A monolithic slab is a floor slab and its perimeter footing (the thickened edge) cast in one pour, and its rebar layout has three parts: continuous bars running around the thickened edge, a mat of bars across the slab, and bent bars or laps that tie the two together at the edge and at the corners. Because the thickened edge carries the walls, the 2024 Ontario Building Code treats it as a structural slab that has to be designed under Part 4, so the bar sizes, spacing and edge dimensions come from the designer's drawings, not a standard table.
This post explains how a thickened edge slab is typically reinforced, the Ontario code limits that decide whether you can use one, and how to get the steel detailed and ordered so it goes together in a single pour.
What a monolithic slab is (and is not)
You will see several names for similar things:
- Monolithic slab or thickened-edge slab: a slab on grade with a deeper section around the perimeter that acts as the footing for the walls, poured together with the slab.
- Slab with interior thickenings: the same idea with extra thickened strips or pads under interior bearing walls and posts.
- Raft or mat foundation: a thicker slab, often the same depth throughout, designed to spread the whole building load across the soil. Rafts usually carry heavier top and bottom mats and are firmly in engineered territory.
- Floating slab: a casual term for a slab on grade that is not tied to a frost-depth foundation. It is not a code term, so check what the person using it means.
The common thread is that the slab is doing structural work. That is the key difference from an ordinary basement floor, which Part 9 lets you pour without reinforcement.
Ontario code limits: frost and design
Two parts of the 2024 Ontario Building Code (O. Reg. 163/24) decide whether a thickened edge is allowed and who designs it.
It has to be designed. Article 9.16.1.2 says floors-on-ground that support loads from the superstructure must be designed under Part 4. A thickened edge carrying walls and a roof is exactly that. Our guide to slab-on-grade rebar requirements explains how that differs from an ordinary floor slab.
Frost depth still applies. Table 9.12.2.2 sets minimum foundation depths by soil type, drainage and whether the space is heated. In clay or soils that are not clearly defined, for example, the minimum is 1.2 m. A thickened edge is far shallower than that, so it only works where one of the exceptions applies, such as:
- foundations on rock, or on coarse-grained soils where the table sets no limit
- foundations insulated to reduce heat loss through the footings, treated as foundations with no heated space (Sentence 9.12.2.2.(2))
- foundations designed for lesser depths, or where local experience shows lesser depths are satisfactory (Sentence 9.12.2.2.(5)), which is how frost-protected shallow foundations are approved
- certain small accessory buildings that are not masonry, not more than 1 storey and not more than 55 m² in building area, among other conditions (Sentence 9.12.2.2.(6))
Garages have their own rules. Detached garages under 55 m², 1 storey and not of masonry or masonry veneer may be supported on a concrete slab at least 100 mm thick (Article 9.35.3.1). Attached unheated garages need foundations below frost level except as Section 9.12 allows, and where a slab-on-ground house has an attached garage slab, a construction joint is required between the two. Our post on garage slab rebar covers garage floors in more detail.
Your building department and designer decide which route applies on your lot. The rest of this post assumes the drawings show a thickened edge slab.
Typical thickened edge rebar layout
Every design is different, but most thickened edge drawings show the same elements:
| Element | What it is | What the drawings set |
|---|---|---|
| Edge bars | Continuous bars running the full perimeter inside the thickened edge, often at the bottom and sometimes also near the top | Number, size, position, laps |
| Edge stirrups or ties | Closed or open bent bars around the edge bars, where the design calls for them | Shape, size, spacing |
| Corner bars | Bent L-bars lapped with the edge bars so the steel is continuous around each corner | Leg lengths, size |
| Slab mat | A grid of bars each way across the slab | Size, spacing, height in the slab |
| Slab-to-edge bars | Slab bars bent down into the edge, or separate L-bars lapped with the slab mat | Shape, lap length |
| Interior thickenings | Extra bars in thickened strips or pads under bearing walls and posts | Bar count, length, ties |
| Re-entrant and opening bars | Diagonal or trim bars at inside corners, door openings and penetrations | Size, length |
A few points come up on almost every job:
- The edge continues under door openings. For garages, Article 9.35.3.1 requires foundations under the garage door portion as well, and on drawings the edge bars usually run straight through the opening. Step the edge down if the slab drops at the door, with the bars bent to follow.
