About

A concrete apron (often called a blind area) is a protective strip poured around the perimeter of a building. Its primary function is to divert rainwater and snowmelt away from the foundation, preventing soil erosion and water damage to the basement. It also serves as a clean pathway for maintenance.

Calculating the material for an apron is slightly more complex than a simple rectangle because it wraps around the structure. The most common error is double-counting the corners (calculating overlapping strips). This calculator computes the exact geometric area of the frame around your house, ensuring you order the correct amount of concrete.

Formulas

The calculator determines the perimeter area using the house dimensions and the apron width.

House Perimeter:
P = 2(L + W)

Apron Area (Geometric):
Area = (P × Width_apron) + (4 × Width_apron²)

Volume:
V = Area × Thickness

The term $4 \times Width 2$ accounts for the four outer corners that extend beyond the simple perimeter strips.

Reference Data

Parameter	Standard Recommendation	Why?
Width	0.6 - 1.0 meters	Ensures water falls beyond the backfill zone.
Slope	2-5% (away from wall)	Critical for gravity drainage.
Thickness	10 cm (4 inches)	Sufficient for foot traffic.
Expansion Joints	Every 2-3 meters	Prevent cracking from thermal movement.
Gap to Wall	1-2 cm (filled with sealant)	Allows independent movement of house and apron.
Reinforcement	Wire Mesh (ø3-4mm)	Holds the strip together if ground settles.
Base Layer	10cm Sand + 10cm Gravel	Prevents frost heave.

Frequently Asked Questions

The blind area should slope away from the house at a rate of at least 2% to 5%. This means for a 1-meter wide apron, the outer edge should be 2 to 5 cm lower than the edge against the house.

No. The apron and the foundation settle at different rates. You should leave a small expansion gap (1-2 cm) between the apron and the wall, filled with a flexible waterproof sealant or bituminous strip.

This tool is designed for a simple rectangular footprint. For complex shapes (L or U), break the perimeter into straight lines. Calculate (Total Length of exposed wall * Width) + (Number of Outside Corners * Width²).

If the apron is narrower than the roof overhang, water dripping from the roof will erode the soil right at the edge of the concrete, potentially undermining the apron itself. Ideally, the apron should extend 20-30cm beyond the roof overhang.