Residential Foundations: Concrete Slab Vs. Conventional Crawlspace

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By Andrew M. Dennis, P.E.

Whether to build a residence on a concrete slab or conventional crawlspace foundation is a common decision homeowners must make.  This decision can be made knowingly based on the merits of each foundation type or unknowingly by building or purchasing a property for reasons unrelated to the foundation type.

The design and construction of either a concrete slab or conventional crawlspace foundation should be tailored to the geography and climatic conditions that it is expected to perform under.  The choice of foundation type must take into consideration the properties of the soil under it and the loads placed on top of it.  Its long-term performance is greatly affected by the critical aspects of the presence of proper lot grading, adequate site surface water drainage, and a proper landscaping plan.

While concrete slab and conventional crawlspace foundations present about the same number of advantages and disadvantages as outlined below, the real value and benefit of either is ultimately determined by how well it has incorporated the critical performance aspects outlined above.

Concrete Slabs

Some residential concrete slabs are placed as monolithic slabs with integral footings.  These monolithic slab footings are thickened areas of the slab, usually located beneath bearing and exterior perimeter walls of the slab.  Another common form of slab-on-grade construction includes a ground-supported slab with perimeter masonry walls that are supported on spread footings placed below grade.

The lot on which a concrete slab is placed should be graded before foundation construction begins.  This will prevent damage to the finished foundation done by any heavy earthmoving equipment used in completing the site grading plan.  Lots must be graded to drain surface water away from the house slabs and walls.  Not all residential lots are developed in conjunction with engineered site grading plans; however, it is highly recommended that considerable thought be given to this often overlooked aspect of homeownership.  No matter what type of foundation is proposed, following a properly engineered grading plan will ensure that lot surface drainage is diverted away from the building site.

Concrete slabs and footings should be supported on undisturbed natural soils or engineered fills.  Any fill soils that support slabs or footings should be designed, constructed, and tested in accordance with accepted engineering practice.  A well-constructed slab should have a capillary break between it and the ground to prevent groundwater or moisture from wicking up through it.  This capillary break is usually built beneath the slab by using a layer of gravel in conjunction with a plastic vapor barrier.

Advantages of Concrete Slabs

After removing the topsoil, concrete slab foundations can be built on grade over undisturbed soil, if conditions allow.  Therefore, the required labor, excavation, and forming costs can be held to a minimum.  These benefits can translate into an earlier project completion or expedite an already late project schedule.  A slab is also the more preferred choice in regards to pests and other vermin being prevented from getting under the house.  A leaking toilet or shower pan will not rot a concrete slab floor.  A slab can also help insulate a house, saving money on  heating bills.  Houses built atop slabs are usually closer to grade and require only one or two steps at the exit or entry doors.  This can be most convenient for those with disabilities or who are in a wheelchair.

Disadvantages of Concrete Slabs

A slab limits access to any of the heating, ventilating, or air conditioning (HVAC) systems or ductwork typically installed beneath the floors of a house on a conventional crawlspace.  The need to repair any utilities in the slab, such as plumbing or electrical, typically requires slab removal, which is expensive.  Poor construction or finishing techniques can create  uneven spots and unlevel floors.  If the lot landscapes and drainage plan are not coordinated, tree roots seeking out water in times of drought can damage slabs.  Water or moisture can rise through any crack in the slab and can damage the floor finishes.  Concrete slab foundations can be considered cheap and negatively impact a house’s resale value.

Advantages of Conventional Crawlspace

If the crawlspace is of adequate size, the heating and air conditioning ductwork and plumbing can be installed in it and be easily maintained.  Any required repairs to these systems will be less expensive due to better accessibility.  A crawlspace elevates a house, making it less susceptible to termite damage.  Crawlspaces can be constructed on sloped lots using tiered concrete footings; however, the lot’s grading plan must provide adequate drainage to prevent damage from adverse surface water drainage.

Disadvantages of Conventional Crawlspace

Excess moisture in a crawlspace is a common occurrence and can have severe consequences if not addressed in a timely manner.  Stormwater and gutter overflow slopes toward the house and into the crawlspace and are sources of moisture that must be prevented or can damage the wood floor system above.  A sheet of plastic with seams overlapped and taped works well as a vapor barrier to prevent the upward transmission of ground moisture.  Adequate ventilation of the crawlspace must also be planned for and implemented in order to prevent the harmful effects that condensation can cause to the wood floor system above.