Care & Feeding Of Crawl Spaces
What Is A Structural Basement Floor?
Inspectors in other parts of the country tip their heads to the side and quietly whisper among themselves when I talk about crawl spaces underneath basements. "Poor Garet, he's gone off the deep end. No one would be crazy enough to build something like that!" It takes a lot of explaining for them to understand why we do it.
With few exceptions, most modern houses in the Colorado Front Range are built on drilled piers to avoid problems with expansive soils. This anchors the foundation in deeper soil or rock that's less prone to moving up and down. But concrete floor slabs in basements are still resting on the soil and often heave. Some have heaved 6 inches, or even more. By the 1980s some builders started excavating a few more feet of dirt when making the basement, casting the basement walls deeper and making a framed floor in the basement. We call this a "structural floor" (see the illustration). The structural floor does an excellent job of protecting the basement floor against damage from expansive soils. Unfortunately it can also generate other problems.
Check Your Crawl Space
You should periodically check your crawl space to see how moist it is (whether it's below a structural floor or not). Yes I know it's not a pleasant place; that's why I charge more when I look at houses that have them. But many people have a moisture problem in their crawl space and don't even know it. Standing water is a major problem, and needs to be corrected immediately! As a rule of thumb, if the soil is moist enough to soak your clothes when crawling through it then it's too wet.
Water entry can be a problem in any crawl space. For some houses water entry can affect the foundation, but the bigger issue is usually rotting of the structure. Some builders have started using steel floor joists to ward off this problem, but if there's enough moisture the connections will rust, and the flooring on top the joists will still rot. The real solution is to install drain pipes several feet deep around the perimeter. For many houses the drain pipes are installed only a foot or so deep and only go part way around the house. This may work just fine if groundwater conditions are relatively mild, but can be disastrous in severe situations. Sometimes adding a vapor retarder plastic over the soil surface will correct the problem. In other cases the drain lines need to be re-installed deeper (an expensive repair).
Sump Pumps
If you've got a sump but no pump, you should check it periodically (particularly after major storms). If there's water in the sump then it needs a pump. If you already have a pump you still need to check it periodically. Remember that just like any other simple mechanical device, a pump is prone to failure and won't last forever.
Trash
Another problem with structural basement floors is that it's a convenient trash pit during construction. I often find wood scraps and other assorted trash laying on top the soil. Add a little moisture from the ground and it will soon rot. This is like throwing out a "WELCOME" mat for termites and other critters (yes, we do have termites in Colorado). Anything organic should be removed.
Power Ventilators
Powered fans are the normal way of ventilating the crawl space under a structural floor. Once again, you need to periodically check to make sure that it's working. The control is usually mounted near the access hatch through the floor, and should be set at about 50% relative humidity.
Exterior Grading
Wait a minute, I thought we were talking about the crawl space! Well as it turns out, moisture problems in crawl spaces are often related to poor grading outside the house (although there can be many other factors). Ideally, the ground surface should slope away from the house for at least the first 5 feet, and irrigation should be minimized in this area. Drainage from downspouts should be directed well away from the house.