Sonoma County's Permeable Landscaping Discrepancy

"You should leave. You should get out of here"

That's the response I got to asking the permit department why artificial turf was not considered permeable, and why decomposed granite was. Healdsburg has a notoriously tough permit department, and I respect that. Cotati is also a notoriously tough department - to the point they've slapped fines and fees on projects that went off without permits that were worth more than the house. Regardless, I go in the Cotati building, and it's nothing but smiles between the staff and myself.

Let me be clear. I respect tough. I am a tough employer and expect a lot out of everyone from my clients to my employees. The expectation between acceptable and unacceptable are black and white every day. What I do not respect, is inconsistency.

Most builders know that decomposed granite is an approved underlayment for artificial turf. You can substitute decomposed granite for 1/4 fines on a baserock layer, but still the point stands, neither of which, when brought to 90% ASTM compaction rates, are very good at absorbing water. However, one is considered permeable, and one isn't? Make it make sense!

Besides concrete, I'm of the opinion that almost any landscaping can have a "permeable" option. Let's explore some of our most common installs, and what makes them "permeable" versus "impermeable".

Pavers

This is a typical paver installation detail. What they're referring to as "gravel" is typically class 2 baserock. It's not very permeable, because it contains high amounts of bentonite clay. In fact, class 2 baserock is essentially the same mixture of gravel and sand that makes up concrete, but with cement (the ingredient that makes concrete get hard and provides the creamy smooth finish) is swapped out for bentonite clay. If you shape it into a depression, compact it like you should, and fill it with water, it will hold the water for quite a long time. This is not a permeable paver design.

Adding to the impermeability is the sand between the pavers. Sand is permeable, but polymeric sand (which contains a water activated glue) that is required between the pavers isn't. This sand creates the "finished" look, and can help prevent shifting or heaving.

Here's the permeable version, penned by the same artist. Notice the lack of polymeric sand, much deeper base layer, and different base rock. "Open" base means this base is devoid of sand or fines. When compacted to ASTM compaction rates it still posesses voids that allow water to trickle through to the bottom. Instead of sand, contractors are to use little rock chips, and instead of regular pavers with their 1/8 inch spacing between the interlocking spacer nubs, permeable pavers have 3/8s spacing between the interlocking spacer nubs. This helps prevent tiny debris from clogging the gaps.

On extremely wet jobsites, or jobsites where the driveway is accepting lots of runoff water or gutter water from a house, a contractor or architect may demand the installation of PVC drain pipes in the base, essentially turning the permeable paving into a french drain. Water seeps into the pipes, gets carried away, and prevents the foundation from getting full.

Also, note the filter fabric lining the foundation. This is to prevent silt and dirt from filling the foundation and ruining the permeability of the pavement.

Flagstone can easily be installed on this same base construction. Instead of mortar or polymeric sand being used in the joints, permeable flagstones get installed with pea gravel, chip rock, or plain ole normal sand in the joints instead.

Asphalt

Normal residential asphalt is used as a cost effective concrete substitute. Asphalt is just like baserock, but with hot tar as the active ingredient that binds the sand and gravel together. Expect a good asphalt job to include about two inches of asphalt and half a foot or more of baserock. Asphalt isn't very strong by its lonesome, so having a super-strong baserock layer is the key to an asphalt job well done. Highway 101, an asphalt freeway, has a couple of feet of baserock and multiple layers of hot asphalt to withstand the constant stream of traffic.

Contrast that, with this architect's permeable asphalt install. Permeable asphalt contains NO fines - it is only tar and course rocks, allowing for little voids for the water to pass through. The rest of the install has undeniable parallells to the permeable pavers - a deep open base layer, geotextile fabric to ward off the silt intrusion, and no "fines". This architect required a drain pipe in case there's a really big rain and the permeable base is at risk of overflowing.

Turf

Here is a typical turf install. You'll see a compacted soil layer, a compacted class 2 baserock layer, a skinny bedding layer of decomposed granite or fines, and your precious outdoor carpet of turf. It's affordable, it's tough, and it lasts decades, but it isn't permeable.

Similar rules apply between other permeable paving and permeable turf. Turf's got to be near perfectly smooth to guarantee a professional seamless install, and open baserock is a rough, chunky product. You will feel the stones underfoot, and won't be happy with your turf install. Therefore, a handful of smooth bedding layers for turf exist. One of them is Romex Trass Bedding, a cementitious permeable crust which the turf is stuck down to with construction adhesive and concrete anchors. It's time consuming and expensive, but it is a truly permeable install. Dump buckets and buckets of water on this all you want, you'll never see the puddle, especially considering the PVC drain pipe in the base layer.

Decomposed Granite

This is what Sonoma County as a whole believes to be permeable. We've covered the standard and permeable install of pavers, flagstone, asphalt, and turf. Now, what do you see here? Compacted subsoil, compacted base, compacted decomposed granite. I see a whole lot of compacted materials with fines in them - and if you have read this far, you're screaming at your screen, "That doesn't look permeable to me" and I agree!

A truly permeable alternative I would recommend is any of these plastic grid paving systems that are filled with 4-6 inches of gravel and turn relatively loose gravel into a surface so tough you can drive on them without sinking. Bodpave, Gravelpave, Gravel Grid, any of these products are valid choices and share the same gravel-locking capacities.

For a cost sensitive install, these can be installed right on dirt, and the surface will be reasonably permeable. For an extra-permeable install, the gravel grid system can be installed on a deep bed of open-grade base without fines - employing the same construction tactics we discussed over and over in this article.

Best of all - this system allows you to use any gravel you like - and it's less messy than granite due to decomposed granite's high quantity of little fines that love getting stuck in your shoes and depositing themselves all over your carpet or expensive hardwood.

Thank you for finishing our article! If you want any of these installs in your home, feel free to get a free quote by calling 707-755-0612, or emailing winglewichsam@gmail.com .