FHB Logo Facebook LinkedIn Email Pinterest Twitter X Instagram Tiktok YouTube Plus Icon Close Icon Navigation Search Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Subscribe
Log In
  • How-To
  • Design
  • Tools & Materials
  • Restoration
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
Log In

Fine Homebuilding Project Guides

Foundations and Masonry Work

Guide Home
  • Concrete & Masonry Basics
  • Building Foundations
  • Waterproofing Foundations
  • Retrofits and Add-ons
  • Masonry in the Landscape
  • Brick
Q&A Spotlight

Dealing with a Faulty Basement Slab

A homeowner weighs the merits of repair or replacement of a concrete slab that was poorly installed.

By Scott Gibson
  • X
  • facebook
  • linkedin
  • pinterest
  • email
  • add to favorites Log in or Sign up to save your favorite articles
Slab with a problem: This concrete slab is showing a number of shortcomings when compared to the job specs. The question is whether it can be repaired, or must it be replaced? Photo credit: Bulrush.

Bulrush is watching as his new home is being built, and things went only as far as the basement slab before problems cropped up.

“The slab has multiple issues, both errors and general poor-quality work,” Bulrush writes in this recent Q&A post. “I’m looking for opinions on how serious the various issues are, whether mitigation options exist, and whether I should push for the entire slab to be torn up and replaced.”

According to the specs, a slab of between 3-1/2 in. and 4 in. was to be poured over 2 in. of extruded polystyrene foam and a polyethylene vapor barrier. For starters, the concrete sub used 4-mil poly, not 6-mil poly, raising concerns over the long-term durability of the plastic.

The slab is not flat, Bulrush adds, with a noticeable variation across the entire slab and “significant variation” across small distances. He wonders how this will affect the luxury vinyl plank flooring he hopes to install.

Nor does the concrete have the fiber reinforcement that was called for. “I also suspect the concrete mix had a lot of water,” Bulrush says, “based on the amount of uncontrolled cracks and unevenness of the slab. The concrete sub poured the slab when temperatures ranged into the high 80s and low 90s (°F) but didn’t take any steps to keep the concrete damp as it cured.”

There’s more. Multiple pipe penetrations are completely cracked through the concrete so the pipes can now be wiggled. Power-screeding seems to be of poor quality with many areas showing concentric arcs—not only visually, but actual ridges.

Bulrush is considering the steps he’d have to take to fix the problems, including grinding the floor flat, filling cracks with polyurethane, and applying some kind of vapor barrier on top of the slab. What else can he do?

That’s where we start this Q&A Spotlight.

The “tear it out” option

Bing 0328, a general contractor for 30 years, doesn’t mince words:

“I use subs whose quality far exceeds minimums,” he says. “That’s just the way I do things. Those same subs know if it isn’t done right it gets redone. I would tell the GC to have it torn out and done right. Hope you haven’t made any payments yet, that’s your leverage.”

CT Yankee, who has been in the concrete industry for a half-century, agrees. He guesses that the concrete sub was in over his head, or was never made aware of the specs.

“If it were my house and if I know that the concrete sub was fully aware of the specs up front, then it’s a no-brainer do-over,” Yankee says.

If the general contractor was responsible for hiring the concrete sub and directing his work, then the GC now has the burden of making it right, he adds.

Bulrush says that while a tear-out may look like the most logical option, it has its downsides. For one, the delay would mean that he loses his place on the framer’s schedule. “I also probably burn some of my ability to insist on fixes later in the build,” he adds, “not contract-wise, but I don’t see a happy ending if the working relationship is dead.”

Are there any alternatives?

Adding a new slab

CT Yankee says that grinding the slab so it’s both level and flat may not be possible without compromising its thickness.

“You may be able to achieve ‘flat’ but never really achieve ‘level,'” he says. “An alternative (if ceiling height isn’t critical) would be to put the 6-mil poly over the existing slab and then place a 4-in. minimum slab on top of it.”

The idea is appealing, but there are some obstacles. The new layer of concrete would cover the bottom plates of steel columns used to support the first-floor framing. And some of the exterior walls are built at slab level for a walkout basement, so a full 4 in. of new concrete would completely cover the existing bottom plate.

