Concrete patios in London
A patio in London is a simpler build than in most of our service area, and the simplicity hides the one thing that actually needs attention.
Flat ground means no terracing. No cutting into a slope, no retaining wall, no drainage stone and drain line behind a structure holding back an upper grade. That is a substantial saving compared with the escarpment cities, and it means the budget goes into the patio itself rather than into the site work around it.
What flat ground does not give you is drainage. Nothing here drains by accident, and London sits on clay, which holds whatever water is left on it. So falls have to be designed into the slab and a discharge point chosen deliberately, and the surface has to be pitched away from the house and kept below the sill. A patio that ponds is a patio that stays wet, grows algae on the shaded side, and works on the freeze-thaw cycle every winter at the exact point where it meets the foundation.
The other thing worth settling before the pour is everything that has to pass through it. Footings for a covered structure or pergola, because a four-inch patio slab is not designed for point loads and cannot be adapted afterward. A sleeve for a gas line to a grill. Conduit for lighting. Each is inexpensive at the right moment and each means cutting a finished surface if it is remembered later.
Heritage applies to part of the city. In a conservation district a patio visible from the street can be an exterior alteration requiring a heritage alteration permit, while a rear terrace screened from public view generally is not. On a valley-adjacent lot the Upper Thames River Conservation Authority may also regulate site alteration under Ontario Regulation 41/24. Both are quick to confirm and awkward to discover late.
Beyond that it is standard flatwork: four inches over a compacted granular base, control joints cut at the right spacing so the cracking lands where it was planned, and an isolation joint where the slab meets the foundation.
How a London patio drains has to be designed rather than assumed, because the ground will not do the work. Under most of the city there is very stiff silt and clay with very low permeability sitting over compact sand, which means water arriving at the slab does not soak away, it runs along the surface and collects in the granular base underneath if that base has nowhere to discharge. A patio built over a sub-base with no outlet is a reservoir with a lid. Falls across the slab away from the house, and a base that drains to a chosen point rather than into the surrounding clay, are what keep it from freezing and lifting each winter.
We also pour the driveways and pool decks that get done in the same season, and cover the rest of our London service area.
East along the 401 the ground and the yards change, covered on concrete patios in Cambridge. Further east the tri-city lots and permit process differ again, dealt with on stamped concrete patios in Kitchener.
