One common observation I’ll make when inspecting a home is buried downspouts. It’s considered an observation because many times a buried downspout will perform exactly as planned and evacuate the water away from the foundation like it is supposed to. However, it is difficult to assess how well the buried downspout is performing and there can be hidden concerns.
Why is is a problem? Water around the foundation can cause a multitude of issues. Moisture intrusion thru the foundation walls and into a basement. Water introduced to the crawlspace where it can contribute to a host of issues revolving around moisture content in the flooring and the potential for microbial growth. Additional, water around the foundation can undermine the foundation itself causing structural issues that can be very problematic and expensive to fix.
There are a couple of things to consider when assessing buried downspouts. What is the material used below ground? The most ideal options would be a single run of solid corrugated tile (the black flexible pipe with ribs) or glued together sections of 4″ or 6″ PVC. Often I will see corrugate tile but in a manner that looks like it could be pieced together below ground. The issue with this approach is quite simply that the sections will pull apart, especially within the first few years as the earth settles, and then water will be allowed to distribute against the foundation unbeknownst to you the homeowner. Another observation I will often make is when perforated tile is used. The problem here is that perforate is designed to go at the base of the foundation, covered with rock and a filter screen, to allow what water does make it that far to enter the tile and be transported quickly away from the home. A last resort. Using it as a drain only allows the water to distribute in the one are in which we are trying to avoid. There have been many times where a builder will tie the downspouts in with the perimeter drain. This is very problematic because it introduces a large amount of water around the foundation when the intent is to avoid this at all costs.
A couple of other things to consider. Often the ground around a house will settle in the first few years. Inevitably this movement will pull the buried drains away from the downspouts and the runoff will proceed to distribute around the foundation. I see this scenario several times a week. The cheap accordion style extensions are not rated for buried use. They are not recommended and will often fail. Make sure the proper materials are used when burying a downspout.
In summation, buried downspouts are fine as long as they are installed correctly. Solid single runs are black tile or PVC are most ideal and less likely to fail. Allowing for the inevitable settling within the first few years is prudent. Make sure the discharge locations are observable so that you can check to see if they are working properly.