Avoiding Disaster Through Proper Design of a Raised Paver Patio

Today I was out on the web looking at the websites of my local competitors, something I do periodically, seeing what kinds of things they were up to.  As I browsed one local competitor’s site in particular (I’m not going to name names, but they do business in Appleton, Neenah and the rest of the Fox Valley, just as we do), I saw the proud walk-through of the construction of a raised paver patio.  Construction that sadly is destined for failure, despite the contractor truly wanting to build something that lasts.

The problem I’ve so often seen in my 30+ years in the industry is a fundamental lack of understanding of engineering principles and proper construction methods.  I want to break down what I saw (without using this contractor’s actual photos to protect their privacy), and lay out for you, our prospective client, what the proper construction of an elevated paver patio should look like.

In the above picture you see a side view of the flawed patio design.  Initially this contractor poured a slab at or below grade upon which they would build the patio.  So far, so good. 

The problems begin when roofing felt or rubber liner was placed over siding, OSB sheathing and sill plate to “protect” it, and then that siding and sheathing was used as a retaining wall to support the crushed stone sandwiched between the two slabs.  Not good. Siding and 5/8″ OSB were never designed to retain soil or crushed stone, and in time the wood elements behind the felt will bow from the pressure and rot from the moisture.  We’ve seen this far too many times.  In fact here we are below, in a project from a few years ago, replacing rotted sheathing in a North Appleton home in preparation for a new, raised patio:

In addition to the bowed, rotted sheathing that will need to be replaced, the problems the client owning the patio diagrammed above can expect in the coming years are as follows:

  • Buckled siding at the point the siding extends past either side of the patio.
  • Rotted floor joists and/or rim joists.
  • Settling of upper concrete slab (imagine a PB&J sandwich, where the slabs are the bread and the crushed stone is about an inch and a half of jelly; because the siding and sheathing can’t retain that stone under that pressure, it will settle at the house, squishing the jelly toward the house, the slab settling on top of it).

And because this is a slab atop another slab, any repairs that are needed will be expensive, because the upper slab will require removal and the crushed stone near the house removed before a repair can be made.

The graphic below shows how this patio should have been constructed, using this contractor’s initial bottom slab.


The primary difference between these two is the construction of a 4-sided “box” retaining wall atop that lower slab.  This allows the patio to be constructed 1/2″ away from the sheathing, giving an air gap that will allow moisture to drain and evaporate.  Aluminum or fiberglass (Groundbreaker) flashing is placed just up under the bottom of the trimmed siding and extends below the last wood element on the home, the sill plate.

The infill of the patio area could be crushed stone or sand, but the preferred material in this situation is sand; we’re using compaction equipment atop a concrete slab, and some of that force involved in getting adequate compaction is translated laterally, potentially pushing those retaining wall block that have been glued or mortared, creating adhesive failure and a slip plane that those block will now move along.

Better is to use easily compacted fill sand, with layers of geotextile along the perimeter, and geogrid or geotextile layers sandwiched between layers of sand, building up to the paver layer.

The result is a paver patio that is retained on all sides with retaining wall block, supported from below in an ICPI-approved fashion using geogrid/geotextile.  It’ll stand like that for decades, and should a repair ever be needed, it’ll be quick and easy, instead of costing thousands and requiring the tear-down of much of the patio.

Here’s a patio we built on Doty Island using this superior method, including construction plans and a progress picture:

Above you see aluminum flashing protecting sheathing, and markings for our crew to properly mortar up the retaining wall block on all sides.  Caution tape was placed on the doors for safety.

Below are the instructions given to our foreman re: mortaring.

And the final product, a beautiful set of steps leading to an even more grand patio (completed as phase 2 the following year), below.

I urge you to give careful consideration when the patio project you’re designing involves multiple levels or layers, or involves tying into your home in any way.  Make sure your builder has a valid dwelling contractor license and is familiar with proper building techniques and has an understanding of the engineering involved in this type of project.

~Jeff Pozniak