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How to Install an Open Graded Base

For years and years contractors have been installing their bases with ASTM D 2940 which typically consists of fine material in addition to stones up to 3/4″ size. On top of this for leveling they used a concrete sand. These days, a new way to install bases is becoming increasingly popular and will soon be the leading way to install bases.

Open-graded base systems are superior in every way to installing the traditional base for interlock. It is now endorsed by leading manufacturers in the industry and has been used for years in the industry, though not without some friction from some contractors.

With so many advantages over a traditional base installation, it makes no sense why companies would not switch over to installing an open-graded base. In this post, we wanted to discuss the advantages and why you should be installing this system over the traditional system. We also want to walk you through the step-by-step process in installing an open-graded base so that you have something to refer to.

What is an Open-Graded Base?

An open-graded base is one that allows water to permeate through it with little resistance. The lack of fines in the open-graded aggregate does not allow for it to hold on to the water. Instead, the water permeates through the base material.

This base is typical in a permeable base installation and has since been introduced and used in any patio and walkway installation to allow for the permeation of water.

The open-graded base is installed using two different types of aggregates:

  1. ASTM #57

  2. This is 3/4″ open-graded crushed rock with no fines that is used for a recommended minimum of 6″ to 8″ after installing a geotextile fabric to separate the subsoil from your base. It has a high compaction rate without even needing to tamp it, so you can lay more in your lifts before compaction. We recommend a maximum of 4″ lifts before compacting with this stone.

  3. ASTM #8

  4. This is a 3/8″ or sometimes found as 1/4″ open-graded crushed rock with no fines that is used up to 1″ of your final bedding layer for leveling. It has a near 99% compaction rate without having to tamp it, so you can screed and lay your stones with the final compaction coming when you compact the polymeric sand into your interlock. It is incredibly easy to work with as a leveling stone.

Both of these stones should be angular crushed. Be sure to verify this with your supplier before ordering as this is crucial to the compaction of your base material. We have seen stone suppliers claiming to have an ASTM #8 stone that is crushed, but it is more circular almost looking like a pea stone. This will not compact as well as an angular crushed stone.

Why Install an Open-Graded Base?

With the increasing demand for permeable solutions, an open-graded base provides just that for contractors and homeowners alike. It is extremely versatile and provides so many benefits for your business. It will save you time and money, and as a hardscaper that is all we ever want. It will also save you from callbacks to your clients house.

Here is why you should be installing open-graded bases:

  • Permeability

  • Allowing any water that penetrates through your stone and polymeric sand to permeate through your base is the best way in ensuring the freeze and thaw cycle of your climate will not effect your project. With such a harsh winter in a lot of North America, we need to do everything we can to make sure that our patios and walkways will be able to withstand the winters.

    The open-graded base is designed to do just that as water permeates right through the base instead of the traditional base where the fines hold on to the water for longer periods of time. In the winter, this water expands with the freezing causing the heaving of a hardscape project. With no water being held by the open-graded aggregates, you project will experience little to no heaving in the winter months.

    Screeding ASTM #8 Stone

  • Ease of Installation

  • With such a high compaction rate of the two open-graded aggregates, it makes for a much easier installation of the base and leveling layer. You can lay more in your lifts before compaction, which in a patio or walkway will save you one or two lifts depending on how deep you install your base. In addition, with no fines present in these materials there is no mess if it rains and less to clean up. They are much easier to work with from an installation perspective which makes them the perfect material for any hardscaper.

  • Workable in Any Weather Condition

  • If it rains and you are installing a traditional base, you may be out of luck. Your fines will wash out or your project will become a mess quickly. With the open-graded aggregates, you can work in any weather condition. That is right, you can work in the rain without a fear of your bedding layer washing away or becoming a mess. This means more time for your crew throughout the year and more money in your pocket.

    How many days do you think you waste due to rain? Especially in the Spring and Fall seasons, there are many rain days that delay or suspend projects. Using an open-graded base system, you can now have your crews working through those days when necessary.

  • Washout

  • In a traditional base project with fines and sand as a bedding layer, there is a certain amount of washout that occurs and is inevitable. Especially in raised patio installations, there will definitely be washout occurring through the retaining wall of your raised patio. This washout will lead to a sinking of your patio. Open-graded aggregates are too large to washout through your raised patio or in any application. There is minimal lose, which means your project will not sink over time due to this loss of base or bedding layer.

  • Insects and Weeds

  • Insects love the fines in a traditional base installation. However, in an open-graded base system they are less likely to be burrowing through it as the stones are too large for them and there is no burrowing to happen in these aggregates. This is a major selling point for most customers. In addition, weeds are less likely to establish roots in these aggregates and are much easier to pull out if they do.

