A single mat slab has one grid of rebar, usually in the upper part of a slab on grade or near the bottom of a simple span. A double mat slab has two grids, one near the top face and one near the bottom, so it can resist bending in both directions and control cracking on both faces. Most residential slabs on grade that are reinforced use a single mat. Suspended slabs, mat foundations, thick industrial floors and slabs over heavy loads often need two. The drawings decide which, and the two layouts are not interchangeable.
This post explains what each layout does, which slabs get which, and what changes on site when the drawings call for top and bottom steel.
What a rebar mat is
A mat is a grid of bars running in two directions at right angles, one layer laid on top of the other and tied where they cross. So even a "single mat" contains two layers of bars:
| Term | Meaning |
|---|---|
| Layer | One set of parallel bars running in one direction |
| Mat (grid) | Two layers at right angles, tied together |
| Single mat | One grid in the slab |
| Double mat (T&B) | A grid near the top face and a grid near the bottom face |
| EW | Each way: the same bar and spacing in both directions |
| BEW / TEW | Bottom each way / top each way |
On drawings, a double mat slab might read 15M @ 300 T&B or show separate callouts like 15M @ 250 BEW and 10M @ 300 TEW. Detailers label the four layers B1, B2, T2 and T1 from the bottom up. Our post on reading slab reinforcement details explains the callouts.
Why a slab needs one mat or two
Steel goes where concrete would crack in tension. Which face is in tension depends on how the slab bends.
A slab that bends one way needs steel on one side. A simple span between two walls sags in the middle, so the bottom is in tension and the bars go near the bottom. A slab on grade mostly needs steel to hold shrinkage and temperature cracks tight at the surface, so its single mat sits in the upper part of the slab.
A slab that bends both ways needs steel on both faces. A suspended slab that is continuous over beams, walls or columns sags between supports (tension at the bottom) and hogs over them (tension at the top). A raft foundation pushes up between columns and down at them. A slab on soft ground under heavy point loads can bend either way depending on where the load sits. In all of these, one mat cannot be in the right place for every condition.
Thick slabs also have a crack control reason for two mats. A single grid near one face does little for cracking at the other face of a deep slab, so designers put a layer of distributed steel near each face.
Our post on where rebar should sit in a concrete slab goes deeper into position and cover.
Which slabs get which layout
| Slab | Typical layout | Why |
|---|---|---|
| Residential slab on grade (basement, garage), where reinforced | Single mat in the upper part of the slab, or mesh | Holds shrinkage cracks tight at the surface |
| Driveways and exterior flatwork, where reinforced | Single mat | Same; joints and concrete quality matter more |
| Thickened-edge or monolithic slab | Single mat in the field, extra bars in the thickened edge | Edge acts as a footing |
| Simple one-way suspended slab | Bottom mat, with top bars over supports | Bottom tension at midspan, top tension over supports |
| Continuous or two-way suspended slab | Bottom mat plus top steel over supports, often a full top mat | Bending reverses over beams, walls and columns |
| Mat foundation or raft slab | Double mat | Bends both ways between and under columns or walls |
| Thick industrial floor or equipment pad | Single or double mat per design | Heavy loads, crack control on both faces |
| Parking decks, tank bases, transfer slabs | Double mat, engineered | Reversing loads, exposure and crack control |
Two points are worth stressing. First, "top bars over supports" are not always a full top mat; in many suspended slabs, top steel is placed only in bands over beams, walls and columns, and the drawings show where each run starts and stops. Second, apart from one narrow Part 9 case (small cold-room slabs in basements), suspended slabs in Ontario are designed by an engineer under Part 4 of the Building Code; our post on rebar for suspended and elevated slabs covers what those drawings show.
For comparison, walls follow a similar logic. CSA A23.3 (the 2004 edition is the one we checked) requires bearing and shear walls thicker than 210 mm to have reinforcement in two layers, with exceptions for exterior basement walls and retaining walls. Slabs have no single thickness trigger like that; the number of mats comes from the design. As a practical signal, the RSIC Manual of Standard Practice notes that two layers of welded wire in slabs on grade are usually specified only on slabs over about 200 mm thick.
