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Do I Really Need a Concrete Basement Floor?

A reader turning a partial basement into living space will have to remove the slab that's already there. Does he have to replace it with more concrete?

Posted on May 5 2014 by Scott Gibson

Rob Rosen is diving into a basement remodel, a job that involves digging out and removing a concrete slab to provide more headroom so the basement can be turned into usable living space.

He'll reinforce the footing and foundation as needed, but when it comes time to build a new floor for the basement, Rosen wonders whether he can go with something other than a concrete slab.

"I would love input from someone who has put a vapor/water barrier, some foam insulation between 2x4 sleepers and then used some form of wood flooring that would work for higher humidity situations," Rosen writes in a Q&A post at Green Building Advisor.

There's nothing that Rosen can find in the Washington State building code that would prevent him from trying it.

"Concrete is soooo not environmentally correct if not necessary," he writes.

Can Rosen swap wood for concrete and still get a durable, structurally sound basement floor? That's the issue in this Q&A Spotlight.

Stopping the migration of moisture from the ground is key

Keeping ground moisture out of the basement is important, but it is the rigid foam insulation and the vapor barrier that actually do the work, not the concrete, says senior editor Martin Holladay. In general, Holladay writes, the rigid foam goes down first, followed by the vapor barrier. "Above the polyethylene, the finish materials are up to you," he says, "and and your local code inspector, of course." [Editor's note: As pointed out by Mark F. in Comment #5 below, a basement slab often has a structural function: It prevents the foundation walls from being forced inward by soil pressure. GBA readers should consult an engineer before omitting a basement slab.]

In terms of durability and cost, Holladay favors concrete, but he adds: "If you are willing to replace any components that rot, or if you don't mind introducing pressure-treated lumber (with associated chemicals) into the interior of your house, go ahead and experiment. If these materials rot in the future, you'll need to replace them. You'll have to consider the wooden components as sacrificial layers."

Agreed, writes Howard Gentler. Although a concrete slab would do a good job of supporting any lumber above it, it won't block any moisture by itself and Rosen might well save both money and effort by using wood instead. "I think you can do what you are considering," he says.

Rosen replies that he was considering adding "ribbons" of concrete on 8-ft. centers to support 2x floor framing. "The 2xs would be supported by the earth and the concrete in case things shifted," he says. "I don't believe that I can use pressure-treated wood on interior living space[s], and don't really know that code guys would be happy with this setup... only because it's atypical."

Use the right lumber

If Rosen uses pressure-treated material as part of the assembly, Flitch Plate says, it should be stamped for ground contact, and it should be allowed to dry before it's installed.

Plate refers Rosen to specifications for treated lumber published by Wolmanized. Although lumber treated with chromated copper arsenate (CCA) has been banned for most residential applications, it's still permitted for permanent wood foundations. The specs indicate that it should have a chemical retention level of 0.60 pounds per cubic foot (pcf). In contrast, above-grade pressure-treated lumber would need a retention level of only 0.25 pcf.

"That is a lot of copper, by weight, making it heavy and dense — a real bear on saw blades," Plate writes. "Use only stainless-steel fasteners; preferably hex heads and star drives. Fasteners are expensive. Galvanized is not reliable, and who wants to use a hammer anyway, these days."

For the vapor barrier, Plate suggests a cross-linked polyethylene material called Tu-Tuff.

Pressure-treated material is expensive, Gentler adds, and in this case it may not be necessary at all.

"I don't think you are planning for the wood to be in actual ground contact, just near it, but above layers that will be excluding most moisture," Gentler says. "It sounds like you will be using concrete piers for support of the wood. Make sure something non-permeable is between the wood and concrete, since the concrete wicks moisture to the wood."

Trouble ahead with resale?

One thing to consider, advises Robert Hronek, is whether a concrete-less basement would affect the resale value of the house. "You will be making a big investment, but I would be concerned about the resale value and the marketability of the home," Hronke says. "You might call an appraiser, a real estate agent and, if you have a good relationship, a lender."

If a lender balked at offering a loan, Rosen wouldn't be able to sell the house. "Personally, I would walk out of the house and not make an offer," he says. "Ask the [real estate] agent how hard it would be to sell. Ask the appraiser how hard it would be to appraise."

Plate agrees that resale and mortgage issues are a "different and valid concern." But while pressure-treated material has earned a "bad rap," basements are fully code compliant if they are built to engineering and materials standards.

