Article No: 212

2007-06-04 10:24:19
Tech Talk: Concrete Homes: Working with the Trades
By: Ed Sauter, CFA Technical Director


 
Part II:  This six-part series focuses on details of today’s concrete homes. Over the past two years, this column has provided considerable information on the general benefits of concrete homes, the various methods used for construction and the performance characteristics. We now focus on strengthening the understanding of the decisions, details and results that can affect the quality achieved in the above-grade concrete home industry delivered by removable forms (RCFs). Since strongly entering the market nearly a decade ago, this method of construction offers an ever-expanding variety of architectural and practical construction solutions for today’s homeowner and designer.

There are literally thousands of decisions to make when you plan a new home. Decisions ranging from the type of finish on the doorknobs to what type of system you will employ to construct the building shell. If you made the decision to build your home of concrete, there are just a few additional decisions to make. Notable among them is who your non-concrete subcontractors will be. You need to make sure that the electrician, the plumber, the carpenter, in particular the finish carpenters are “on board” with your decision to build with concrete. Placing wiring and plumbing in concrete presents a unique challenge to the “residential” subcontractor while attaching things to concrete walls is just a little different than nailing into wood studs or drywall. I’ll attempt to give you some advice in this article to make the transition to concrete just a little simpler. For purpose of this article I am assuming that you are building the entire shell of concrete, including the floor and roof deck, and that interior bearing walls will also be of concrete. Non-load-bearing walls will be of conventional construction. Constructing the exterior and bearing walls of concrete plus the decks will provide more than enough mass for temperature moderation as well as sound dampening, storm protection, and lower maintenance. Using “conventional” drywall on non-load-bearing walls means that the home can be more easily remodeled in the future if another owner decides they didn’t like your layout.

Preplanning is absolutely essential when building a concrete home. With conventional wood construction, some of the critical decisions, such as the final location of electrical outlets and switches, can be delayed until after the framing contractor is finished. No so with the concrete home since many of these items will be cast directly into the concrete wall.

Electrical
There are several options when determining the locations of outlets and switches, and where the wires that supply those devices will be placed. The first, simplest, and least costly approach is to avoid putting them in concrete walls if it is prudent to do so. In many instances, a switch or outlet will function just as well if it is moved around a corner to a conventional wall. This will not always be the case, however, and with building codes dictating the spacing and frequency of outlets, you will always have some outlets and switches in concrete walls. There are two options for concrete walls. The first is to place EMT conduit in the walls with pull wires during the forming operation. Plastic conduit with metal boxes seems to be the material of choice. The location of the conduit must be coordinated with the structural reinforcement, which is typically in the center of the wall. If the reinforcement occupies the center of the wall that means the conduit will be closer to the face. A good suggestion is to put the conduit closer to the outside face of the concrete. The concrete cover over the conduit will be thin and hairline cracks are a real possibility. While the cracks have no impact on the structural performance of the structure, they can cause consternation with the owner if they are visible. By placing the conduit toward the outside face of the wall (this assumes you are using a system with exterior insulation) the hairline crack will not be visible.

You will need to use deep junction boxes to push the conduit to the outer edge of the wall but that gives you more flexibility for connections within the box. The use of conduit is foreign to many residential electricians so you may have to contact a “commercial” contractor for your electrical wiring needs. In some jurisdictions, the forming crews can actually place the conduit in the wall leaving the actual running of wire to the electrician at a later date. This will save considerable time (and cost). A licensed electrician should be used for the installation and hook-up of the actual wiring. The conduits can terminate in junction boxes or be extended to areas in the attic or in frame walls for simpler connections.

Another option, which can add a delightful architectural feature to the house, is to run a decorative soffit around the perimeter of rooms. You can then terminate the conduit at the surface of the wall just below the ceiling and run wires around the perimeter of the room. The same soffit can also be used to run ductwork and to introduce indirect cove lighting or recessed can lighting. If you use 3/4-inch conduit you can use Romex for the wiring to avoid using conduit in areas other than the concrete walls and ceilings. It is more efficient structurally if the conduit runs vertically as opposed to horizontally since the walls are typically designed to span in the vertical direction.

The same approach will apply for ceiling lights and fixtures. The conduit for the wires should be installed with knowledge as to where the reinforcement will be placed within the ceiling. The direction of ceiling/floor conduit is also important. It should run in the direction of the span where possible. If that is not practical, coordinate its location with your structural engineer to minimize the impact on the structural system.

Since mistakes are costly and time-consuming to fix, it is a good idea to draw an elevation of each concrete wall showing exactly where the outlets and switches will be. This reduces the likelihood of missing or improperly positioning an embedded item. Junction boxes can be secured a number of ways. They can be wired to the reinforcing, the form ties. Another creative approach is to cover the box with a standard junction box cover and attach it directly to the form with a screw through the face of the form. This method allows positioning of the switch or outlet anywhere along the face of the wall instead of a location adjacent to a form tie or reinforcing rod.

