PAVER REPAIR • BISMARCK, ND
Frost Heave: Why Pavers Lift in North Dakota
Pavers lift in North Dakota because water in the soil under them freezes, grows into layers of ice and pushes the ground up. It takes three things: soil that holds water, freezing temperatures and a supply of water. Bismarck has all three. Take away the water and give the pavers a deep, well-drained gravel base, and most heaving stops.
The good news: pavers can be pulled up and reset. Unlike a cracked concrete slab, you don’t have to throw them away.
THE CAUSE
What Frost Heave Is
Pavement Interactive, a pavement engineering reference, lists the “three elements necessary for ice lenses and thus frost heave”:
- Frost-susceptible soil, meaning soil with a lot of fine particles
- Subfreezing temperatures
- Water
Here’s how it works. Water gets pulled up through fine soil toward the freezing layer and freezes into flat sheets of ice called ice lenses. As those lenses grow, “the overlying soil and pavement will heave up.” When it thaws, the melting ice leaves the soil soft and wet, and it “tends to lose bearing capacity.” That’s when pavers sink and rock.
Remove any one of the three, and frost heave drops off. We can’t change the weather. We can change the soil right under the pavers and how much water gets to it.
LOCAL CONDITIONS
Why Bismarck Patios Get Hit Hard
Deep frost
The City of Bismarck lists the required frost depth as 48 inches below finished grade. A patio base is a few inches deep, so it sits right in the freezing zone every winter.
Fine, heavy soil
Many Bismarck yards sit on clay that holds water.
Spring melt on frozen ground
Bismarck averages about 50.5 inches of snow a year. When it melts over ground that’s still frozen, the water has nowhere to go but sideways, often right under the patio.
Freeze, thaw, refreeze
Warm March afternoons and cold nights mean the water under a patio can freeze and thaw over and over.
DIAGNOSE THE PROBLEM
What Your Pavers Are Telling You
| What you see | Likely cause |
|---|---|
| One spot lifts every winter | Water collecting there, often from a downspout, a sprinkler or a low spot |
| Pavers sink or rock in spring | Soil went soft as it thawed, and the base was too thin to bridge it |
| The whole patio is wavy | Base too thin, not packed in layers, or no fabric over the clay |
| Edge pavers spread and drop | Missing or failed edge restraint |
| Joints are empty | Joint sand washed out, so pavers lost their grip on each other |
| Pavers next to the house lift or tilt | Roof water or a slope running back toward the house |
| Mortared joints cracked and crumbling | Mortar can’t flex with frost |
BEFORE A REBUILD
What You Can Do Yourself
Move the roof water
Extend downspouts past the patio. The City of Bismarck asks homeowners to send gutter water into lawns or planted areas, not onto hard surfaces.
Check the sprinklers
Make sure no heads spray onto the patio or soak its edges.
Refill the joints
ICPI says to sweep dry joint sand into the joints and compact “until the joints are full.” Full joints keep pavers locked together.
Fix the grade around it
Soil should slope away from the house and away from the patio edges, so water doesn’t pool against them.
Wait for the thaw
Some lifting shows up only while the ground is frozen. Mark the high spots with chalk or flags, then check them again once the ground has thawed and dried in spring.
PERMANENT REPAIR
How Heaved Pavers Get Fixed for Good
If spots keep moving year after year, the fix is under the pavers. We follow the concrete paver industry’s construction guide, ICPI Tech Spec 2.
Pull up and stack the pavers
ICPI’s Tech Spec 10 says base problems can be reached “by removing and later reinstating the same pavers.” You keep your pavers.
Dig out the soft base
Wet, mixed-in soil and thin gravel come out.
Lay geotextile fabric
ICPI recommends it “where there is freeze and thaw, or over clay and moist silty subgrade soils.” It keeps clay from pushing up into the gravel.
Build a thicker gravel base
ICPI’s minimum for patios is 4 inches over well-drained soil. It says colder climates with wet or weak soils need bases “at least 2 to 4 in. thicker.”
Compact it in layers
of 4 to 6 inches, as ICPI specifies.
Slope it at least 1.5%,
about 3/16 inch per foot, away from the house.
Screed about 1 inch of bedding sand,
reset the pavers and compact them.
Install edge restraints
ICPI says they “maintain continuity and interlock among the paving units.”
Fill the joints and compact again
Fix the water source,
like a downspout or low spot, so it doesn’t happen again.
For full patio builds, see paver patios in Bismarck.
WHAT TO AVOID
Mistakes That Make Heaving Worse
Mortaring the joints
ICPI says mortared joints “have a high risk of deterioration” with freeze-thaw cycles. Flexible joints let pavers move with the ground.
Building on frozen ground
ICPI says base material shouldn’t be “placed over a frozen soil subgrade,” and frozen or soaked sand shouldn’t be used.
Skipping the fabric on clay
Without it, wet clay mixes into the gravel and the base gets weaker every year.
One thick dump of gravel
A base that’s packed once at the top stays loose at the bottom.
Pointing downspouts at the patio
It’s the first water source to check.
Questions
Frost Heave FAQ
Answers about lifted pavers, spring repairs, gravel bases and North Dakota frost.
Is your patio uneven?
Call and explain what you see. We can help you find the next step.
Why are my pavers heaving?
Water in the soil under them is freezing into ice layers that push the ground up. Frost heave needs fine soil, freezing temperatures and water. In Bismarck, the usual causes are a thin gravel base, clay soil, no fabric under the base, and roof or sprinkler water soaking under the patio.
Will heaved pavers go back down in spring?
Sometimes, partly. As the ice melts, the soil softens and loses strength, so pavers can settle back, but often not evenly. If the same spots lift or rock every year, the base needs to be rebuilt.
How deep is the frost line in Bismarck?
The City of Bismarck lists the required frost depth as 48 inches below finished grade. Burleigh County also lists a 4-foot frost line. A patio base is only inches deep, so it relies on drainage and a deep gravel base rather than depth.
Can heaved pavers be reused?
Yes. ICPI says the base can be repaired by removing and later reinstating the same pavers. That’s one of the biggest advantages of pavers over poured concrete in North Dakota.
Should I fix heaved pavers in winter?
No. ICPI says base material shouldn’t be placed over frozen soil, and frozen sand shouldn’t be used. Mark the problem spots in winter and fix them once the ground has thawed and dried.
Are pavers or concrete better for frost?
Pavers handle frost better in most yards. ICPI says the joints between pavers help reduce the cracking common in concrete, and that pavers can resist deicing salts better than conventional paving. When pavers do move, they can be lifted and reset instead of torn out.
Bismarck Landscape
Pavers Lifting or Sinking?
Send us a photo of the problem spot, or have us look at it after the thaw. We’ll find where the water is coming from and tell you whether it needs joint sand, a downspout fix or a rebuilt base. Call Bismarck Landscape.
See our contact options. For drainage problems around the patio, see yard drainage and grading in Bismarck.