Exactly, adds Donald Endsley. "The use of concrete is mainly because it can handle bulk water, and water vapor issues, IMHO," Endsley says. "As long as those are 100% taken care of I see no reason wood won't work."

Endsley's grandfather, in fact, built a house in Florida with a wood foundation. "Parts of that foundation were still there when he sold the place," he says. "It lasted 90 some years, but only because that soil was extremely well drained. The parts that had to be replaced were due to adding plumbing to the house and having it subsequently leak."

If galvanized steel is OK, why not pressure-treated lumber?

Jack Woolfe offers a link to Polycore Canada, which makes a basement flooring system that can be used in place of concrete. Factory-made sections of galvanized steel and expanded polystyrene insulation have an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of 18, according to the company's web site. With no large equipment and no specialized tools, a crew of three could install a 1,500-square-foot floor in a single day.

"Polycore Canada makes a floor system similar to what you've described, except they use galvanized sheet-steel sleepers instead of pressure-treated wood," says Woolfe. "I suspect PT wood would work okay too."

Our expert's opinion

Here's what GBA technical director Peter Yost had to say:

I think the default basement floor is concrete because it’s a self-leveling material with really high compressive strength and it does not care about being wet. I am not saying it has great moisture-managing properties, but as an inorganic material, it can wet and dry repeatedly without compromise to its other properties (contraction/expansion, compressive strength, etc.).

Any floor assembly can be set up to handle soil moisture and soil gases (such as radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles.). Basement floors can get wet from soil moisture or an internal leak, so I would not install any basement floor system that is inherently more moisture-sensitive without having a lot of confidence in historic high water table information, foundation perimeter drainage system, and leak protection for hard-piped appliances: clothes washer, dishwasher, and ice-makers.

Having done more than one basement retrofit (including my own home) involving casting a concrete slab, neither the expense nor the difficulty were issues I considered, mainly because other framed systems seemed just as much if not more cost and work.

On the other hand, assuming equivalent strategies for managing moisture and radon, the Polycore Canada system looks pretty slick. There is no reason that any framed system won’t work, so long as it sees the same or similar conditions as other below-grade framed systems: protection from liquid water (bulk and capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules.) and vapor permeability to allow drying to the interior.

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May 5, 2014 11:42 AM ET

am i reading this wrong ???
by Jin Kazama

Why would anyone on this website assume to install a basement floor layer without insulation under it ?

I thought exactly of this about a year ago after reading some threads here on the Q&A, but i can't recall what exactly was the topic ...
About why we are still using concrete as basement floor only to get it covered afterwards.

Let's assume that we should not be pulling a basement floor on less than 2" of rigid insulation,
why would you care about ground moisture management if you have already installed 2" or more
+ a suitable air/water barrier such as plastic poly , on the grave base ???

I would surely be scared of using a wood base on a retrofit with a wrong footing height Vs water levels, but on a new construction with a properly designed height setting and good drains ...

and some of the $$$ spent on the unecessary concrete could be used toward a thicker insulation

I will personally try to analyse this possible arrangement in all my future projects,
unless someone can provide a reason not to .

May 6, 2014 8:50 PM ET

Polycore looks very interesting but what about bulk water?
by Keith H

Anyone have any experience with polycore? It looks very interesting but it's not clear how it deals, if at all, with bulk water.

May 6, 2014 9:28 PM ET

Code Considerations
by Malcolm Taylor

I'm not that familiar with your code, but doesn't it have some concerns with restraining the bottom of foundation walls? What does this in the absence of a slab?

May 7, 2014 10:44 AM ET

sandwich the poly between layers of rigid.
by Robert Swinburne

Is there anything wrong with sandwiching the poly between layers of rigid foam to protect it? This would apply to under slabs as well.

May 7, 2014 5:46 PM ET

Floor system
by Mark F

Per IRC, basement floor system must restrain inward wall pressure unless the wall pinned at the base and also helps resist rotation of the wall. Good luck.

May 7, 2014 6:39 PM ET

Artifcial Flooring?
by Dan Weinhold

How would tongue and groove "boards" of a composite deck flooring such as Trexx, etc., work in the basement? Probably laid directly over a vapor barrier over rigid insulation?

