Floor Slabs  

by Mark Postma, PE, Carl Walker, Inc.
Revised by the Chairs of the Building Enclosure Councils with assistance from Richard Keleher, AIA, CSI, LEED AP and Kenneth Roko, AIA of The Facade Group, LLC
Also assisting was Judd Peterson of Judd Allen Group



The base floor within a building may simply be a cast-in-place concrete slab-on-grade with limited design considerations for structural support or environmental control functions. The base floor may also be comprised of a mud or structural foundation slab complete with waterproofing and wearing slab with the overall system designed to carry structural hydrostatic pressure loads and maintain a controlled environment. Floor slabs are often the source of leakage into the building with slab cracking of common concrete materials being a primary cause. Issues of controlling soil gas emissions such as radon may also be of importance.

Because the cost penalty to correct a foundation or slab on grade waterproofing failure is either extraordinarily expensive (up to 7 times the initial cost of waterproofing) or practically impossible to correct at all after construction is complete, it is better to err on the side of caution on the initial installation. Approach critical areas that will later be buried in construction with extreme conservatism. The recommendation is to raise the quality of approach one degree more than the existing condition reports suggest, that is, use the higher quality material and detail it with additional reinforcements and belt and suspenders precautions applied at each level of risk perceived.


This section provides specific description of materials and systems common in floor slab systems. Descriptions and guidelines are provided in the following sections:

  • Finish Floor Coverings
  • Concrete Floor Slab
  • Aggregate Drainage Layers
  • Under Slab Vapor Retarder
  • Waterproofing Membrane
  • Protection Board
  • Prefabricated Drainage Layers

Finish Floor Coverings

Depending on the interior space the finish floor covering may be the exposed concrete surface itself or various floor coverings such as wood, vinyl floors or carpet. Many adhesives used in applying floor coverings are sensitive to moisture requiring the use of a waterproof system or lengthy drying times if a poly vapor retarder is used.

Concrete Floor Slab

In typical office environments, the concrete floor slab itself is comprised of 4" to 6" thick concrete reinforced with one layer of welded wire fabric at mid depth, unless below the water table, when hydrostatic heads may exert upward pressure, requiring stronger construction.

Under Slab Vapor Retarders or Waterproofing Membrane

Under slab vapor retarders may include polyethylene sheets, polyolefin sheets, high density bonded polyethylene, and asphalt/polyethylene composite sheets or polymer modified bitumen sheets. Poly sheets are usually 15 mils thick with taped seams, edges, and penetrations. Vapor retarders should be selected in accordance with ASTM E 1745 and E 1993 and installed and inspected in accordance with ASTM E 1643.

Where high water tables create contact with the slab on grade, it is necessary to waterproof the slab on grade to resist hydrostatic pressures. A mud slab can be utilized to facilitate the installation of vapor retarder membranes and waterproofing membranes. Mud slabs are usually a 2" to 3" unreinforced concrete slab with a float finish. They provide a flat surface for the membranes, which are then fully supported and much less likely to be punctured by subsequent construction activities.

Elevator pit waterproofing is always recommended regardless of soil conditions, as a precaution.

Capillary Break Layer

Capillary break layers under floor slabs are typically comprised of 6 to 8 inch thick layer of 3/4 inch granular material that is gap graded to increase drainage rates. The granular material serves as a capillary break and a place to "store" the water until it can be absorbed back into the surrounding soil.


Figure 3 contains an overall schematic that characterizes the four functions i.e. Structural Support, Environmental Control, Finish, and Distribution as they relate to the below grade enclosure element of floor slabs.

Floor Slab Schematic

Fig. 3. Floor Slab Schematic

The four function categories, i.e. Structural Support, Environmental Control, Finish, and Distribution, are expanded in general terms for floor slab systems.

Structural Support Functions—The floor slab of the below grade building enclosure must be designed to carry downward vertical gravity loadings as well as any upward soil or hydrostatic pressure loadings.

Downward vertical gravity loadings exist from the dead weight of the floor slab and any occupancy live loads. In many deeper structures the floor slab may also be a mat foundation slab carrying significant building column and wall loads.

