slab grade

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Building Envelope Design Guide - Floor Slabs Mark Postma, PE 06-01-2009Introduction

The base floor within a building may simply be a cast-in-place concrete slab-on-grade with limited designconsiderations 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 theoverall 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 alsobe of importance.Description

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 CoveringsDepending 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 floorcoverings 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 concretereinforced with one layer of welded wire fabric at mid depth.

Aggregate Drainage Layers

Drainage layers under floor slabs are typically comprised of 6 to 8 inch thick layer of ¾ inch granularmaterial that is gap graded to increase drainage rates.

Under Slab Vapor RetardersUnder slab vapor retarders may include polyethylene sheets, asphalt/polyethylene composite sheets or

polymer modified bitumen sheets. The most common used vapor retarder is 2 layers of 6 milpolyethylene sheets. Two layers are used to reduce puncturing and provide redundancy at seams.Fundamentals

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.

Fig. 3. Floor Slab Schematic

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

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





Downward vertical gravity loadings exist from the floor slabs dead weight and any occupancy live loads.In many deeper structures the floor slab may also be a matt 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 loadson the foundation results in an upward pressure on the floor slab.

In areas such as crawlspaces and unoccupied basement areas the structural support component involvinga 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 includesenvironmental control loadings such as thermal, moisture, insects, and soil gas. The interior environmentthat 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 moderatethese 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. Forcases 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 nature water table

level. The second component of the moisture control system includes a granular drainage layer below thefloor 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 dryconditions, the granular drainage layer and exit drain 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 floorfinishes unaffected by vapor migration. However, most building codes require a vapor retarder as a

minimum amount of moisture protection. In these cases, the vapor retarder should consist of a doublelayer of 6 mil polyethylene 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 inshrinkage restraint.

Waterproofing membranes are needed in situations with hydrostatic pressure or moisture sensitiveinterior environments. Waterproofing membranes are typically applied to a mud slab cast on a granular

drainage layer or applied to compacted earth. Protection of the waterproofing membrane from damageduring 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 allterminations 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 todetailing at all terminations 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 isdependent 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 surfaceof 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.Applications

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

• Base Floor Slab—Typical System

• Base Floor Slab—Waterproof System

Base Floor Slab—Typical SystemA 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 wellcompacted yet well draining granular drainage system placed directly on unexcavated, undisturbed

ground. The granular drainage system provides a collection area for moisture to accumulate and dissipateas well as a firm support for slab loadings. A 2 layer - 6 mil polyethylene vapor retarder is placed between

the granular drainage system and the concrete slab to minimize moisture vapor transmission or soil gastransmission into the occupied space. The concrete floor slab itself provides structural support for floor

loads and suitable backup for floor coverings and finishes. With interior space use of parking there is noneed for the polyethylene vapor retarder unless desired for minimizing cracking and curling from concrete

slab shrinkage.

Base Floor Slab—Waterproof SystemA typical base floor slab where the design criteria include controlling moisture migration and water vaportransmission 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 drainage system placed directly onunexcavated, undisturbed ground. The granular drainage 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 someinstances 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 matt foundation slab

and protected with protection board. In this case a wearing floor slab is poured on top of the protectedwaterproofing system.

Below-Grade PenetrationsA general element that is common to all buildings yet frequently not fully detailed or addressed during

design is penetrations. These penetrations are any openings in floor slab that once waterproofed providean 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 commonpenetrations, 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 assteam penetrations or other features that require special treatment.

Isolation and Expansion JointsIsolation joints do 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 reducingleakage. 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 alsohighly 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.The joint seal systems, although the primary line of defense can provide effective seal against any minor

or surface water migration. Eliminating the possibility of a build-up of water head against all joint sealsystems 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 forflow 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.

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