Flagstone Crack Filler: Repair Flagstone Patios and Paths

A flagstone patio or garden path has a relaxed, natural look, until a few stones crack, some joints crumble, and weeds move into every gap. Fixing it depends on how the flagstone was laid in the first place. A flagstone crack filler takes care of cracks through the stones themselves; the joints between them need a different approach depending on whether they're mortared or dry-laid.

For small, stable cracks in a solid flagstone, a no mix concrete crack filler suited to stone fills the crack neatly with no mixing. Loose stones and crumbling joints take a little more work, and I'll cover both.

The Short Answer

Fill small, stable cracks in individual flagstones with a stone-suited crack filler: clean, fill, level and mist. Repair mortared joints by repointing with fresh mortar, and refill dry-laid joints with polymeric sand, gravel or soil depending on the look you want. Reset or replace flagstones that rock, have sunk, or are broken into pieces.

Mortared vs. Dry-Laid Flagstone

Mortared flagstone

Stones set in mortar on a concrete slab, with mortar joints between them. Cracks in the slab below often show up through the stones and joints above.

Dry-laid flagstone

Stones set on a compacted gravel and sand base, with sand, gravel, soil or plants in the joints. Stones shift more easily but are simple to lift and reset.

Knowing which you have tells you how to fix the joints.

Why Flagstones Crack and Loosen

• Freeze-thaw, especially in porous stone like sandstone
• Natural seams in the stone that split under stress
• A cracked slab underneath a mortared patio
• Settling base under dry-laid stone
• Thin stones used where they carry weight
• Roots pushing up from below

What Needs Fixing?

A crack through one stone

If the stone is firmly set and level, fill the crack. If it's broken into pieces or loose, reset or replace it.

Crumbling mortar joints

Rake out loose mortar and repoint with fresh mortar.

Weedy dry-laid joints

Pull weeds and refill with polymeric sand, fine gravel or soil.

Many loose or cracked stones on a mortared patio

The slab underneath may be cracked or settling. A hardscape contractor can tell you whether a section needs rebuilding.

Choosing a Flagstone Crack Filler

• Suited to natural stone and masonry
• Precise application to avoid staining the stone face
• Weather-resistant once cured
• Works on flat surfaces, which flagstone always is

The Crack Repair Filler by VALYA is ready to use, water-activated, and suitable for stone, masonry, brick, concrete and asphalt. For background on how water-activated fillers work, see our guide to powder concrete crack filler. It's also our pick as the best concrete crack filler for concrete steps or walks next to the patio.

How to Fill a Crack in a Flagstone

1. Clean out the crack with a thin screwdriver and stiff nylon brush.
2. Blow or vacuum the dust.
3. Let the stone dry fully.
4. Fill the crack with the pointed applicator.
5. Level it flush with a putty knife and wipe excess off the stone right away.
6. Mist lightly with water and let it set.

How to Reset a Loose Flagstone

Dry-laid

1. Lift the stone with a flat bar.
2. Add base sand and level it.
3. Set the stone and tap it down with a rubber mallet until it's level and doesn't rock.
4. Refill the joints.

Mortared

1. Chip out the old mortar under and around the stone.
2. Set the stone in fresh mortar, level it, and fill the joints.
3. Keep it damp and shaded while it cures, per the mortar directions.

Mistakes With Flagstone

• Filling a crack in a loose stone instead of resetting it
• Using rigid filler in dry-laid joints, where stones need to move a little
• Smearing mortar or filler across the stone face
• Acid cleaners on limestone or other sensitive stone
• Ignoring a cracked slab under a mortared patio

FAQ

How do I fix a cracked flagstone?

If it's solid and level, clean and fill the crack with a stone-suited filler. If it's broken or loose, reset or replace it.

What should I put between flagstones?

For mortared patios, mortar. For dry-laid patios, polymeric sand, fine gravel, soil or low-growing plants.

Why do my flagstones keep coming loose?

On mortared patios, often a cracked or moving slab underneath. On dry-laid patios, a settling or washed-out base.

Can I use the same filler on stone and concrete?

If the product is suited to both. The VALYA filler works on stone, concrete, masonry and brick.

Wrapping Up

A flagstone crack filler fixes small, stable cracks in individual stones. Repoint mortared joints, refill dry-laid joints, and reset loose stones on a good base.

If your flagstone patio or path has small cracks, take a look at the Crack Repair Filler by VALYA and see if it fits your project.