  • Increased Sales

  • We already talked about the efficiency factors of installing an open-graded base, as well as allowing you to use your crews through rain days which will in turn make you more money, but it will also increase sales in your business. Everything we discussed above are selling points for your customer.

    When you meet with your customer, inform them of this base installation method and let them know the benefits of this. It will quickly become apparent to them that you are the contractor that they will want to hire. Most other contractors are likely to not have switched to this method yet. Or perhaps some of them have, but they did not communicate this base installation method to their customer properly.

    This is where communicating the advantages of the open-graded base for their installation will help you sell more projects. It has helped us dramatically improve sales and most of our customers were never explained this method by any other contractor that they met with. By communicating this to them and making sure that they understand that this is the best base installation for them, they will be able to use that information when discussing their project with any other contractors.

  • Fewer Callbacks

  • With all of these advantages over a traditional base, it all leads to having fewer callbacks. This means less headaches, wasted time, and angry customers. This means more time to invest into current and future projects, happier customers, and better reviews for your business which leads to even more business coming in.

Steps to Install an Open-Graded Base

Every step here is crucial in preparing an open-graded base and should not be skipped. Without the proper preparation of your base in any scenario will cause problems for you in the future. Ensuring that your base is properly prepared will provide you with the ease of mind that your project will not move for years to come.

  1. Planning, Preparation, and Excavation

  2. With any project, traditional base or open-graded base, there is a planning and preparation process. This process is no different. Planning out your project with your client and preparing the necessary tools required to complete this project remains the same. As always, make sure that you are calculating the depth of your base, plus the depth of your bedding layer, plus the height of your pavers to know how much you will be excavating. We recommend a minimum of 6″ to 8″ for your ASTM #57 stone (3/4″ Crushed Stone) and a 1″ bedding layer of ASTM #8 (3/8″ or 1/4″ Crushed Stone). Most interlock for patios and walkways are 2 3/8″ in height, but some are a little less or a little more. Make sure to verify this before your excavation.

  3. Soil Identification and Amendment

  4. Most likely in North America you have one of two subsoils: clay or sand. In both scenarios an open-graded base is an excellent solution. You may want to consider doing a soil amendment before continuing with your base preparation. This allows the water bearing ability to increase.

    In a clay subsoil, water will get trapped in the clay which can lead to future problems with your base. By completing a soil amendment, you will allow some of that trapped water to filter further through your subsoil to allow for a stronger base.

    To do a soil amendment you need either a lime material or portland cement. We use 50 pounds for every 100 square feet that is being amended. With this spread around, we can then add some ASTM #57 (3/4″ Crushed Stone) in a single layer on top of the lime or portland cement. You can then compact your subsoil. By doing this, you will have broken up the clay and allowed for the water to continue through your subsoil and thus strengthening the base of your project.

    With how much clay is in our area, every job we come across we are doing an amendment. We make this very clear to our customers as this is a process that is rarely discussed or even done by any hardscaping company in our area. By communicating this with our customer and explaining why we do it, we can sell more jobs.

  5. Installing Geotextile Fabric

  6. With the soil amended, we can now install our geotextile fabric. This fabric comes in either woven or non-woven. Each has their own application. The woven geotextile is typically made of polypropylene woven together. This fabric is much stronger than the non-woven fabric and has a greater bearing ability. Using this in situations such as a raised patio where there is a larger amount of granular material that it is supporting or a driveway are the best applications of this material. This material impedes the passage of water, but water is still able to pass through it. It is important, with either of these materials, that your excavated subsoil is tapered away from any structure where you do not want water to build.

    woven geotextile fabric

    A non-woven fabric is a felt-like product that allows water to flow through at a faster rate than the woven fabric. It is important that you purchase at least the medium strength non-woven fabric which is thicker than the lesser strength fabric. It is ideal for walkways and patio base preparation.

    nonwoven geotextile fabric

    The installation of fabric is incredibly important in the preparation of any base as it provides extra support for your base material, as well as it acts as a barrier from allowing any contamination of your base material from your subsoil. This in turn prevents any future heaving from freeze-thaw cycles.

    Fabric is completely necessary in any open-graded base installation as the aggregates used in this installation have no fines to protect any further contamination of organics. If organics are able to flow through the open-graded aggregates, then this will cause your project to have problems in the future. Separating your soils from your base is important for this matter.

    Make sure that the fabric extends up the sides of your installation and wraps up and onto the top 6″ that extends past the edge of your interlock installation where the edge restraint will be installed. This prevents any organic material from the top and sides to filter down into your project.

