Walk through a few unfinished basements along the Front Range and you may spot something odd: a wall that stops a little short of the joists above it, with long nails bridging the gap. That gap is on purpose. It is called a floating wall, and in a lot of Colorado basements it is the most important detail in the framing.
Here is why basements here move, how a floating wall handles it, and what to look for before the drywall goes up.
Why Colorado basement floors move
Many Colorado homes are built on clay soils that swell when they get wet and shrink when they dry out. The Colorado Geological Survey calls expansive soil “Colorado’s most significant geologic hazard” and notes that most of the homes in the state sit in areas of potentially expansive clay. According to the Survey, these soils can expand up to 20% by volume when exposed to water and push with a force of up to 30,000 pounds per square foot.
Builders on expansive soils treat the structure and the basement floor differently. The Survey’s guidance on swelling soils says bearing walls should be founded like the exterior walls, while the floor is left free to move. In many Colorado homes the basement slab is built as a “floating” slab, which means it is separated from the foundation walls, columns, stairs and utilities so it can rise and fall without damaging the structure. A Colorado Geological Survey publication on building over swelling soils describes it this way and adds that some cracking of the floor slab can be anticipated.
A slab that is built to move is good news for the house. It becomes a problem only when someone finishes the basement and ties new walls rigidly to that slab and to the floor joists above it. When the slab heaves, a wall locked at both ends has nowhere to go. It pushes up on the floor above, and you see it as cracked drywall, doors that stick, and gaps opening upstairs.
How a floating wall works
The idea is simple: one part of the wall is fixed, the other part can slide, and there is a planned gap between them.
The idea is simple: one part of the wall is fixed, the other part can slide, and there is a planned gap between them.
In many Front Range basements the wall sits on the slab and slides up into a gap at the top. One plate is fastened to the floor joists, the wall’s own top plate sits below it, and long spikes or nails run through both so the wall stays straight while the slab moves it up and down. The Colorado Geological Survey’s guidance describes the other common version, where a non-load-bearing wall hangs from the ceiling with a void of several inches between the wall and the floor slab. Either way, the slab can move without lifting the house.
How big the gap should be is not a number to guess. It comes from your plans, from a soils or engineering report if your home has one, or from your local building department. If you’re not sure which applies to your house, ask before framing starts, not after. Our post on getting a permit for a basement finish covers who to call.
Two details matter at the bottom of the wall too. Wood that sits on concrete is usually pressure-treated, because a slab can wick moisture. And the fasteners have to suit the design: a wall meant to slide cannot be nailed solid at both ends.
Everything else in the basement has to float too
A floating wall only works if nothing else ties the slab to the house. The Survey’s guidance says homeowners and contractors finishing an existing basement should follow the same precautions a builder does. Before the walls get covered, check these five things:
- The gap is clear. No shims, blocking, or drywall screws should bridge it. Drywall on a floating wall should be fastened to the wall, never to the fixed plate or the ceiling framing above it.
- Doors have room to move. Door frames need head space above them, the same as the walls around them. Otherwise the slab pushes the frame into the header and the door binds.
- Pipes can flex. Water, sewer and gas lines that pass through the slab or a wall should be able to absorb movement. The Survey mentions flexible pipe, U-shaped connectors, and rubber gaskets and collars.
- The furnace connection can flex. The Survey suggests a flexible connection between a basement furnace and its ductwork, so a moving floor doesn’t pull on the ducts.
- Stairs and trim are isolated. Stairs are on the Survey’s list of things a floating slab should be separated from. At the floor, baseboard can be attached to the wall or to the floor but not both, and the gap gets covered with trim instead of being filled.
None of this is exotic. It is the kind of detail that’s easy to do while the framing is open and expensive to fix once the drywall is painted.
Does every basement need floating walls?
Not every house sits on the same soil, and not every basement slab is built the same way. Some homes have a structural floor instead of a slab on the ground, and some building departments ask for specific details. What your basement needs depends on the house and on the department that issues your permit.
A good starting point is to find out what you have. Look for a soils report or builder paperwork from when the house was built, check whether the existing slab is separated from the foundation walls, and ask your building department what it expects for a basement finish at your address. A framer who works in the area regularly should ask the same questions before quoting.
If you’re still sorting out the layout itself, our guide on what to decide before the framers arrive walks through room sizes, soffits and door locations.
The short version
Colorado’s soils move, and basement slabs here are often built to move with them. A floating wall lets your new rooms ride along without pushing on the house above. When you hire someone for basement framing, ask how they handle the gap, the doors, the pipes and the stairs, and walk the framing with them before anything gets covered.
Sources
- Colorado Geological Survey, Expansive Soil and Rock
- Colorado Geological Survey, Special Publication 11, Home Construction on Shrinking and Swelling Soils (Holtz and Hart)
