Basements can be great, if they are well designed; if you just went out and dug a basement sized hole in your yard, chances are it would fill up with water sooner or later. Part of our job is to make sure that does not happen.
Basements are a relatively inexpensive way to add space to a new home, as well as provide a place for utilities, storage, and even living space. Generally, homes on a full basement foundation or crawlspace are easier to insulate and therefore, often more energy efficient. The problem with building basements in RI and southeastern MA is water. If you’re building on Aquidneck Island, for example, there is a very high likelihood you have a high groundwater table (aka Seasonal High Groundwater Table (SHGWT)). Residential building codes (IRC, section R400) require that any new foundation slab be elevated above the groundwater table by at least 1 foot. This has been the code for some time but the regulation is now being dutifully enforced. The takeaway is, if you’re planning on building a new home with a basement, you need to have a soil evaluation done by a PE or Soil Evaluator during the design phase.
This allows us to determine what the maximum water table depth was at the highest point in the past few hundreds to thousands of years. It’s a relatively conservative measure, but it is a safe measure. Buildings with basement slabs above this height are very unlikely to have groundwater infiltration to the basement. More likely than not, you won’t be able to elevate the basement above the water table if you have height restrictions or site limitation. If you’re not able to set your basement slab above this elevation, it may not be a deal breaker. Often, we are able to design and install sub drains (aka French Drains) on the inside and/or outside of the building perimeter. On sloping lots, we can outlet these drains “via gravity” slope so the drain requires no independent power to operate. That’s the gold standard. On flat parcels, it’s often impossible to do this, so a sump pump is required. We do not install these and if you’re in this situation we strongly recommend working with a basement waterproofing company (examples, not necessarily recommendations: Pioneer, Trident, etc) so they can install battery, standby generator, or water pressure driven backup systems that will allow the pump to run if the power goes out. In addition, it’s important to work with these firms to help determine the ideal specification for damp-proofing. There are many options outside the scope of this blog. Make sure the term “redundant” is used (and understood), which often means installing an exterior membrane, an interior sealant, and pump(s) with a dedicated backup. Basements flood during storms, which is when power goes out. A sump pump doesn’t do any good without electricity so a generous backup battery, generator, or even a pump that can run off water pressure are a must. The asphalt damp proofing rolled or sprayed on to the average building is often not sufficient in many cases. It’s the minimum allowed by code and is not enough to “guarantee” a dry basement. Any good basement system has multiple redundancies.
While failure is not typical (for a well-designed and installed system), the consequences of failure are high, so an ounce of prevention outweighs a pound of cure. Many municipalities (predominantly dense coastal cities, like Newport and Narragansett) have prohibitions on sump pump discharge in many areas, due to over-capacity drainage systems. In these cases, it is possible to install a basement but it needs to be undertaken with extreme caution and under the direction of a licensed engineer and waterproofing company. The foundation can be made fully sealed (like a boat), but the foundation must be specifically designed to resist buoyant forces so the house doesn’t float! This is a very expensive practice and is typically reserved for very tight or high-priced parcels where space is at a premium.
Now, just because you’ve had an expert design your basement with interior and exterior damp proofing and sub drains doesn’t mean your home free. The designer and excavators still need to take care to follow the basics for any new or existing home to make sure roof and surface water are carried away from the structure. These include:
- Installing gutters, downspouts, or similar features
- Carrying downspouts at least 10’ past the foundation
- Sloping the grade away from the building at least 6″ drop in the first 10’
- Making sure the sub-base is adequately compact to prevent sink-holes and making sure that these are properly filled if they do occur
- Use a membrane waterproofing system for high water table or critical areas (such as a “dimpled” foundation membrane, e..g Delta-MS or similar.
- Bonus points for directing your roof water to a properly designed Best Management Practice (BMP) feature, like a rain garden or drywell. (more on that soon)
- Installing a fixed dehumidifier, with a pump that discharges outside at least 10′ from the foundation (route it out near a downspout). We require this on homes we build, it’s key.
If you have recently bought a new home, and are having trouble with any of the following, then be sure to check out the RI Consumer Protection Statutes for residential construction, specifically Site Drainage and Erosion: http://www.crb.ri.gov/consumerprotection/index.php
Your contractor is responsible for at least a year from the date of sale or occupancy.
TLDR: Get a soils expert to identify your water table and soils type before planning a home with a basement. Involve multiple experts, engineers, soils scientists, and waterproofing contractor to make sure your basement has redundant waterproofing measures. Follow basic guidelines to keep water away from the building, and be aware of the consumer protection statutes.
Get in touch with us for more info on basements: