Building a new house in Palo Alto

Building a new house in Palo Alto

Monday, September 17, 2012

Building a Rat Slab

As you may recall, we left off my last post, How to build a foundation & please your inspector looking like this:

Step #1: Shovel dirt out of the middle of the house & lightly slope ground towards a designated drainage area.
What you may not have noticed was the gargantuan dust bunny flying in the middle of the shot. (I suppose you were all too busy admiring the blue condoms.) Well, the flying dirt was being shoveled out of the middle of our house as part of the process of building a rat slab. A what? Yes, a R-A-T slab. That was exactly my response when my builder friend, James Witt said that it's a sign of a well-built home. I'm sorry, but whoever coined the name flunked Marketing 101 because I have definitely never ever seen a "rat slab" featured in those glossy brochures they hand out at open houses.  

Despite its ugly name, a rat slab is an upgrade worth considering when building a house on a perimeter foundation. It's one of those inside baseball upgrades that builders put into their own home but maybe not yours. So if you're buying a new home, ask whether the house has's a telltale sign that the builder cared about more than just the bottom line. A rat slab is basically a thin layer of concrete that is poured over a vapor barrier in the crawlspace of your house. The floor is sloped towards a hole that contains a sump pump and automatically drains water out. A rat slab is not required by Palo Alto code but here are some of the benefits:
  • Helps keep pesky ground critters from getting into the house (rodents & subterranean termites) 
  • Keeps water from percolating up from the soil which prevents mold & water damage under the house (better indoor air quality)
  • Creates a cleaner underfloor environment & makes crawling under the house easier (your contractors will thank you)
We have about 1900 square feet of living space on our ground floor and the quote our general contractor received for building a rat slab was $4600 for labor and materials. As someone who has never in my life ventured into a crawlspace, I was a bit skeptical about spending the extra money to make it nice. I also spoke to a neighbor who confirmed that she's never had problems with moisture under her house. There are some areas in town where the water table is high and a rat slab is a no brainer, but Green Acres doesn't seem to be one of them. Nevertheless, when your building idol (who's been in the biz for 40+ years) tells you to just do it, you just do it.

Here's a photo documentary of the making of the rat slab. It took one afternoon to dig out the extra dirt and slope the floor, then another day to lay the ground cover and cement. Yours truly even put down her camera and helped with the digging. (She is still sore from the effort but has a nice tan to show for it.)

*Builder's tip from James Witt:  Have your concrete provider add fibers to the cement. When concrete is poured thin it has a tendency to crack, the fibers will make it stronger.

Step 2: Cover the entire ground area with Visqueen plastic sheeting to act as a vapor barrier.
Step 3: Pour and spread the cement about 2 inches high.
Step 4: Quick! Smooth the cement before it dries.
Step 5: Admire your new roller rink.
Step 6: Put your paw prints at the entry way.
Thomas & Astro showed up at the end of the day. Rats, the slab is already dry! 


  1. we had the concrete ready mix plant add fibers to the cement because when the concrete is poured thin like that it better to have the fibers make it stronger

    1. Ah, good point, James! I've updated the post with your tip.

  2. No worries that Astro & T showed up late, right? Will there be another wet-concrete opportunity later? I do love the fact that for the most part, only you guys will know the Kay + Cisco mark is there, hidden away...

    1. Yes, our lefties will have a chance to leave their marks when we poor the garage floor! BTW, a little bit of Olavson trivia that didn't make it into the post...both Thomas and Astro are lefties whereas Cisky and I are righties.

  3. I have so much to catch up on! Loved the photo at the end. So cute, your little southpaw, righty family. PS Did you out James unwittingly in these comments. (PS Do you love my wordplay; I am so easily self amused.)

  4. What do you think about pouring a rat slab in an already existing 1960 dirt crawl space? Having a mice problem in a summer home? Have had estimates on encapsulating with 6ml poly.

  5. What do you think about pouring a rat slab in an already existing dirt crawl space? Is it cost effective and can it even be done properly?

  6. while called "rat-proofing its primarily for moisture blocking we have pumped concrete under existing crawl spaces over visqueen ( plastic ) its miserable work. the better way is called steggo-wrap
    for rat problems its more about blocking all the tiny (as little as 1/2" ) holes that rats find their way under the house


  7. James, please share your opinion. We have rats digging underneath our foundation to get into our crawl space. They have ruined our Flex ducting for the second time in less than 5 years. We have to get all the ducting replaced. We are in the Bay Area. Do you think metal ducting is a better option for rodent proofing than Flex ducting in a crawl space? On top of everything else we have to seal the house. We got a quote today for almost 13K to build a rat slab in the crawl space. This doesn't include the cost of replacing the ducting. This is silly I know, but I've been crying off and on today just thinking about the cost and what the rats have done. It's a lot to process. Do you think a rat slab is worth the cost? Is there any other way to seal the foundation so they can't get underneath? Any advice is greatly appreciated! Kathleen

    1. Hi Kathleen,
      Here's what James said...

      i would for now only worry about getting / keeping them out from under i wouldn’t do a slab on existing home just for rats ( water and rats maybe )
      i have a rat whisperer guy if you need
      where are you?
      more about what to do later
      main thing be glad its not north carolina

    2. Hi Kathleen... what did you end up doing? We have the same problem. I am now replacing all of our ducts and was told that the rats are burrowing into our crawl space. The person replacing the ducts suggested spraying in concrete over all of our crawl space. Ugh...
      Thanks for any advice.

  8. Hi, I am having a similar problem with rats digging under the foundation into the crawlspace. I have a half basement/half crawlspace and would like to pour a "rat slab" to alleviate this problem along with helping with radon/mold/mildew. I am curious why you don't recommend pouring a "rat slab" in an existing structure? I have also gotten a quote for encapsulation of the crawlspace but the slab seemed like the better option.
    Thank you,

  9. We live in CA, so I can't speak for your state, but I was advised not to pour a rat slab because of earthquakes, it could create a mold issue, water might not properly drain even with a dump pump, etc. Pouring a rat slab does seem pretty permanent. I've been advised to seal the house (which we've done) set traps (which we've done) and move HVAC system to attic which we can't do.