure more work in this down economy, and into the future, is to do more work on the contracts you have. Your men are on the job, your equipment is there, the concrete pump is there—but you only cast the foundation walls and footings. Some of you have made the jump to abovegrade concrete housing and most of you have at least contemplated it.
If you aren't quite ready to jump in with both feet, why not go part way—cast the deck (the main floor of the house) in concrete. You might not be able to increase the number of your projects this year but you can increase your gross sales. One contractor who has built several decks said concrete decks cost around 30% more than a traditional wood deck. So why would an owner spend more to get a deck and what is in it for you?
Why include a deck
There are many good reasons for owners to consider concrete decks.
- Safe room. Clients get the ultimate safe room—a basement with 6 to 8 inches of concrete on three or four sides and 6 to 8 inches above. A room with those specifications can resist just about anything Mother Nature can throw at it.
- Noise reduction. Concrete floors are quiet. Six inches of concrete has tremendous sound absorptive qualities at all spectrums of the audible scale.
- Fire protection. A fire that starts in the basement (say, the furnace room) will take much longer to spread and can easily be contained if there is a horizontal concrete layer separating the fire from the balance of the habitable space.
- The elimination of mold and varmint damage. Mold needs three things to grow: mold spores, moisture, and a food source. Concrete is not a food source for mold growth like wood or drywall, so this is not a concern. As for wood eaters such as termites, mice, and carpenter ants—if you find one that eats concrete then we've got much bigger problems to worry about.
- Thinner floors. A 6-inch concrete deck can span up to 20 feet without intermediate support. It would take at least a premium grade (Douglas Fir-Larch #1) 2x10 spaced 12 inches o.c. to span that distance—and wood won't provide any of the other qualities discussed. The continuous connection between a concrete deck and a standard 8-inch foundation wall produces a strong structural detail at the wall/ floor juncture, improving the structural efficiency and performance of the floor system.
- Radiant floor heating. In-floor heating is the ultimate in comfort and efficiency. Concrete floor systems are the best material to use with radiant heating systems.
- Decorative concrete floors. Stains, stamps, diamond polishing, and a host of other concrete finishes will add durability, beauty, and functionality to your home.
- Energy efficiency. The most efficient method of casting foundation walls and decks is in a single-placement operation. The resulting monolithic cast will provide the most airtight, structurally integral basement wall/ deck available on the market with little, if any, material waste.
Decks benefit contractors
What's in it for you, the contractor, in addition to more work and money from the same project?
- Top restraint. You will get the best support possible for the top of the wall so there won't be any more callbacks because an excavator pushed in a wall during the backfilling operation.
- Anchorage to deck. You won't have to be concerned about anchor bolts giving way during backfilling.
- End wall bracing. There won't be any more issues about the possible lack of “required” bracing for the joists parallel to the end walls.
- Performance. There are fewer problems with cracks because the top of the wall is not only fixed, it gives greater load capacity to the wall itself.
- Floor diaphragm. If you are building walkout basements, there is always concern because there is no soil opposite the backfilled wall to resist the force of the soil. With a concrete deck, you have the ultimate diaphragm, which can transfer load to the transverse or perpendicular walls.
The ACI 332, Residential Concrete Work Committee is contemplating the inclusion of decks in future versions of the standard. Empirical tables will likely be part of the output of the committee reducing or eliminating the need for an engineer on many decks, or if an engineer's approval is required, the cost can be substantially reduced.
What do you need to begin pouring decks as well as walls? Fortunately, the form manufacturers have anticipated this need and have all of the accessories and tools required to form, brace, and support the decks. Decks typically are cast at the same time as the walls, which makes casting a one-step operation. The two major needs are a ledge or support to form the transition from the wall to the deck and to support the deck form. The second is the temporary intermediate beam and shoring system to support deck form panels.
How does it change your operation? You will spend more time on the job (therefore more compensation) and you will develop crews who can be as efficient with wall/deck combinations as they are with wall/footings (relative to cost or man-hours).
One aspect of building concrete decks that is more important, compared to building with wood, is planning. With wood, you simply drill and cut openings where required for plumbing chases and vents. Wiring is installed after the wood deck is already in place.
