A frost apron is a strip of gravel, concrete, or paving directly around a foundation that prevents frost from creeping under the foundation and lifting it up. It is a protective border that ensures the ground immediately next to the foundation does not freeze deeply. Especially with shallow foundations (such as for garages and garden sheds), a frost apron is a simple but effective protection.
Why is a frost apron needed?
Water in the ground expands when it freezes — by approximately 9% in volume. If the ground under a foundation freezes, the foundation can be pushed upward. This is called frost heave or frost damage.
With a deep foundation (60-80 cm below ground level), this is not a problem because the foundation sits below the frost line. But with a shallow foundation or pier foundation, frost can get under the foundation.
A frost apron works as follows:
1. The strip around the building insulates the ground
2. Frost penetrates less deeply next to the foundation
3. The frost line is effectively “raised” around the building
When do you need a frost apron?
| Situation | Frost apron needed? |
|---|---|
| Strip foundation (60-80 cm deep) | No — already below the frost line |
| Shallow pier foundation | Yes — frost can get underneath |
| Concrete slab on ground | Yes — slab does not sit deep |
| Garage with shallow foundation | Yes |
| Garden shed on piers | Yes, or set piers deep enough |
How to make a frost apron
Method 1: Gravel bed
The simplest method:
1. Dig a strip around the foundation, 40-60 cm wide and 30-40 cm deep
2. Fill with coarse gravel (16-32 mm)
3. The gravel drains water away, so there is less water that can freeze
Method 2: Insulation boards
More effective than gravel:
1. Dig a strip 60-80 cm wide around the building
2. Lay XPS insulation boards (pressure-resistant) at approximately 20-30 cm depth
3. Cover with soil or paving
4. The insulation prevents frost from penetrating deeply next to the foundation
Method 3: Paving
A practical solution:
1. A path or patio around the building
2. The hardscape (tiles, pavers) combined with a sand bed slows frost penetration
3. Optionally combine with insulation underneath for optimal effect
Dimensions
- Width: Minimum 40 cm, preferably 60-80 cm
- Insulation depth: 20-30 cm below ground level
- Insulation thickness (XPS): Minimum 50 mm, preferably 80 mm
Frost apron for garages and foundations
When building a garage on a concrete slab, a frost apron is a cheap insurance:
- The concrete slab often sits only 10-20 cm in the ground
- A frost apron of 60 cm wide with XPS insulation prevents the edges of the slab from being lifted by frost
- Combine the frost apron with good drainage (slope away from the building)
Related terms
- Strip foundation
- Frost line
- Concrete slab (floor slab)
- Pier
- Slope (drainage gradient)