How To Fill in Gaps in a Flagstone Patio (Multiple Ways)

A beautiful and safe flagstone patio is a great area for casual hangouts outside of your home. However, sometimes gaps can appear between the stones themselves: leaving you wondering how to fill in the gaps in a flagstone patio.

Gaps in a flagstone patio can be filled in using smaller rocks, mulch, sand, plants, and polymeric sand.

If you spy cracks or gaps between the flagstones of your patio, you’ll be happy to know there are many decorative options to fill them in. In this article we’ll cover what to use when filling in the gaps and how to fix the gaps themselves.

What Do You Use to Fill in Between Flagstone?

There are many materials to use when you’re filling in the gaps between each flagstone. Take a look at the list below for the most optimal choices:

  • Sand – Sand is a cheap and easy-to-apply option when filling in the gaps between flagstones. One downside to the sand is that it will need to be replaced, as time causes sand to erode.
  • Rocks – It may sound strange to fill in the faps between rocks with more rocks, but gravel, smaller stones, and crushed gravel are great options to fill in the gaps. They are practical, preventing weed growth, as well as aesthetically pleasing. 
  • Mulch – Crushed bark and wood mulch usually come in vibrant reds or rick browns, which look great against the neutral backdrops of flagstones. Mulch is known to repel insects as well as weed growth.
  • Plant Life – There are certain types of plant life besides weeds that should be welcome filling in the gaps between patio flagstones. Moss and grass turf not only look great, but can cover larger distances between flagstones.
  • Polymeric Stone Dust – The most useful option to fill in the gaps between flagstones, polymeric stone dust accomplishes all the benefits of the above options while also being great for filling in less uniform, varied flagstone gaps.

Deciding on which filler to use for your patio is a decision that must take into account not only aesthetics, but the amount of recurring work you’ll need to put into the patio as gaps may return.

What is the Best Product to Put Between Flagstone?

The best product to put between flagstones when there are gaps in your patio is polymeric sand.

Polymeric sand is a type of gardening product which is made not only from regular sand, but an acrylic binder. It prevents the growth of weeds, the infestation of insects, and does not erode as easily as regular sand.

Check the list below for benefits of polymeric sand:

  1. Polymeric Sand is Easily Installed in a Patio
  2. Polymeric Sand Prevents Infestation
  3. Polymeric Sand Does not Disappear Easily

Let’s take a closer look at these advantages which make polymeric sand the best product to put between flagstones!

Polymeric Sand is Easily Installed in a Patio

The only things needed to install polymeric sand in a patio is a garden hose, a broom, and the polymeric sand itself!

The binder mixed into the sand causes polymeric sand to solidify once it comes into contact with water. Simply use your broom to sweep polymeric sand into the cracks between each flagstone. Then, hose the flagstone patio down. The binding will solidify, and your installation is complete!

Polymeric Sand Prevents Infestation

Polymeric sand’s binding component means that ants and other mining insects will not be able to chew through your patio on their way to your picnic lunch or other outdoor gatherings.

For the same reason, weeds and other harmful plant life will have a hard time growing under the patio or between the flagstones.

Polymeric Sand Will not Disappear Easily

Polymeric sand’s hardening component (definitely it’s greatest advantage!) also keeps it from eroding like regular sand or even gravel and mulch might. Rainfall typically leads to the washing away of less stable material, but polymeric sand is made to harden in water.

As a result, polymeric sand takes much longer to wash away than it’s lesser filler rivals.

How Do You Fix a Gap in a Flagstone Patio?

A gap in a flagstone patio can be solved using any kind of filler material that will keep out insects and weeds as well as closing up the space between each flagstone. Check the list below for more specific filling methods!

  • Level – Be sure that the area you are attempting to fill in is as level as it can be. This means that no matter what you plan on filling gaps with, you need to be sure that there aren’t fragments of your flagstones jammed into the gaps.
  • Pour Material – Whether you’re using wood, mulch, polymeric sand, or even seeding your gaps with turf, you’ll need to pour the material into the gaps themselves. It isn’t necessary to remove every flagstone unless you’re planning on using mortar.
  • Clean Up – Again, no matter what material you use, there will be some cleanup necessary! If stray pieces of mulch are left scattered across your flagstones, or gravel is poured helter-skelter around the patio, it won’t matter that the gaps are filled in.

Can You Put Mortar Between Flagstone?

According to Landscaping Network, putting flagstone fragments into wet mortar is possible! As long as you have an existing base, such as one made of compact gravel, sand, or more concrete.

After placing the flagstone into wet mortar, there will still be some gaps, and additional mortar will need to be applied as a kind of grout.

Make sure before attempting to apply mortar that any existing mortar, or cracked pieces of flagstones, are removed. This can be done with a chisel or a screwdriver. 

Once this is finished, mix three parts of sand into one part of cement. Add water at a slow pace until the mixture is at the rough thickness of peanut butter.

With this mortar mixture, fill in the gaps between each flagstone, making sure to thoroughly seal the area. If you mess up a little, and there is excess mortar, be sure to have a sponge handy and wipe the extra mortar away before it dries!


Filling in the gaps of a flagstone patio can be accomplished with smaller, poured materials such as plant life, gravel, sand, or polymeric sand. In terms of stability and prevention of weed and insect invasion, polymeric sand is the best product to fill gaps in with on your flagstone patio!

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How To Fill in Gaps in a Flagstone Patio (Multiple Ways)
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