How to Backfill a Timber Retaining Wall
Today I do not want to talk to you about dirt. I want to talk to you about this stuff: 57-stone. This is a course stone about 3/4 – 1″ and it’s great for drainage. We’re building a paver patio that’s going to extend off of the house to just a portion underneath this deck. So, what’s important here is this wall’s pretty short. It’s about 3 ½ ft tall at its highest point, so you might be tempted to try to get away without drainage. We’ve seen a lot of jobs where contractors will just backfill with dirt only.
I do not recommend in any situation but specifically where water is going to be sloping down and has a potential to seep in behind this wall. What we’ve done here is we’ve put in a boat load of this 57-stone instead of using dirt. In a normal drainage system, you’re going to have about 12 inches of dirt that you’re going to wrap in a fabric and allow that to create a separation between your gravel and your dirt.
So eventually they don’t combine. Water always continues to filter through there and you keep that french drain piping clean. In this situation, we’re just going to keep going with stone and here’s why. Yes, it’s more expensive but it also is self-compacting. It’s been raining. It’s the middle of the winter season and the dirt that we’re buying is exposed to all of the elements. If I were to buy dirt it would be very difficult to compact and it would just create a lot of labor problems for me.
If I bring in the stone, we dump it in we roll over, and it’s compacted. In most situations where I have this slope off of the house, a patio that’s going to be roughly 15-16 feet off of the house, and I have to build a small wall to support that in most of these situations I’m going to use this drainage stone to support it because I’m not going to have to worry about sinking and settling in compaction and coming back later. The other thing that we’re doing is waterproofing all of the posts with a black tar product that will seal out moisture and prevent them from rotting prematurely.
One extra step we could take would be to raise the posts up above and put them on standoff plates. Here’s the downside that. This is a really big deck and this was going to make the project really out of the budget. These posts are designed to last 20 years, so adding the waterproof coating is extending the life of the post. Also putting 11 posts on a standoff place means we’d have to pour about 11 new footings. We’d have to jack up the deck temporarily and it was just almost cost prohibitive to do the job at that point.
So, when in doubt when you’re back filling a retaining wall, go the extra mile get this 57-stone or 3/4 drainage stones. We’re surrounding all of these posts with that stone to create good drainage around each one of them. Yes, dirt is cheaper and that’s where the benefit of it stops. It’s unpredictable to work with. Stone is very predictable. It’s self-compacting. The labor is simple. So whatever you think you’re spending extra on the stone, I can assure you that you will save it in the long run on the labor.