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 one...it'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!