HPB vs. Granular A: What’s the Best Base for Interlock?

If you're researching how interlock should be installed in Ontario, you've probably come across two materials repeatedly: HPB and Granular A.

You may also hear one contractor say they install interlock using HPB while another says they use Granular A.

So which one is better?

The first thing to understand is that HPB and Granular A aren't necessarily direct substitutes for each other.

They are different aggregates with different properties, and depending on the type of interlock system being built, they can perform different roles.

At 4 Season Solutions, we use open-graded base systems for many of our interlock projects, typically incorporating angular clear stone for the structural base and a thinner layer of HPB as the bedding material.

But that doesn't mean Granular A is a bad material or that every interlock project should be built the same way.

The better question is whether the entire base system is appropriate for the property, drainage conditions and loads it needs to support.

What Is HPB?

HPB stands for High Performance Bedding.

It is a small, angular, washed aggregate commonly used as a bedding material underneath interlocking pavers.

Unlike aggregates containing a mixture of larger particles and fines, HPB contains relatively uniform stone with very little fine material.

That gives it an important characteristic:

Water can move through the voids between the stones.

The angular particles also interlock with one another when consolidated, creating a stable bedding surface for pavers.

For many of our projects, HPB is installed as a relatively thin bedding layer over the structural aggregate base.

That's an important distinction.

When we say we use HPB underneath interlock, we don't necessarily mean the entire excavation is filled with HPB.

What Is Granular A?

Granular A is a dense-graded aggregate containing a range of particle sizes, including fines.

Those smaller particles help fill the voids between larger pieces of aggregate.

When properly placed at the correct moisture content and compacted in appropriate lifts, Granular A can create a very dense and stable base.

It has been used successfully underneath interlock and many other construction applications for years.

The presence of fines, however, means it behaves differently around water than an open-graded aggregate.

Instead of having large interconnected voids that allow water to move relatively freely through the base, a dense-graded system relies much more heavily on proper grading, compaction and drainage management.

That doesn't make it wrong.

It simply means it is a different type of system.

So, Is HPB Better Than Granular A?

Not necessarily.

This is where the question can become misleading.

If someone asks whether they should use "HPB or Granular A under interlock," they're often comparing individual materials when they should really be comparing complete base systems.

For example, one system might involve:

prepared subgrade → geotextile → angular clear stone → HPB bedding → pavers

Another might use:

prepared subgrade → Granular A installed and compacted in lifts → bedding material → pavers

Both approaches can perform well when they're properly designed and installed.

The important question isn't simply:

"Do you use HPB or Granular A?"

It's:

"How are you building the entire foundation underneath my interlock?"

Why We Use Open-Graded Base Systems for Many Projects

For many of our projects in Southern Ontario, 4 Season Solutions prefers an open-graded base system where site conditions and the application make it appropriate.

After excavating to suitable material and preparing the subgrade, the system can incorporate geotextile separation fabric followed by angular clear stone.

The clear stone forms the structural portion of the base.

Because the aggregate contains very few fines, water can move through the voids within the stone rather than being held within a dense base.

The angular particles are consolidated so they lock together and create a stable foundation.

A thinner layer of HPB is then installed above the structural base to create the final bedding surface for the pavers.

We like this type of construction for many Ontario applications because our hardscapes have to deal with rain, snow, melting water and repeated freezing and thawing.

Water management matters.

But an open-graded system still has to be designed properly.

Simply using clear stone and HPB doesn't automatically create a good interlock installation.

Granular A Can Still Make an Excellent Base

It's important not to turn this into an argument that Granular A is somehow an inferior construction material.

It isn't.

A properly constructed dense-graded Granular A base can perform very well.

The quality of that installation depends heavily on preparation and compaction.

Granular A generally needs to be installed in manageable lifts and compacted properly rather than dumping a large amount of material into the excavation and compacting only the surface.

Moisture conditions can also affect how effectively dense-graded aggregate compacts.

When installed correctly over a suitable subgrade with proper drainage and grading, it can create a strong foundation for interlock.

The problem isn't Granular A.

The problem is when any base material is installed poorly.

Is HPB a Replacement for the Entire Base?

This is an important distinction because homeowners sometimes hear "HPB base" and assume HPB should simply fill the entire excavation.

That's not how we typically approach our installations.

For many of our open-graded projects, angular clear stone provides the structural base and HPB provides the final bedding layer.

Those materials are doing different jobs.

The structural base needs to support and distribute the loads placed on the finished surface.

The bedding layer creates the final surface on which the pavers are installed.

How thick each component needs to be depends on the project.

A pedestrian patio and a driveway carrying vehicles shouldn't automatically receive the same construction specification.

The application matters.

What About Screening or Limestone Dust?

Another material homeowners sometimes encounter is screening, stone dust or limestone screenings.