May 7, 2014 9:20 PM ET

reply to Robert Swinburne
by charles CAMPBELL

The reason poly should be above the insulation is so that the poly acts as a "slip sheet", allowing the concrete to slide as it cures. I believe the American Concrete Institute says so.

May 8, 2014 12:01 AM ET

Edited May 8, 2014 12:03 AM ET.

ThermaSteel by LeeCor Systems
by steven medlin

This project could utilize the ThermaSteel panels from LeeCor Systems for the floor application. These are typically used for foundation walls, suspended slabs as well as the traditional SIP applications. It's a great way to quickly install these portions of the project with air/vapor barriers and eliminate thermal bridging issues. We have used these for deep energy retrofits in St. Louis with great success.

May 8, 2014 5:53 PM ET

Bracing the bottom of a basement wall
by Corian Johnston

I am amazed that the article does not mention this at all and there is only one comment about it. The basement floor slab, or other suitable structure, is required to prevent the walls from sliding in just like the floor framing above and the sill anchors. Typical basement walls are not designed to work without both of these and by the time you redesigned and reinforced them to work, it would probably be far less costly to just put the concrete slab in. The original question also said they were lowering for floor which means the base of the wall and footing may have even less required bracing than it did originally. As an engineer, I get this question quite often and while anything can be engineered to work, within reason, sometimes you just have to look at why we have been putting in those concrete slabs for all these years. Put in the insulation, add some pex tubing, place the concrete and enjoy your new floor without having to worry about it. By the way, lowering the floor slab makes the wall taller and therefore subject to higher stresses, cracking and failure.

May 8, 2014 5:56 PM ET

Restraint, etc
by Terry Steiner

I have built 2 portions of this house using PWF (permanent wood foundation) techniques and most of what is posted previously I would agree with. On the need for the floor to restrain the walls from being pushed inward by soil pressure, a PWF wooden floor needs to be designed to support that aspect. Depending on the soil and conditions, that pressure can be pretty significant. The floor joists need to be firmly against the foundation walls and as continuous as possible across the floor area to handle the stress. In addition, blocking is definitely needed to prevent the joists from twisting from the force from the walls. Realize the force is greater in the center of a long wall. A plywood floor has to be well attached to the joists to support the joists and also prevent the joists from bowing from the wall force.
The gravel base should be well compacted and as thick as possible, at LEAST 4" with the excavation sloping to drainage points. I put in drain tile across the floor to help ensure any water stays near the soil interface. I would actually skip the cement piers as they can be a conduit for moisture into the floor structure. If the gravel is well compacted, sleepers under the joists laid across the gravel and foam seem to do fine and can minimize the span for the joists (but they are actually limited by the wall forces if I remember correctly). A nice thing about this method of making a floor in a retrofit situation is getting the material down there, particularly if the site makes it tough to get a cement truck chute in place).
But be sure that the drainage is excellent! Water will find a way past the poly even though the poly is good for water vapor. Once you get water ON the poly it will stay for a LONG time.
Good luck!

May 19, 2014 4:33 PM ET

Like a crawlspace
by Jason Burk

Wouldn't the end result of using a wood joist flooring system in lieu of a concrete flooring system be essentially the same as comparing a slab on grade to a crawl space (without much space below it)?

Feb 11, 2015 7:10 PM ET

by Thomas Hoffmann

Terry stated " If the gravel is well compacted, sleepers under the joists laid across the gravel and foam seem to do fine and can minimize the span for the joists" Can you clarify? Did you place the gravel bed over the foam and then the sleepers on the gravel bed?

Apr 28, 2017 12:43 AM ET

Edited Apr 28, 2017 12:44 AM ET.

Jason, i guess that would be called a no-crawl space?
by Ethan T ; Climate Zone 5A ; ~6000HDD

I, too am interested in reducing concrete content of buildings, and don't see why a crawlspace without a slab is possible but a wood floor replacing a slab on grade isn't.

Apr 28, 2017 4:12 AM ET

Response to Ethan T
by Martin Holladay

The answer has to do with the height of the walls. The forces being resisted by a basement slab are greater than those being resisted by a crawl space slab (assuming the crawl space has a slab), because the basement walls are taller.

-- Martin Holladay

Apr 28, 2017 10:04 AM ET

My guess is that just a
by Jon R

My guess is that just a footing provides enough resistance to the greater lateral force at depth to make the middle of the wall the failure point. But I'm not a structural engineer.

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