What does the 2012 IRC code state on the subject of drainage of intrusive water from the home?
Know that in my experience here, that departments of Building Safety and their building officials along with Planning Departments sometimes have modified these requirements to requirements that are more stringent. It is very important to review these with your building official to determine what is ultimately needed.
In section R401.3 it requires that the lot be graded away from the building to a point where either water can gather in a safe place from the home or into a drainage system for storm runoff. The ground is not allowed to be closer then 8 inches from untreated wood framing of a home.
(Most builders I know keep the finished line of soil surface to be a minimum of 8 inches below the top of their foundations, instead of the 6 inches that the code states as a minimum. They do this because they don’t want to use treated sheathing along the foundation perimeter line when sheathing edges come within 8 inches of grade.)
From that point, the code requires dirt to have a minimum fall of 6 inches in the first 10 feet away from the home. If that is not totally possible because of another building or a property line, then a sloped swale or drain is then needed to divert water away. Any impervious surfaces such as asphalt or concrete within that ten feet of a home need to slope away a minimum of 2 percent. Or in other words they need the grade to fall in ten feet distance a minimum of two and a half inches.
Unless the ground has a minimum peculation rate of 4 inches per hour, all foundations into the ground that separate habitable or usable space from the earth, must have a perimeter foundation drain system installed. They must discharge their contents by either a gravity method or a mechanical method to a point that is approved by the building official. If the drain is a perforated pipe, it needs to either be wrapped in a water permeable membrane or surrounded by rock or stone larger then the holes in the pipe. That pipe if not wrapped, needs to have at least two inches of gravel underneath it and covered by not less then 6 inches of that same gravel on the other sides and top. A filter membrane is then required on top of the rock. This is to prevent silting of the pipe drain system. 2012 IRC Section R405
Waterproofing and dampproofing concrete or concrete block foundations is covered in the 2012 IRC in Section R406. These are two separate types of systems to keep water from penetrating a foundation. All foundations that surround enclosed or habitable spaces (example would be basements, storage areas and “under-floor spaces” aka crawl spaces must have one of these systems applied to the foundation from its top of footing to the finished grade. Waterproofing is a more severe method of keeping water away. Its required when you have a water table that is high or another source of water that is severe. Dampproofing is for all other light to non existing water issues. There is a list of approved products for each in this code section. All joints in membrane waterproofing must be lapped and sealed with an approved adhesive.
I would say from my experience is that most foundations of homes are dampproofed as opposed to waterproofed. I see a lot of dampproofing using the top item on the list in the code. That is bituminous coatings. The methods listed in waterproofing involve a lot more materials, man hours to install and can be intense in their details. However, if a home is to be built in an area where water is a problem, then waterproofing systems must be used. You need to discuss this issue with the building official since waterproofing is going to be much more expensive system to apply, but certainly worth it to prevent the damaging effects of water.
The other remedy of course is to not have any enclosed spaces or habitable space below the finished outdoor grade in the home. Basically this only happens if you build a home on a slab over grade. This eliminates the code requirements for perimeter drains, dampproofing or waterproofing systems all together.
Gary Miller
Principal and Architectural Designer,
Timbertree and Stonecastle Designs, LLC