Coverage is the sheet minus the lap, on both axes
A sheet that measures 8 by 20 feet does not cover 160 square feet of slab once it is part of a run. Every joint takes a lap, and the lap comes off one edge of each sheet in each direction, so the sheet that follows only advances the coverage by its length minus the lap. At an 8 inch lap that is 19 feet 4 inches of advance for a 20 foot sheet and 7 feet 4 inches for an 8 foot width, which is 141.8 square feet of effective coverage against 160 bought — a little over 11 percent given up before any offcut.
The arithmetic for the count follows from that. Subtract one lap from the field dimension, divide by the effective advance, round up. The subtraction at the front is there because the first sheet in a run contributes its whole width, and only the ones after it are reduced.
Laps are the small half of the waste
Lapping costs about 11 percent of a 20 by 8 foot sheet at an 8 inch lap, which is the figure people expect. What they do not expect is the rounding. A mesh field 29 feet 8 inches long, covered by sheets that advance 19 feet 4 inches each after the first, needs exactly 1.5 sheets and therefore gets two, and the surplus half sheet is spent whether or not it is used. Multiply that across both directions and a 702 square foot field takes 1,280 square feet of sheet — 82 percent over, of which only a small part is lap.
That is why the results carry two numbers: the order figure, which places every piece whole and is how mesh is normally bought and counted, and the floor, which is what the same field would take if every offcut were re-used somewhere. On the default slab those are eight sheets and five. The gap is a sheet size that does not suit the slab rather than a lap requirement, and the cheapest thing to try before ordering is turning the sheet the other way round.
Laps get written in cross wires, not inches
Specifications for welded wire reinforcement very often express the lap as a number of cross wires rather than as a dimension, because what the lap has to do depends on where the wires fall rather than on how far the sheets overlap. On mesh with 6 inch spacing, a lap that reaches past two cross wires is somewhere between 6 and 12 inches depending on how you count and where the sheet was cut.
This page takes an inch figure and shows you what it comes to in squares at the spacing you entered, so you can check it against a specification written the other way round. It supplies no lap of its own, because the requirement depends on the style of mesh, the wire spacing and the code your jurisdiction has adopted, and none of those are things a calculator should be guessing.
Sheets or rolls
| Flat sheets | Rolls | |
|---|---|---|
| Lies down | Immediately | After the curl is flattened and weighted |
| Waste | The part sheet at the end of a run | The remainder on the last roll |
| Handling | One piece at a time, two people | Rolled out, then cut to strips |
| Suits | Most slab work | Long continuous runs |
The curl is the practical difference and it is worth more than it sounds on a small pour. A rolled sheet that has not been flattened lifts at the ends, sits at the wrong height under the concrete, and takes people and weights to hold down. On a garage-sized slab the labour to deal with that can exceed whatever the roll saved.
The support question, which is the whole question
Mesh sitting on the subgrade contributes nothing, and this is the single most common defect in slab reinforcement. It has to be at whatever height the design specified, which means chairs or supports at a spacing that keeps it there while people walk on it and while concrete lands on it.
The method of hooking it up with a rake during the pour is the alternative everybody has seen. It works while somebody is genuinely doing it and it stops working the moment the pour gets busy, which is exactly when it matters, and the parts nobody was watching are the parts where the mesh stayed on the ground. Supports cost very little against the cost of a slab with the reinforcement in the wrong place.
What height it belongs at, and whether mesh is the right reinforcement for the slab at all, are decisions from the drawing and from whoever designed it. This page counts sheets and weighs them. The sharp wire ends left by cutting are at shin height across an open slab, and wet concrete burns skin chemically over hours rather than on contact — both worth naming before a pour and neither a procedure here.
Questions people ask
How many sheets of wire mesh do I need for a slab?
Work in effective coverage rather than sheet size. Subtract the lap from the sheet length and from the sheet width, subtract one lap from each field dimension, then divide and round up in each direction. A 30 by 24 foot slab with a 2 inch setback and an 8 inch lap, in 20 by 8 foot sheets, comes out as two sheets along by four across — eight sheets for a mesh field of 702 square feet.
How much overlap does welded wire mesh need?
Whatever the specification and the adopted code require, and this page supplies no figure. The requirement is often written as a number of cross wires rather than as an inch dimension, because what matters is which wires fall inside the lap rather than how far the sheets overlap, and it depends on the mesh style and the wire spacing. The calculator converts whatever inch figure you enter into squares at your spacing so you can check it against a spec written the other way.
Why is the mesh I bought so much more than the slab area?
Laps first, then offcut, and the offcut is usually the bigger half. Laps alone cost about 11 percent of each sheet at an 8 inch lap on a 20 by 8 foot sheet. But 20 foot sheets across a 29 foot 8 inch field means two sheets deep for a run that only needs one and a half, and that surplus is what pushes a 702 square foot field to 1,280 square feet bought — 82 percent over. The calculator prints the floor you would reach if every offcut were re-used, which on that slab is five sheets rather than eight, so you can see how much of the gap is genuinely lap and how much is a sheet size that does not suit the slab.
Sheets or rolls?
Sheets for most slab work, rolls for long continuous runs where the roll can be rolled out in the direction it wants to go. The deciding factor is usually not price but the curl: rolled mesh has to be flattened and held down before anything else can happen, and on a small slab the labour that takes can cost more than the roll saved. Sheets lie down as they come out of the stack.
Does the mesh have to be up off the ground?
At whatever height the design specified, yes, and mesh lying on the subgrade contributes nothing. That is what supports are for. Pulling it up with a rake during the pour works while somebody is genuinely doing it and stops working when the pour gets busy, which is when it matters most. What height it belongs at, and whether mesh is the right reinforcement here at all, come from the drawing and from whoever designed the slab.