    When laying the fabric, you will want to overlap the fabric by 2 – 3 feet when installing more than one piece. In addition to this, you will want to overlap the fabric so that the water following the slope of the subsoil will not slip underneath the fabric and will instead flow over. You will lay your fabric preferably horizontally or parallel with the foundation of the structure you are laying to starting at the furthest point and working your way towards the structure. This ensures that the fabric will shed water away from the structure and land on top of the next piece of fabric following the overlap instead of slipping underneath.

  7. Installing the Base

  8. Using the two open-graded aggregates that were discussed earlier in this post, we will prepare the base of our project by installing a 6″ to 8″ minimum of compacted ASTM #57 (3/4″ Crushed Stone) and a bedding layer of 1″ of ASTM #8 (3/8″ or sometimes 1/4″ Crushed Stone).

    When we are completing this step of the process, we install lifts of 2″ to 4″ and then compact. With the already high compaction rate of the ASTM #57 stone, you are able to install higher lifts before you use your plate compactor. This saves you time in the installation.

    Generally we install an 8″ base using ASTM #57 stone and install in 4″ lifts. After our first 4″ lift, we will install a biaxial geogrid to add extra support to our open-graded base. It is important if you do this step that you use the biaxial geogrid instead of the uniaxial. The difference being the function of them. Biaxial provides strength in both directions, up and down plus left and right, whereas the uniaxial provides strength in just one direction and must be installed in that direction when using it for a retaining wall.

    Installing the geogrid will provided added security and is an extremely inexpensive insurance for your project, not to mention another selling point for your customer as this is still not adapted by most contractors. Your geogrid will NOT overlap. It will be installed side-by-side if you are installing more than one piece. The geogrid will prevent any settling of the ASTM #57 stone from falling through and is especially helpful around the perimeter of your installation where most projects will begin to fail.

    Once our geogrid is installed, we can then install our next 4″ lift to complete our ASTM #57 8″ base and compact it using our plate compactor. With this complete, we can then install our bedding layer of 1″ ASTM #8 stone, screed, and lay our interlock. It is not necessary to compact your screeded bedding layer. The final compaction of the polymeric sand will provide the compaction required to settle your interlock.

    A lot of contractors have asked us about migration of the ASTM #8 stone into the voids of the ASTM #57 stone. This does not occur, as the voids are not as large as 3/8″ or 1/4″ chips that are being laid on top of it. Any void filling on the surface of your ASTM #57 stone base is completed in the screeding process of the ASTM #8 stone.

    Screeding ASTM #8 Stone

  9. Laying Interlock and Installing Polymeric Sand and Edge Restraint

  10. Interlock can now be laid on your bedding layer as with any installation. The best part of using these open-graded aggregates is that this process can be completed easily in the rain without ruining your bedding layer. Once your interlock has been laid and the stones are dry and there is no rain for the next 24 hours, you can then install your polymeric sand. If you walk on the stones, you will notice that they do not feel firm in the bedding layer. This will be experienced with any installation method and the final installation of the polymeric sand and compaction will provide the locking that interlock provides.

    A lot of contractors have asked whether the polymeric sand will migrate through your bedding layer. This will occur and there is a small loss of sand in this process, however it is almost unnoticeable and will not compromise the integrity of your installation. Some contractors install a non-woven geotextile fabric on top of their bedding layer before installing their interlock. However, we do not recommend this as it is an added cost and involves more time as well as being a pain to have to fix any minor adjustments on the go.

    Before the final compaction, you will need to install an edge restraint. The traditional edge restraint that will be used is a plastic edging that allows for a 10″ spike to be driven into them every foot or so. These spikes then corrode and stick into your base providing a stable restraint. However, these work in a traditional base where the spikes adhere to the fines within the base. In an open-graded base system, these spikes will not adhere to your base and this will cause them to pop-up over time.

    plastic edge restraint for pavers

    In an open-graded base, it is recommended that you install a concrete edge restraint where you will mix concrete and create a tapered collar from 4″ to 6″ away from the edge of the paver that starts on top of your base and tapers up to about 1″ of the height of your interlock. Some contractors mix fibers into this or even rebar to provide added strength to this concrete collar. An alternative product that is on the market for this particular purpose is Perma Edge. This is installed the same way as concrete, but it provides a more permeable solution and is much more flexible than concrete allowing for better performance during freeze-thaw cycles.

    Concrete Edge Restraint

    The final compaction will settle the stones minimally, between 1/16″ up to 1/8″. This may need to be taken into consideration before installing your interlock.


We hope that this was an easy to read guide on open-graded bases and that you are more confident than ever to get out there in the field and apply this to your business. With so many advantages and no drawbacks to installing the open-graded base, it is a no-brainer to be offering this to your clients.

It will help you become more efficient and it will improve sales with fewer callbacks from customers. With many of the leading hardscape manufacturers now promoting this method of installation, there is nothing that should be holding you back from beginning to install the open-graded base.

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