While summers are often spent on urban rooftops building decks, the cooler months may find us below street level, finishing — sometimes after digging out — row home basements. But the primary consideration when working on roof decks or basements remains the same: keeping unwanted water out of the house. I'll address roof decks in a later post, but since we're approaching basement season I thought I'd spend some time down there.
Let me first say that I'm going to outline a very thorough and extensive approach — if your basement isn't prone to leaking, every one of these steps probably isn't necessary. We always consider each basement individually, since there's no point in spending a lot of time and money if the basement is already pretty dry. Also, if ever you're concerned about water in the basement, I'd recommend staying away from laminate or hardwood floors and sticking with tile or other water resistant materials just in case. OK, now to the tech-speak:
Rainwater gets into a basement from two directions — down through the stone/block foundation walls and up through the floor. The most effective way of waterproofing a basement, of course, is digging up the foundation from the outside and doing a major waterproofing job (install exterior perimeter drains, waterproof the walls then correctly backfill, etc. See Part 3 of this blog for more information on this process). In the city, though, I don't think your neighbors in the adjoining homes would really appreciate that …
Almost all finished basements should have a French drain (also called perimeter drain) system that feeds into a sump pump. A French drain is basically a trench (or several), covered with gravel or rock, and containing a perforated pipe that redirects surface water and groundwater coming from below.
In a basement, this system is pretty much pointless unless it drains into a sump pump that sends the water into the sewer system. We recommend that sump pumps be airtight to minimize odor or moisture seepage, that they be installed with a battery back-up system in case of power loss, and that they're checked on a fairly regular basis for clogs or obstacles.
We would then cover the entire floor with a 3/4"-thick drainage mat and run it up the wall about 14 inches. The floor is then covered with 2 inches of extruded polystyrene (XPS) insulation, turned up at the perimeter.
When the new slab is poured, we will have created a sort of rigid-foam bathtub. Check in next post to see how the bathtub helps keep the walls dry …