“Sounds like the framers are already at work,” CT Yankee replies. “Do the columns have the screw adjustment at the bottom? Perhaps they can be inverted to put the screw adjustment at the top. Very often I see lally columns whose bases are fully encapsulated by the floor slab concrete. Not a problem until they rust to the point of requiring replacement (seen that as well).”

Adding a leveling compound

Bulrush also is thinking about a self-leveling material on top of the existing slab to even up inconsistencies and provide a flat surface for the finish floor. He’s thinking specifically about products made by Ardex, as described at this website.

“Seems like something like that would at least result in a flat and level floor, which is the most immediate problem,” he says.

CT Yankee, however, argues against an epoxy slab coating. “It may left over time or not cure properly,” he says. “As I mentioned previously, a full do-over would be best.”

Our expert’s opinion

Mike Guertin added these thoughts:

I’m sorry to hear about the problem. It’s frustrating that even with the specifications spelled out, the requirements were not met. I wonder if the floor finishers were experienced with placing concrete over plastic. It can be a tricky process, especially if water is added to the concrete. Excess water has to bleed and dry off the top instead of draining through the bottom into the soil beneath. Excess water makes finishing more time consuming and difficult. That’s not to give the concrete crew an excuse—they and the builder should have known what precautions to take and what skills were needed for a successful job.

I don’t think the 4-mil poly will deteriorate and lose its effectiveness at resisting water-vapor transmission. Since you have 2 in. of XPS beneath the plastic, I think the vapor transmission through the foam-and-plastic sandwich will be slight. And the small amount of moisture that does make it through can be handled with periodic ventilation and/or dehumidification.

If the concrete finishers did water down the concrete a lot (and if the surface is already showing cracks it sounds like they did), the current cracks will continue to get wider and more cracks will likely develop. Current and future cracks will telegraph through any applied concrete surfacer and that may be a problem for some finish flooring materials applied directly over the slab.

The other remedy solutions that have been offered—removing and placing an entirely new concrete slab and placing a thin slab over the existing one—are viable choices. If you opt to place a new slab over the old then adding a layer of plastic or other membrane to decouple the old slab from the new one will reduce the chance for cracks to telegraph through.

Another option to provide a good base for flooring is to overlay the slab with two layers of Advantech sheathing screwed together. The work can be done anytime so it won’t interfere with the framing operations; and the sheathing can bridge over gentle humps and dips so the existing slab only needs spot treatment and not a full resurfacing. The slab doesn’t need to be perfectly level, unless that’s of paramount importance to you. The ridges and small high spots can be knocked down with a grinder and low spots can be filled with self-leveling concrete surfacer made to fill to a feather edge. Cracks that propagate through spot leveler won’t matter since the sheathing will cover them. Any type of flooring—including ceramic tile, LVT, laminate, and engineered wood—can be installed over the subfloor sheathing.

Scott Gibson is a contributing writer at FHB and Green Building Advisor. Mike Guertin is a Rhode Island builder and an editorial adviser at FHB. Reader comments may have been edited for clarity.


RELATED STORIES

  • Concrete: Summer vs. Winter Mixtures
  • Fixing Concrete Cracks—Basement Slab Repair
  • A Wood Floor that Can Survive Anywhere

Fine Homebuilding Recommended Products

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

100-ft. Tape Measure

Laying out mudsills and foundations for new homes and additions requires a bigger tape measure than your everyday carry.
Buy at Amazon

Smart String Line

String lines — sometimes called dry lines — are invaluable for straightening walls and a million other things. The reel makes it easy to stretch and wind the string.
Buy at Amazon

Plate Level

A plate level allows to you plumb walls accurately because it registers off the plates without touching potentially warped or twisted studs. This one can reach the plates on 12-ft. tall walls.
Buy at Amazon
Previous: What to Do About a Cracked Concrete Block Foundation Next: 2 Ways to Detail Durable Porch Piers

Guide

Foundations and Masonry Work

Chapter

Retrofits and Add-ons

Sign up for eletters today and get the latest how-to from Fine Homebuilding, plus special offers.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Foundations and Masonry Work

Foundations and Masonry Work

Trusted, comprehensive guidance from the pros for working with concrete and masonry

View Project Guide

View All Project Guides »

Become a member and get unlimited site access, including the Foundations and Masonry Work Project Guide.