Floor slabs may also resist upward soil or hydrostatic pressure loadings. Upward soil pressures may be applied to the floor slab in situations where it is acting as a matt foundation and the building point loads on the foundation results in an upward pressure on the floor slab.

In areas such as crawlspaces and unoccupied basement areas the structural support component involving a concrete slab may not be needed. In these areas, environmental control functions may still need to be addressed.

Environmental Control Functions—The exterior environment that the floor slab is subjected to includes environmental control loadings such as thermal, moisture, insects, and soil gas. The interior environment that the floor slab is subjected to includes environmental control loadings such as thermal and moisture. The performance of the floor slab system depends on its ability to control, regulate and/or moderate these environmental control loadings on the interior of the floor slab to desired levels.

As with foundation wall systems, the control of moisture is likely the most important environmental control function. Moisture control is dealt with in a drainage and barrier type of design approach. For cases with hydrostatic pressure from ground water levels the first phase of control of moisture can be accomplished through pumping and dewatering systems to artificially drive down the natural water table level. The second component of the moisture control system includes a granular aggregate capillary break layer below the floor slab to allow an area for moisture to accumulate and dissipate or to be pumped out or drained into an exit drain or sump system. In many floor slab situations with low water table elevations or dry conditions, the granular aggregate capillary break layer (with exit drain if required) will control the majority of the water. There may be no need for an active pumping system.

The key question that remains is whether to provide a waterproof membrane or a vapor retarder below the floor slab. A vapor retarder resists vapor migration in the absence of hydrostatic pressure. Waterproofing resists both vapor migration and hydrostatic pressure. Generally, a vapor retarder can only be eliminated on well drained sites with water tables well below the floor slab surface and the use of floor finishes unaffected by vapor migration. However, most building codes require a vapor retarder be installed between the granular drainage and the floor slab. This layer has the added benefit of minimizing shrinkage stresses and cracking in the floor slab due to the reduction in shrinkage restraint.

Waterproofing membranes are needed in situations with hydrostatic pressure or moisture sensitive interior environments. Waterproofing membranes are typically applied to a mud slab cast on a granular aggregate capillary break or applied to compacted earth. Protection of the waterproofing membrane from damage during construction is critical. Protection is typically provided with a protection board application directly to the waterproofing membrane soon after membrane installation. Detailing of waterproofing at all terminations and penetrations are critical. Top side waterproofing of floor slabs is not recommended for any situation.

Other environmental loading conditions may include soil gas such as radon. Migration of soil gas into interior environments can be controlled through the proper use and detailing of a polyethylene type of vapor retarder or a waterproofing membrane. Proper laps, protection during construction and attention to detailing at all terminations, edges, and penetrations are critical to fully control migration of soil gas.

Finish Functions—With floor systems the only finish of concern is to the interior space. This finish is dependent on the interior use whether it be a controlled office environment or a non-controlled parking environment. Typical finish systems may include carpet, tile or adhered flooring. The proper control of vapor migration loadings is critical with tile or adhered flooring applications that need proper adhesion. In some applications such as interior parking or storage space the interior finish is simply the interior surface of the concrete floor slab. In others, such as crawlspaces, the finish may be the vapor retarder.

Distribution Functions—The floor slab may contain distribution systems such as electrical feeders, electronic conduit, mechanical piping or heating systems.


There are two main types of base floor detailing that are distinguished by the requirements of the interior space and the exterior environment:

  • Base Floor Slab—Typical System
  • Base Floor Slab—Waterproof System

Base Floor Slab—Typical System

A typical base floor slab where the design criteria includes controlling water vapor transmission into the interior space but is not concerned about waterproofing the base floor due to hydrostatic pressure loads can be referred to as an imperfect barrier system. The components of the system include a well compacted yet well draining granular aggregate capillary break system placed directly on unexcavated, undisturbed ground. The granular aggregate capillary break system provides a collection area for moisture to accumulate and dissipate as well as a firm support for slab loadings. A vapor retarder (see Description, above) is placed between the granular drainage system and the concrete slab to minimize moisture vapor transmission or soil gas transmission into the occupied space. The concrete floor slab itself provides structural support for floor loads and suitable backup for floor coverings and finishes.