Spacing rules for a rebar grid
Whether a slab has one mat or two, the bar size and spacing in each layer come from the drawings. CSA A23.3 sets the outer limits for structural slabs. In the 2004 edition:
- Minimum reinforcement in slabs is 0.002 times the gross concrete area in each direction (Clause 7.8.1), spaced no farther apart than five times the slab thickness or 500 mm, whichever is smaller (Clause 7.8.3).
- Principal (main) reinforcement is limited to three times the slab thickness or 500 mm. Our post on maximum rebar spacing under CSA A23.3 explains those clauses.
In a double mat slab, each mat has its own spacing and the two mats do not have to match. A raft might carry heavier bars at the bottom under columns and lighter bars at the top, or the reverse. Our rebar spacing guide for slabs shows how to lay out a grid from a callout.
What changes on site with a double mat
A second mat is more than twice the bar. It changes how the slab is built:
- Supports for the top mat. The bottom mat sits on chairs or blocks; the top mat needs high chairs, continuous supports or standees that hold it at the specified height above the bottom mat. Our post on high chairs for double mat slabs and walls covers the options.
- Working out the heights. Each mat is set by its cover to the nearest face, and the space between mats is what is left. With 15M or larger bars in a thinner slab, four layers plus cover can use up most of the depth, so check the stack before ordering supports.
- Placing sequence. Bottom mat first, then supports, then the top mat. Embedded items, sleeves and conduit go between the mats before the top steel closes off access.
- Edge bars. At free edges, U-bars or hooked bars often tie the top and bottom mats together. They need to be on the bar list.
- Walking on the steel. A top mat gets walked on during placing and pouring. Enough supports and ties keep it from being pushed down, which would reduce the effective depth of the top steel.
- Concrete placement. Two mats leave less room for the concrete to flow, so aggregate size, slump and vibration need attention, especially at laps where bars double up.
Common mistakes
- Treating a double mat as a single mat in the middle. Putting all the steel at mid-depth puts it in the wrong place for both faces.
- Swapping top and bottom callouts on a slab with different bars in each mat.
- Top steel stopped short of where the drawings show it extending past supports.
- Top mat crushed down by foot traffic or pump hose movement because supports were too far apart.
- Laying a single mat on the base and pulling it up during the pour. It ends up at the bottom, where it does nothing for surface cracks.
If you are unsure whether a slab needs one mat or two, the answer is on the structural drawings or with the engineer. It should never be decided by eye on site.
Getting slab mats detailed and cut
Whether your slab has one mat or two, we prepare the bar list and placing drawings from your structural drawings, cut and bend the bars, tag them by mat and layer, and deliver across the GTA and Ontario. Installation is available. Send us your drawings for a free take-off and quote, and see our rebar products for what we supply.
Frequently asked questions
What is a rebar mat in a slab?
A rebar mat is a grid of bars in two directions, one layer laid across the other and tied at the crossings. A single mat slab has one grid; a double mat slab has one grid near the bottom and another near the top.
When does a slab need two mats of rebar?
When the slab bends both ways or needs crack control on both faces: suspended slabs over supports, mat foundations and rafts, thick industrial or equipment slabs, and slabs designed for heavy point loads. The structural drawings decide; it is not a field choice.
What does T&B mean on slab drawings?
T&B means top and bottom: the slab has a mat near each face. Each mat usually has its own bar size and spacing callout, such as 15M @ 300 T&B or separate TEW and BEW callouts.
What is the spacing for a rebar grid in a slab?
Spacing is set on the drawings. For structural slabs designed to CSA A23.3, principal bars can be no farther apart than three times the slab thickness or 500 mm, and minimum slab reinforcement no farther apart than five times the thickness or 500 mm.
