One of the most common questions a residential architect is asked is, “Would it be better to have a concrete slab or a wood framed floor with a crawl space?” When choosing what type of floor system will be best for a project, several factors need to be considered. Site topography, where the house will be located on the site, seasonal groundwater levels, the number of floor levels the home will have, the type of heat desired, and budget constraints all need to be weighed before making a decision.
A concrete slab on grade works well on relatively level sites with a first floor (or basement) level that will be relatively close to existing grade. Since fill will need to be imported into the house footprint to bring the grade up to the underside of the slab, floors that will be high off the ground are not logical candidates for a slab on grade floor. On the other hand, if your site has a high water table, is subject to flooding, or has surface water nearby, a slab on grade floor might be your best bet. Since a concrete slab is always poured over granular fill that can’t wick water upward, they typically work well on sites where a crawl space might be susceptible to flooding, provided the floor level is high enough to be above any potential surface runoff.
Concrete slabs are also the most efficient for hydronic radiant heating systems, as the thermal mass of the slab helps hold heat. Concrete slab floors do require that all subgrade utility rough ins be completed before the slab is poured and be accurately located. Retrofits for plumbing or mechanical changes later can be difficult and costly.
Wood framed floors with a crawlspace (or basement) below have the advantage of access to space that can be utilized for running utilities (plumbing, electrical, and ducts) and for storage. They also work well on sloping sites or where the floor level will be significantly above the existing grade. Wood framed floors require more labor and material to build, and have potential for squeaks, creaks, and vibration if they are not properly designed and constructed. A gypcrete overlay can add rigidity and a solid feel to a framed floor, but is only practical if it is used as part of a radiant heat system.
On wet sites or where flooding potential exists, crawl spaces and basements need to be carefully designed to reduce the potential for moisture problems. They also need to be properly ventilated or heated as if they were living space. The building code has special requirements for crawl spaces or basements that are below the 100 year flood elevation, and in some areas, homeowners insurance rates are significantly higher if a wood framed floor is used where the floor elevation is close to the flood plain elevation.
As a rule of thumb, the installed cost of a slab on grade vs. a wood framed floor with gypcrete are about the same. Installed costs are, however, subject to a lot of project specific variables that can make one system significantly more expensive for a particular application. Factors like existing soil conditions, fill requirements, hauling distances, and ease of executing the concrete pour can affect the costs of slab on grade. Similarly, local labor costs, required floor framing member sizes, and current lumber pricing will determine the cost of a framed floor system.
We are often asked if finish floor options will be limited if one floor system is chosen over the other. The short answer is “yes”, but not significantly. Some wood flooring options don’t perform as well on concrete slabs, and likewise for some hard flooring options on wood framed floors. In general, however, most flooring choices will work on either type of sub-floor and shouldn’t be a major determinant in which system you choose.
At Hendricks Architecture, we specialize in designing western mountain style lodge homes. We design homes with both concrete slabs and wood framed floors.
Tom Russell, Architect, LEED AP
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