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Basements


Basement Floors

Visible damage in the basement can be due to drainage problems on the exterior, failure of the foundation drain system, the movement of the floor or foundation, or any combination.


Basement floors can be slab-on-grade concrete, structural concrete, or structural wood.

Learn about the types of basement floors here.

Concrete Slab-on-Grade Basement Floors

The slab-on-grade basement floor is susceptible to movement and should be expected.  The issue becomes the magnitude of movement.  The amount of potential movement is estimated by geotechnical engineers and typically provided as information in the soils (geotechnical) report.  Even with proper framing voids, basement finishes such as drywall, trim, millwork, flooring, and fixtures are susceptible to slab-on-grade movement. 

Vertical slab movement of 1 to 3 inches is possible for slabs bearing on reconditioned soils with low to moderate swell potential.  In some cases, vertical movement may exceed this range.  Reconditioned soils are material that is removed and altered per engineering requirements to be more stable and placed back.  


A structural floor system is the best solution if movement and associated damage to floors and finishes cannot be tolerated.  Floor slabs should be separated from structural components to allow for the floor to move.  It is essential for isolation joints and control joints to be placed in accordance with applicable Building Codes and standards published by the American Concrete Institute (ACI).


Damage to basement slab-on-grade floors can be associated with soil behavior, foundation movement, defective construction methods, or a combination of these factors.

Width of framing void in basement wall partition measured by engineer

Framing Void

Interior non-bearing partitions and attached furnishings (such as cabinets and shower stalls) on concrete slabs should be constructed with a void so they do not transmit floor slab movement to the roof or overlying floor.  A void of at least 1-1/2 inches is recommended beneath non-bearing partitions.  Over time, the void may require reconstruction to re-establish the void due to vertical slab movement, and the interior finishes may also require repair.

In a finished basement, a framing void is often visible in the utility room if the utility room does not have drywall.

Common Basement Conditions

Visible damage in the basement can be due to drainage problems on the exterior, failure of the foundation drain system, the movement of the floor or foundation, or a combination.

 

Basement floors can be slab-on-grade concrete, structural concrete, or structural wood.  Types of basement floors are discussed here.

 

The most common basement floors are concrete slab-on-grade.

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DRYWALL CRACKS AT DOORS

Diagonal cracks at the corners of doors caused by upward floor movement can start small and increase in width and length over time.

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DOOR DAMAGE

Out-of-square doorframes and sticky doors in the basement are most likely caused by upward floor movement (heave) but can be due to the movement of the foundation also.  If there is foundation movement, cracks should be visible on the foundation walls if not covered with sheetrock.

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DRYWALL DAMAGE

Foundation and basement floor movement can manifest in all different types of drywall cracks and separations;  on the ceiling, walls, around doors, and windows.

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DAMAGE AROUND BASEMENT WINDOWS

Cracks and separations around basement windows can be caused by foundation or floor movement.

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FLOOR TILE DAMAGE - CRACKS

Floor tile is brittle and has difficulty accommodating even minor basement floor movement.  If some movement of the basement slab-on-grade floor is anticipated, a luxury grout-able vinyl tile may be an option for basement slab-on-grade floors.

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BATHROOM FIXTURES

Basement concrete slab-on-grade floor movement can be evident in the separation between the floor and plumbing fixtures like a pedestal sink or toilet.

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WALL TILE AND TRIM DAMAGE

The movement of a basement slab-on-grade floor can damage wall finishes and baseboards.

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MOISTURE PROBLEMS

Drainage problems, roof gutter downspouts, not properly functioning foundation drain, or water leaks can cause moisture problems in the basement.

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DAMAGE AROUND COLUMNS

Columns in the basement need to be properly isolated from the concrete slab-on-grade floor to avoid damage from movement.

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FLOOR HEAVE

Minor floor settlement or heave is common with basement concrete slab-on-grade floors.  It is important for the homeowner to refer to the geotechnical report for the predicted slab-on-grade movement before finishing the basement.

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FLOOR VERTICAL MOVEMENT

Basement concrete slab-on-grade floor movement can be evident in the separation between the floor and baseboard.

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CONCRETE FLOOR STRESS CRACKS

Minor stress cracks in a basement concrete slab-on-grade floor are common and do not necessarily mean that there is a problem.


However, homeowners should monitor all cracks for water seepage or staining, which could be signs of moisture problems.

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FLOOR JOINTS

Concrete cracks and moves – it's just a fact, and providing joints in the concrete surface is a method of directing cracks into locations where they are less visible. 


The problem arises if the crack continues to get wider or if there is significant vertical movement.

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