Although it is possible to drill and saw concrete after placement, it makes more sense to use blockouts and simply eliminate the concrete from locations where stairs, ducts, and chases are required. The location and size of these elements should be included in the plans. Doing this saves time and cost. This planning also helps engineers know where additional reinforcement may be required at the openings.
Wiring can be installed in surface mounted “Wiremold” or similar products, but it is more efficient and aesthetically pleasing to have conduit in the slab instead. The location of lights, outlets, and switches can be easily predetermined. When in doubt, install an extra conduit. Depending on your local codes, the concrete contractor may be able to install the conduit with a licensed electrician making all of the connections.
Steel reinforcement in a structural slab is a critical component to its performance. Depending on whether or not there is an intermediate beam, the reinforcement may be near the bottom or top of the slab. Drawings and specifications prepared by an engineer will indicate the proper size, location, spacing, and cover for the reinforcement.
When spans are longer than 20 feet, there are several options. First, you can thicken the slab. An additional 2 inches of concrete with the appropriate amount of reinforcement will allow you to increase the span by several feet. A second option is an intermediate, integral concrete beam. You also could use a steel beam but a concrete beam can be cast integral with the floor and wall pour. The beam can be supported with steel columns spaced appropriately to the beam depth and loading conditions.
Another option is an intermediate concrete bearing wall, cast monolithically with the exterior walls and deck. Post-tensioning is an option if experts are available in your area. A down economy is a good time to entice specialty contractors who otherwise might not consider residential construction.
Flat slab systems are not the only option available for doing concrete decks. Another innovation for residential construction is a one-way or ribbed slab consisting of a concrete beam spanning one direction, cast integrally with the floor. The advantage of this type of system is that you can span much greater distances. The total depth of the floor system will be greater but the amount of concrete used will be less for greater spans. One system uses sliding pans that can be varied depending on the span length.
Heating and cooling
Another advantage of your concrete home will be less dynamic temperature gradients. The thermal mass inherent in the concrete will absorb heat and once it reaches a given temp, it takes much longer for concrete to change temperature. Although it takes less heating and cooling energy to maintain temperature, space conditioning still is required.
Radiant heating is considered the ultimate heating system by many and with an in-floor radiant heating system you attain this goal without the cumbersome radiators typical in traditional hot-water or steam systems. In-floor heating systems are simple to install in concrete decks. They are cast into the deck in conjunction with the reinforcement and electrical conduit. Preplanning is again important as you cannot attach plates or cut openings where the piping has been placed. A warmed slab can be a real comfort on cold winter days.
Air conditioning can be accomplished using a traditional duct distribution system or with a high-velocity duct system that uses much smaller ducts. Dehumidification is typically the No. 1 need for cooling in a home with concrete decks or walls.
You have several options when it comes to finishing concrete floors. You can carpet the floor, but you will greatly reduce its ability to improve energy performance by isolating the thermal mass of the floor from the habitable space. Carpet also increases the possibility of mold, mildew, and other undesirable things that reside in the piling. If you must use carpet, consider using area rugs, which can be removed, cleaned, and varied as your tastes change.
Placing wood over concrete also reduces thermal performance but not to the same degree as carpet. Tile of any kind applied directly over the concrete is the best way to introduce more variety in appearance for entries, kitchens, and baths while maintaining the mass storage potential.
Decorative concrete has made great strides over the past decade. It has evolved from a curiosity to an art form with contractors specializing in the craft. There are dozens of systems producing an unlimited number of patterns, colors, and textures. They result in a low maintenance, durable, and unique finish that will serve you well for years to come.
The case for considering concrete decks is compelling. Yes, it will cost a little more but as the saying goes, you get what you pay for. In the case of concrete decks, you get a lot more than what you pay for.
Footnote: The Concrete Foundations Association (CFA) Summer Convention will feature a session by Brent Anderson, Brent Anderson Associates, Minneapolis, on designing and constructing concrete decks. Contact Ed Sauter, executive director of the CFA atwww.cfawalls.orgfor additional details.