- Cover differs between the edge and the slab. The bottom and sides of the thickened edge are cast against earth, which takes 75 mm of cover under CSA A23.1. The slab mat sits at the height the designer chose, often well above the base.
- Anchor bolts and hold-downs for the walls are cast into the thickened edge. Their positions have to clear the edge bars, so check the anchor layout against the steel before the pour.
How the slab ties into the edge
The connection between the slab mat and the edge is the detail most often improvised on site, and it should not be.
Common details include:
- Bent slab bars. Each slab bar running toward the edge is bent down into the thickened section. Simple, but every bar is a bent shape and the bend has to match the slope of the edge.
- Separate L-bars. Straight slab bars stop short of the edge, and L-shaped bars lapped with them turn down into the edge. Easier to place, and the L-bars can be made in one shape and repeated.
- Hooked bars over the edge bars. Slab bars hook around the top edge bars to tie the two together.
Laps between the slab bars and the L-bars need the lap length shown on the drawings. Our lap splice length chart explains how lap lengths are worked out under CSA A23.3.
Placing it all for one pour
A monolithic slab is placed in a single pour, so everything has to be in and inspected beforehand:
- Excavate and compact the edge trench and grade the slab base. Thickened edges are often dug with a sloped inner face; the bent bars have to fit that shape.
- Set the edge steel first on precast concrete blocks or sand-plate chairs at the bottom cover, with corner bars and laps tied.
- Place insulation, vapour barrier and under-slab services as the drawings show.
- Set the slab chairs and lay the slab mat at its height.
- Add the slab-to-edge bars, interior thickening steel, re-entrant and opening bars.
- Set anchor bolts and any dowels or embeds, and check everything against the drawings.
Our step-by-step guide on how to rebar a slab covers chairs, tying and protecting the mat during the pour, and how to lay and tie rebar in footings covers the edge steel.
Detailing and ordering monolithic slab steel
Because so much of a thickened edge slab is bent bars (corner bars, L-bars, stirrups, stepped edge bars), it is worth having the steel detailed and cut from the drawings rather than bent on site. A bar list with every mark, shape and length, tagged by location, lets the crew place the edge and slab steel without measuring each piece.
We provide estimating and rebar detailing, prepare bar lists and placing drawings, and cut and bend 10M to 55M bar to your drawings, with delivery across the GTA and Ontario. Engineering services are available for projects that need them; tell us about the project when you ask for a quote.
Send your slab and foundation drawings for a free take-off and quote, or see our services.
Frequently asked questions
What size rebar is used in a monolithic slab?
There is no code-set bar size. Because a thickened-edge slab carries the building's walls, the Ontario Building Code requires it to be designed under Part 4, and the designer sets the bar sizes. On light buildings the drawings usually show 10M or 15M bars.
Can you build a monolithic slab in Ontario?
Yes, but frost is the main constraint. The OBC sets minimum foundation depths by soil type and whether the space is heated, so a thickened edge only works where the design shows it is protected from frost or where the code allows shallow foundations, such as certain small detached accessory buildings.
What is the difference between a monolithic slab and a raft slab?
A monolithic slab is a floor slab with thickened edges, and sometimes interior thickenings, poured in one piece. A raft or mat foundation is a thicker slab designed to spread the whole building load over the soil, usually with heavier top and bottom reinforcement.
Does the rebar in the slab connect to the thickened edge?
Usually. Drawings commonly show the slab bars extended or bent down into the thickened edge, or separate L-bars lapped with the slab mat, so the slab and edge act together. The detail on your drawings sets which.