Start Free Trial

Concrete & Masonry Basics
  • Choosing & Mixing Concrete
  • Concrete & Masonry Tools & Techniques
Building Foundations
  • Foundation Basics
  • Foundation Footings
  • Insulated Concrete Forms (ICFs)
Waterproofing Foundations
  • Waterproofing Strategies
  • Foundation Drains
Retrofits and Add-ons
Masonry in the Landscape
  • Patios, Walkways, and Driveways
  • Retaining Walls
  • Dry Stone Walls
  • Fire Pits
Brick

Fine Home Building

Newsletter Sign-up

  • Fine Homebuilding

    Home building tips, offers, and expert advice in your inbox.

  • Green Building Advisor

    Building science and energy efficiency advice, plus special offers, in your inbox.

  • Old House Journal

    Repair, renovation, and restoration tips, plus special offers, in your inbox.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters

Follow

  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X
  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Privacy Preferences
  • About
  • Contact
  • Advertise
  • Careers
  • Terms of Use
  • Site Map
  • Do not sell or share my information
  • Privacy Policy
  • Accessibility
  • California Privacy Rights

© 2025 Active Interest Media. All rights reserved.

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

  • Home Group
  • Antique Trader
  • Arts & Crafts Homes
  • Bank Note Reporter
  • Cabin Life
  • Cuisine at Home
  • Fine Gardening
  • Fine Woodworking
  • Green Building Advisor
  • Garden Gate
  • Horticulture
  • Keep Craft Alive
  • Log Home Living
  • Military Trader/Vehicles
  • Numismatic News
  • Numismaster
  • Old Cars Weekly
  • Old House Journal
  • Period Homes
  • Popular Woodworking
  • Script
  • ShopNotes
  • Sports Collectors Digest
  • Threads
  • Timber Home Living
  • Traditional Building
  • Woodsmith
  • World Coin News
  • Writer's Digest
Active Interest Media logo
X
X
This is a dialog window which overlays the main content of the page. The modal window is a 'site map' of the most critical areas of the site. Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page.

Main Menu

  • How-To
  • Design
  • Tools & Materials
  • Video
  • Blogs
  • Forum
  • Project Guides
  • Reader Projects
  • Magazine
  • Members
  • FHB House

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Popular Topics

  • Kitchens
  • Business
  • Bedrooms
  • Roofs
  • Architecture and Design
  • Green Building
  • Decks
  • Framing
  • Safety
  • Remodeling
  • Bathrooms
  • Windows
  • Tilework
  • Ceilings
  • HVAC

Magazine

  • Current Issue
  • Past Issues
  • Magazine Index
  • Subscribe
  • Online Archive
  • Author Guidelines

All Access

  • Member Home
  • Start Free Trial
  • Gift Membership

Online Learning

  • Courses
  • Project Guides
  • Reader Projects
  • Podcast

More

  • FHB Ambassadors
  • FHB House
  • Customer Support

Account

  • Log In
  • Join

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Follow

  • X
  • YouTube
  • instagram
  • facebook
  • pinterest
  • Tiktok

Join All Access

Become a member and get instant access to thousands of videos, how-tos, tool reviews, and design features.

Start Your Free Trial

Subscribe

FHB Magazine

Start your subscription today and save up to 81%

Subscribe

We hope you’ve enjoyed your free articles. To keep reading, become a member today.

Get complete site access to expert advice, how-to videos, Code Check, and more, plus the print magazine.

Start your FREE trial

Already a member? Log in

Privacy Policy Update

We use cookies, pixels, script and other tracking technologies to analyze and improve our service, to improve and personalize content, and for advertising to you. We also share information about your use of our site with third-party social media, advertising and analytics partners. You can view our Privacy Policy here and our Terms of Use here.

Cookies

Analytics

These cookies help us track site metrics to improve our sites and provide a better user experience.

Advertising/Social Media

These cookies are used to serve advertisements aligned with your interests.

Essential

These cookies are required to provide basic functions like page navigation and access to secure areas of the website.

Delete My Data

Delete all cookies and associated data