Question
I have a question relating to drainage from my backyard hillside and planters. I have a retaining wall that is about 2 feet high, and runs around my entire backyard. On each side of my house, the wall creates a nice planter that is about 4 feet wide between the retaining wall and the exterior walls of my property. In the back of the house, I have about 15 feet between the retaining wall and the exterior wall, and an incline of about 8 feet creating a hillside for trees, flowers, etc. The contractor that did the retaining wall left weep holes around the entire wall that drain to my grass, and then to surface drainage on the grass. There are no surface drains behind the retaining wall for the planter and hillside.

I have spoken to two different people to do the landscaping of the hillside and planters. One of them says that the weep holes are completely sufficient, and would simply recommend drilling them out a little and installing some crushed stone on the inside of the wall to prevent them from being clogged. The other says that he won't proceed until I have him construct surface drains around the entire inside of the wall, and core under the wall to tap into the drains I have in my lawn. The cost he is quoting me for this is around $2,500, and I would like to avoid paying that if it's not necessary.

I live in Southern California, and the summers tend to be pretty hot. Assuming that I'll have to be watering the hillside and planters a good amount during the hotter months, do you think I need to add surface drainage behind the wall? Or does the recommendation about the weep holes seem reasonable to you?

Answer
Hi Chris -   

It isn't the summer water that will get behind these walls.

I grew up in southern California, and we knew it was spring when houses started sliding off of Mulhulland Drive or Pacific Palisades homes slipping into the Pacific.  I remember bailing out our basement from water seepage and keeping everything elevated off of the concrete floor.  Ah, fond memories!

It is a notorious area for landslides, usually due to excessive rain and poor soils & retaining wall construction.  If you are POSITIVE that no other water sources arriving at your retaining wall than the area you described, then the weep holes are probably sufficient.

If, however, there is any amount of water arriving at the top of these walls from some other source, then I'd spend the extra money and add extra drainage, either in the form of gravel behind the walls (less expensive) or hard pipes to intecept it before it can seep in and press against the wall (more expensive).  

Water is a powerful force, and 1 gallon weighs 8 pounds, but can add up quickly to cause wall failure.  The weight is in addition to the weight of the 8 ft high slope dirt at about 24# per square foot!

The gravel or pipes probably should have been installed to begin with, but "necessary" is a value judgment by you, the homeowner...do you NEED homeowner's insurance?  Probably not, as your house may never burn to the ground, yet you pay lots of money "just in case".

You said the contractor "won't proceed" - are you adding landscaping to this slope?  If I were the contractor, I don't think I'd want to have to pay for a new wall in a few years if the homeowner thought that the new irrigation system caused it's failure.  I'd probably recommend that it get some help, too.

I guess you should never ask a barber if you need a haircut, or a contractor if you need extra work done...maybe you can sign a waiver or "proceed at the homeowner's risk" kind of statement with the contractor if you don''t want to spend the extra cash.

~Marc