These fine materials have traditionally been used as bedding underneath interlocking pavers in some systems.

Our preference for many projects is HPB because of its angular structure and drainage characteristics.

But just like the Granular A discussion, the bedding material shouldn't be evaluated in isolation.

A good bedding layer can't compensate for an inadequate structural base underneath it.

If the excavation is too shallow, the subgrade is unsuitable or the base hasn't been prepared correctly, changing the top inch or two of bedding material isn't going to solve the larger problem.

What Role Does Geotextile Play?

Geotextile separation fabric is another component we commonly incorporate into our open-graded systems.

Its purpose is to help separate the native subgrade from the aggregate installed above it.

Without proper separation, soil and aggregate can migrate into one another under certain conditions, affecting the integrity of the system over time.

But geotextile isn't a substitute for proper excavation.

If unsuitable soil or organic material is present, simply covering it with fabric doesn't automatically create a suitable foundation.

The condition of the subgrade still matters.

The entire system needs to work together.

Why Drainage Matters in Ontario

One of the reasons base construction deserves so much attention in Ontario is our climate.

A driveway or patio can experience heavy rain in the fall, snow throughout the winter, melting periods, refreezing and then spring thaw.

Water is constantly interacting with the property.

If drainage hasn't been considered properly, problems can develop regardless of which aggregate was selected.

This is why we don't think homeowners should choose a base system based on one material name.

Drainage, grading and finished elevations need to be considered as part of the entire project.

• Where will surface water go?

• How does the new interlock meet the garage?

• Where does it meet the house?

• Are there steps or retaining walls involved?

• Does the surrounding lawn slope toward or away from the hardscape?

Those questions can be just as important as whether HPB or Granular A appears on the quote.

Driveways and Patios Don't Have the Same Requirements

Another mistake is assuming that every interlock project should receive the exact same base.

A backyard patio primarily experiences pedestrian loads.

A driveway repeatedly carries vehicles weighing thousands of pounds.

That difference matters.

Driveways generally require more substantial excavation and base preparation than pedestrian patios and walkways.

The pavers themselves may also differ.

For many residential driveway applications, 70 mm pavers are commonly considered, although the appropriate product should always be selected for the specific application and manufacturer requirements.

The base underneath those pavers needs to be designed for the loads it will experience.

This is why a contractor saying, "We always install exactly the same amount of base," isn't necessarily reassuring.

Different projects can require different specifications.

Why Base Construction Can Change the Price So Much

Most homeowners understandably focus on the paver because that's what they'll see every day.

But a significant portion of the cost of an interlocking project can be underneath it.

More substantial base construction can mean additional:

• excavation

• disposal

• aggregate

• machine time

• labour

• trucking

• compaction or consolidation

Once the project is complete, almost all of that disappears underneath the finished surface.

This is one reason two contractors can quote the same driveway and arrive at very different prices.

One contractor may be proposing a completely different foundation than the other.

Two interlock quotes aren't necessarily comparable simply because they're using the same paver.

How Should You Compare Interlock Base Specifications?

If you're comparing quotations, don't stop at asking whether the contractor uses HPB or Granular A.

Ask them to explain the complete system.

• How deep do you expect to excavate?

• What happens if unsuitable soil is found?

• Are you using geotextile?

• What material forms the structural base?

• How much structural base is being installed?

• How is it being compacted or consolidated?

• What material is being used for bedding?

• How are drainage and elevations being handled?

• Does the specification change for a driveway versus a patio?

• What type and thickness of paver is being installed?

  
  

Those answers will tell you considerably more than one aggregate name.

So, What's the Best Base for Interlock?

There isn't one base specification that should automatically be used on every property.

Granular A can create an excellent dense-graded base when it's installed and compacted properly.

For many of our projects, we prefer an open-graded system using angular clear stone for the structural base and HPB for the bedding layer.

But the material selection is only one part of the installation.

The quality of the finished project also depends on excavation, subgrade conditions, separation, drainage, grading, base depth, consolidation, bedding preparation, edge restraint and the paver itself.

That's why we wouldn't choose an interlocking contractor simply because they say they use HPB, Granular A or any other particular material.

Ask how the entire system is being built and why that system makes sense for your property.

Planning an Interlock Project in Pickering, Ajax or Whitby?

4 Season Solutions provides interlocking installation and repair throughout Pickering, Ajax, Whitby, Durham Region and the eastern GTA.

We work on driveways, patios, walkways, front entrances, steps, retaining walls and complete outdoor renovations.

Our approach is straightforward:

Start with a good design, excavate to suitable material, choose a base system appropriate for the property and build the project properly from the ground up.

If you're considering an interlocking project, we'd be happy to visit the property, discuss what you're trying to build and prepare a detailed quote.