Short answer
A concrete slab is chosen for the load it carries. A published Ontario price guide lists basic slabs at about $3.80 to $5 per square foot (an estimate, not a quote). Garage floors and shed pads are commonly about 4 inches thick on a compacted gravel base, while heavier loads such as hot tubs or vehicles need more.
Common slab and pad jobs
- Garage floors. New floors for detached or attached garages, or replacement of cracked ones.
- Shed and workshop bases. A flat, durable base that keeps structures dry and level.
- Hot tub pads. Higher loads and the need for flatness mean thicker, reinforced slabs.
- A/C and generator pads. Small, level pads that keep equipment out of snow and water.
- Bin and storage pads. Simple outdoor slabs for utility use.
- Basement floors. Interior slab work is specialised. See the note below.
What decides thickness and reinforcement
| Use | Typical thickness (general guide) | Notes |
|---|---|---|
| Shed or storage base | About 4 inches | Compacted base and level surface are key. |
| Garage floor | About 4 inches, thicker for heavy vehicles | Slope toward the door so water drains out. |
| Hot tub pad | About 4 to 6 inches with reinforcement | Check the tub's weight with water and people. |
| A/C or generator pad | About 3 to 4 inches | Often small; confirm manufacturer clearance rules. |
| Heavy vehicle or equipment | 6 inches or more with engineered reinforcement | Have a professional specify the design. |
Base, drainage and frost
A slab is only as good as what it sits on. A properly compacted granular base prevents settling, helps drainage and reduces the chance that frost heaves one corner of the slab. Garage and shed slabs in Ontario may also be tied to or isolated from foundations depending on the structure, which is a decision that belongs with a contractor or designer.
Exterior pads should be sloped slightly so water does not pool, and joints should be placed to control cracking. As with other flatwork, air-entrained concrete is the usual choice for exterior surfaces exposed to freezing. See the driveway page for what a good build includes.
Permits and structures
A slab by itself does not usually need a permit, but a garage or shed built on it may, and zoning rules can apply to size and setbacks. The City of London publishes guidance on common homeowner projects, so check before building a structure on a new pad.
Basement floor replacement and structural foundation work are different jobs involving waterproofing, drainage and sometimes engineering. They are outside this page, and a contractor who offers them should be able to show relevant experience and insurance.
Cost
The HomeStars Ontario price guide (updated August 2026, Ottawa-based contractors) lists concrete slabs at about $3.80 to $5 per square foot, with site preparation at about $0.40 to $2 per square foot and reinforcement installation at about $50 to $360. These are ESTIMATE ranges, not London quotes. Small pads often cost more per square foot because of minimum charges for concrete delivery. See the cost guide.
Frequently asked questions
How thick should a garage floor be?
About 4 inches is common for cars, and thicker for heavier loads. A contractor should recommend the thickness and reinforcement for your use.
Do I need a permit for a concrete slab?
Often not for the slab alone, but a structure on it may need one. Check with the City of London.
Can concrete be poured in cold weather?
Yes with protection, but it costs more and most outdoor pads are poured from spring to fall.
How level should a hot tub pad be?
Very level, and strong enough to carry the full weight of the tub, water and people. Check the manufacturer's requirements.
Will a shed slab crack?
All slabs can crack. Joints and a good base help control where and how much.
Can you pour a slab over an old one?
Sometimes a bonded overlay is possible, but a failed base usually means removal and replacement. See repair and resurfacing.