Base Floor Slab—Waterproof System

A typical base floor slab where the design criteria include controlling moisture migration and water vapor transmission into the interior space can be referred to as a waterproof system. The components of the system include a well compacted yet well draining granular aggregate capillary break system placed directly on unexcavated, undisturbed ground. The granular aggregate capillary break system provides a collection area for moisture to accumulate and dissipate as well as a firm support for slab loadings. To provide a solid base material on which to apply the waterproofing membrane a mud slab or compacted earth layer is provided. In some instances with significant hydrostatic pressure or to accommodate building loadings a matt foundation slab is used in lieu of the mud slab. The waterproofing is then applied directly to the mat foundation slab and protected with protection board. In this case a wearing floor slab is poured on top of the protected waterproofing system.

Below-Grade Penetrations and Edges

A general element that is common to all buildings yet frequently not fully detailed or addressed during design is penetrations and edges. These penetrations are any openings in floor slabs that provide an avenue of breech for moisture entry into the building. Sewer pipe penetrations, water line entry penetrations, drain basins in the floor slab or sleeves for electrical, gas or communication are all common penetrations, typically with their own design or detailed features. These features, however, leave much to be desired with respect to sealing and waterproofing. Penetrations can also become quite exotic such as steam penetrations or other features that require special treatment. The edges of slabs need to be made vapor-tight/watertight, too.

When rising water tables frequently contact the bottom of the slab on grade, it may be necessary to consider installing a drain tile system of either parallel, perforated, drain tile pipes or a grid of such piping to drain off the rising water and maintain the water table below the slab on grade, by pumping the drain tile sump away from the building.

Isolation and Expansion Joints

Isolation joints accommodate minor movements between structural elements and/or fixtures that penetrate through or around them. Both a prime and a back-up seal are effective as a means of reducing leakage. Raising the slab profile also works well. As with expansion joints, the detailing of concrete gradients or slope at isolation joints to prevent direct accumulation of any transient moisture is also highly effective. The same rules concerning drainage grid material or a flow path continuation from joints to drain basins should be considered during the design process.

A common ground rule applicable to keeping joint sealant systems leak free is to be certain that the moisture evacuation or drainage systems are properly in place and connected to the sub grade layers. Eliminating the possibility of a build-up of water head against all joint seal systems is considered the main function of sub-drain systems.

Mechanical Floor Drains and Pump Systems

Floor drains in floor slabs require treatment by proper design for back flow valves or special treatment for flow capacity depending upon the use of the structure. Where sump pumps are installed special back water valves or back pressure valves are needed to prevent flow back. The application or installation of pump assemblies and certain sumps requires proper coordination and effective treatment of the discharge system to avoid leakage through mechanical penetrations.


The following details can be downloaded in DWG format or viewed online in DWF™ (Design Web Format™) or Adobe Acrobat PDF by clicking on the appropriate format to the right of the drawing title.

The details associated with this section of the BEDG on the WBDG were developed by committee and are intended solely as a means to illustrate general design and construction concepts only. Appropriate use and application of the concepts illustrated in these details will vary based on performance considerations and environmental conditions unique to each project and, therefore, do not represent the final opinion or recommendation of the author of each section or the committee members responsible for the development of the WBDG.

The details, graphics, and related information shown in the details are intended to illustrate basic design concepts and principles only and should be considered collectively with the appropriate narrative sections of the Whole Building design Guide (WBDG). The information contained therein is not intended for actual construction, and is subject to revision based on changes and or refinements in local, state, and national building codes, emerging building envelope technologies, and advancements in the research and understanding of building enclosure failure mechanisms.

Below Grade Slab—Waterproof System (Detail 1.3.2)   DWG |  DWF |  PDF

Emerging Issues

For emerging issues refer to General Overview section.

Relevant Codes and Standards


There are a large number of standards pertaining to roof systems. ASTM developed the majority of them. The ASTM standards typically pertain to test methods (laboratory and field) and product standards. However, there are a few design and application guides:

Additional Resources


Products and Systems

See appropriate sections under applicable guide specifications: Unified Facility Guide Specifications (UFGS), VA Guide Specifications, Federal Guide for Green Construction Specifications, MasterSpec®


For resources including texts, guides, and web pages refer to General Overview section.

NOTE: Photographs, figures, and drawings were provided by the original author unless otherwise noted.