What if the unforeseen happens? If the outlet or switch is in an exterior wall it is often simplest to run the wire through the back of the wall and extend it behind the insulation. Make sure you either replace the insulation or fill the gap in with expandable foam to avoid a cold spot.

There is a third option. It can be used exclusively or in concert with the other alternatives mentioned above. That option is the use of a product called Wiremold. This product is surface-applied over the concrete after the forms are stripped from the walls. There are several advantages to this approach in addition to the fact that it can be post installed and will not impact the structural design. The biggest advantage is that outlets can be placed at regular intervals along its length. There are at least three versions of the product. The traditional narrow raceway is the least expensive but they also make a “chair rail” and “baseboard” version of the product, which allows the molding to blend with, or be used instead of the traditional chair rail or baseboard.

Plumbing
Plumbing is another trade where a little planning can save a lot of headache and cost. While plumbing can be run in walls it is best, as with wiring, to avoid concrete walls where possible. Locating fixtures on interior non-load-bearing walls is the simplest solution for vents and supply lines.

If the plumbing must be in an exterior wall it makes the most sense to collect the piping in one or more locations and cover them with a chase constructed on the interior of the concrete wall. There is no concern with respect to freezing as is the case with frame walls because you will have at least 4 inches of concrete and foam insulation covering the concrete. If the pipes must be located in concrete walls, then it is most appropriate to provide a block-out for the portion of the wall where the pipes must be run. Because the diameter of plumbing is typically greater than that of conduit it will very likely impact the structural design if not oriented properly. Another reason for the isolation is to allow for slight movement between the concrete and the plumbing as the building expands and contracts in response to thermal change. This movement can crack pipes that are cast directly into concrete. If you must embed directly in the concrete, then plastic piping is your best alternative.

Floor systems offer different challenges. The best option for floors is to provide a block-out for the plumbing where it will penetrate the floor. A dropped ceiling in the space beneath a bathroom can easily hide piping until it is routed to a chase. The locations and dimension of floor block-outs must be considered in the structural design of the flooring system

Carpentry
The last area where the trades will be facing something different is that of carpentry, in particular the finish carpenter. Attaching items to concrete is different than nailing to a wood stud.

“Tapcon” or similar anchors work best for attaching wood members. If you make most of the attachments soon after the walls are stripped, the concrete will still be “green” and easier to attach. A hammer drill will make for much easier drilling of holes, even after the concrete has reached strength. Cabinets, wood trim, and just about anything else can be attached with this anchoring system. In the case of stained wood trim, where you want to minimize filling of holes, finish screws are your best alternative. You can drill the hole and insert a wood plug or masonry anchor for securing the screws.

Homeowner concerns
Most homeowners are at a loss when faced with concrete walls. “How will I hang my pictures?” is a common lament with homeowners. The answers, of course, are quite simple. One solution is to have the finish carpenters apply a picture railing. This is the same type of solution used in many of our museums and finer historic homes. In addition to providing a hanging space, a decorative picture molding at door head height or a foot or so below the ceiling can add a sense of class to any structure. If picture rails are not your solution then the “hammer drill” is the answer. This specialized drill is available at almost any tool store and drills the concrete much easier than a standard drill. A special bit is required due to the hardness of concrete. One of our more enterprising builders actually provides a hammer drill to owners at closing. If you need to move the picture (and fill the hole) it is actually much easier to repair and results in less wall damage than with standard drywall.
What if I want a new opening in a concrete wall? Specialty contractors can again provide the answer. Concrete saws, some of which are mounted on the wall for a straight cut, can make simple work out of cutting concrete walls and there is less patching after they are done. One caveat, however, is that an engineer must be consulted before cutting holes in concrete walls. Most openings that are less than 4 feet will not present a problem but wider openings may require some retrofitting such as a lintel angle—much like installing a header over a new opening in a frame wall.

Still have concerns?
Pre-planning and cooperative sub-contractors will ensure that your new concrete home runs smoothly. I have covered most of the common concerns when it comes to working with the trades typically involved in residential concrete construction. As subcontractors become more familiar with the concrete home, they will be more willing to tackle this new form of construction and prices will come down. Until that time, you might just have to look a little harder to find the right company to work with on your concrete home. If you have other questions, or some good solutions to commonly encountered problems, contact the Concrete Homes Council at (319) 895-0761 or e-mail info@concretehomescouncil.org.

Ed Sauter, esauter@cfawalls.org, is Executive Director of the Concrete Foundations Association and the Concrete Homes Council.