The deep mystery in my basement

October 30, 2017 6:37 PM

OK, this mystery has nothing to do with Halloween, but... why is my basement weirdly inconsistent about flooding during rainstorms?

Where I live, the weather was miserable over the last couple of days. We had about a day-and-a-half of constant rain. The parking lot next door has a pond in it now. However, my basement stayed largely dry, except for a small wet spot in one corner.

On the other hand, about a week ago, we had an overnight rainstorm that lasted maybe four or five hours. In the morning, I found a much larger puddle in the basement.

I've noticed this same, strange pattern over the past ten years that I've lived here. Usually, a light rain will result in very little flooding, while a heavy rain will lead to big puddles on the concrete floor. But there are lots of exceptions where you get the opposite result.

Why the strange inconsistency? This has puzzled me for years now.

posted by akk2014 to Home & Garden (10 answers total)

do you have gutters? Do you keep them cleaned out? where do they discharge? The further from the house the gutters discharge, the less likely your basement will leak. If you don't have gutters, how wide are your eaves, and how permeable is the surface under them?
posted by cosmicbandito at 7:15 PM on October 30, 2017 [2 favorites]

Response by poster: @cosmicbandito: I have gutters. They get cleaned every two or three years (not often enough, obviously). They discharge near the house (except for one that gets routed about 10 feet away from the house).

There's no particular mystery about why my basement leaks. The house is old, there are cracks in the foundation, and the gutters are probably not in the best of shape. The mystery lies in the far-from-perfect correlation between intensity/duration of rainfall on the one hand, and degree of resulting flooding on the other hand.
posted by akk2014 at 7:43 PM on October 30, 2017

How far are you from a body of water with a tide? Could it have to do with tidal time and rainfall time?
posted by kellyblah at 7:49 PM on October 30, 2017

Response by poster: @kellyblah: Not close to any bodies of water with tides (the pond in the parking lot doesn't quite qualify. unless we get a lot more rain). Good idea, though.
posted by akk2014 at 7:51 PM on October 30, 2017

Could be direction of rain and wind factors too. If wind blows rain into side of the house and it runs down against foundation, more likely to leak in. If ground is dry, will also absorb a lot more moisture before leaking into basement.
posted by cosmicbandito at 7:52 PM on October 30, 2017 [4 favorites]

Best answer: My house, which is well above sea level, also floods very unpredictably during rain storms. Even without tides, there may be variations in the height of the water table or the saturation of the ground around your house. This could vary seasonally, and also depend on the amount of recent precipitation, and on whether the storm drains in your neighborhood are backed up.
posted by mbrubeck at 7:56 PM on October 30, 2017 [3 favorites]

Best answer: The factor that most affects basement flooding is the level of the water table. If it is high, even a modest rainstorm could be enough to cause overflow. If it is low, it can absorb a major rainfall without spilling over.
posted by yclipse at 8:00 PM on October 30, 2017 [4 favorites]

Best answer: This is a water table issue. It depends on how deep the water table was before the rain and whether the rain storm brought it up to the level of your basement floor or not. I had the same issue in my house.
posted by Toddles at 9:04 PM on October 30, 2017 [3 favorites]

Best answer: Yeah, a huge factor is how saturated the ground is already. A wet week in October with rain on and off can result in a light sprinkling causing water in your basement; GOUTS of water in mid-July when it's been dry as a bone can all be sucked down by the very dry ground. In the cold climates, frozen ground is another issue. When the snow melts but the ground's still frozen, your basement is awfully attractive to all that water looking for somewhere to go.

But yeah, summertime rainstorms don't really bother me no matter how ferocious, but a springtime sprinkle when the melt is on and the ground is squishy and I check my sump pump every hour!

(Also, extend your gutters, it made a huge difference to how much water we got in the basement and how often our sump had to run when we had the gutters depositing the water much farther away from the house instead of right next to the foundation to seep right back down the basement wall. Those black gutter extender things are fine (and easy to remove in the dry season!), but we had awesome luck building a natural-looking "stream" lined with waterproof fabric with a slight downhill grade, filled with river pebbles, that let out into a depression where we planted a rain garden to absorb the water. It carried the water 25 feet away from our foundation and looked really nice as a landscaping feature. Plus we got tons of butterflies in the rain garden.)
posted by Eyebrows McGee at 9:17 PM on October 30, 2017 [4 favorites]

You may be closer to a body of water than you think - a friend had a similar issue and, it eventually turned out, a street adjacent to his lot was built over a creek that had been channelized into a sewer. The sewer, now a century old, was leaking in heavy rain, saturating the soil and contributing to pushing water through his (deteriorated) foundation wall.

TLDR - there may be more subsurface water than is apparent from the lawn level.
posted by ryanshepard at 8:47 AM on October 31, 2017 [1 favorite]