How do I prepare the ground for a concrete patio in NB?

Proper ground preparation is the most critical step in building a concrete patio that will last through New Brunswick winters — more important than any other single factor, because a properly prepared base prevents the frost heave, settling, and drainage problems that destroy NB patios from beneath.

Start by marking out the patio area and excavating to a total depth of 10-12 inches below your desired finished surface height — this accounts for 6 inches of compacted gravel plus 4 inches of concrete. In areas with clay-heavy soils (common in Fredericton, Moncton, and the Saint John River Valley), excavate an extra 2 inches and replace with additional granular material. Remove all topsoil, sod, roots, and organic material completely. Organic material decomposes and creates voids beneath the slab, leading to settlement and cracking. Do not pour concrete over any organic material under any circumstances.

Granular base is non-negotiable in NB. Spread and compact a minimum 6-inch layer of crushed limestone (also called 3/4-inch clear or crusher run) across the entire excavated area. Compact in 2-3 inch lifts using a plate compactor — do not try to compact 6 inches in one pass. The goal is a firm, stable base that does not shift under foot pressure. A properly compacted granular base serves two purposes in NB's climate: it provides load-bearing support for the slab, and it acts as a drainage layer that allows water to move away from beneath the slab rather than accumulating and freezing.

Slope the sub-base for drainage. As you prepare the gravel base, establish the drainage slope — typically 1/8 to 1/4 inch per foot sloping away from the house. This slope should be built into the granular base and maintained through to the finished concrete surface. Water that pools against your foundation is a serious problem in NB, particularly during spring thaw when saturated soil increases hydrostatic pressure on foundation walls.

Form setting and final checks before pouring. Set your forms (2x6 lumber for a 4-inch slab, with stakes driven into the granular base) to your finished height lines. Check that forms are straight, level in the direction across the patio, and sloped in the drainage direction. Place wire mesh or rebar on chairs at mid-slab height before the pour — not on the ground. With NB's short working season, May through September is the ideal window for this work. Avoid starting excavation when the ground is saturated from spring thaw (typically April in southern NB, May in northern areas like Bathurst and Miramichi) — wet, soft soil does not compact properly.