The engineering drawings are the sheets nobody frames on the wall and everybody should read. They show the slab, the beams under it, the steel inside it and the bolts holding the frame down. They are also the one part of your house you cannot see once the concrete goes off, so the drawing is your only record of what was supposed to be down there. Read it before the pour, and stand on the block with it in your hand on the day.
A supervisor reads the footing plan alongside the soil report, because the two have to agree. The report says how reactive the ground is, and the slab drawing has to be the system that class demands. If the report says H2 and the drawing is a thin class M raft, someone has priced the wrong slab, and you find out with a crack, not a phone call.
The engineering is usually a separate set from the architectural plans, stamped by a structural engineer. The key sheets are the footing or slab plan, which is a bird's eye view of the slab with the beams drawn as thick bands under it, and the sections and details, which are slices through the slab and footings showing the depths and the steel. There is normally a notes sheet too, full of small print about concrete strength, curing and steel cover, and that small print is where half the important numbers live.
Find the title block and check it is for your house, your lot, and the current revision. Slabs get redesigned when the soil report changes, and pouring off a superseded drawing is a genuine disaster. The revision letter or number on the sheet should match the one the builder says is current.
Which sheet is the footing plan, and is this the latest revision for my lot?
On a waffle or raft slab the thick bands running across the footing plan are the beams, the deep ribs of concrete that stiffen the floor so it rides over moving ground. The edge beam runs around the whole perimeter, and internal beams grid across the middle. The drawing labels each beam with a size, something like 300 deep by 110 wide, and the more reactive the soil the deeper those beams get. On a sloping block the low side gets a drop edge beam, a deeper beam that reaches down to make up the fall, and that is a number worth finding because it is often where a "flat block" quietly stops being flat.
Count the internal beams against the class. A stable block has few. A class H slab has a dense grid. If the class is high and the grid is sparse, ask why.
What class is this footing plan designed for, and does the beam depth and grid match it?
Concrete is strong when you squash it and weak when you pull it, so steel goes in to take the pull. The drawing calls up a mesh over the whole slab, written as something like SL82, and heavier trench mesh or individual bars running along the bottom of each beam. The number in the mesh code is roughly the wire size, so SL92 is heavier than SL82. The beam bars will be a count and a size, like 3-N12, meaning three twelve millimetre bars.
The part that gets missed on site is cover: the steel has to sit in the middle of the concrete, not on the ground and not poking out the top, or it rusts and the slab loses its strength. That is what the plastic bar chairs are for, and the drawing sets a cover, usually around 30 to 40mm from any face. On the day of the pour you can literally see whether the mesh is lifted on chairs or lying in the dirt. If it is in the dirt, stop the pour.
Is the mesh on chairs at the right cover, and is the vapour barrier under everything and taped?
The engineering also shows how the house is fixed to the slab and how the slab is fixed to the ground. Hold-down or anchor bolts are set into the wet concrete where the frame's bottom plates land, and the wall bracing plan later relies on them, so their spacing on the drawing matters. On reactive or problem sites the drawing may call up piers, bored holes filled with concrete and steel that reach down to firm ground, and the plan shows where each one goes and how deep. Piers are expensive and they are easy to leave out or make too shallow, so their depth on the drawing is a number to keep.
If your slab is on piers, the pier depth is checked and signed off before the pour, usually by the engineer or a certifier. Ask for that record. It is the proof the holes went as deep as the drawing said.
Are there piers on this design, how deep, and who signs off that they were bored to depth?
The whole point of reading the drawing is the walk you do before the concrete truck arrives, at the stage builders call pre-pour or steel inspection. With the footing plan in your hand, check the outline matches the setout, the beams are dug to the depths on the section, the mesh and beam steel are the sizes called up and sitting on chairs, the vapour barrier is under it all and taped at the joins, the plumbing and conduit that pass through are sleeved, and any piers are drilled and filled. Photograph all of it, wide and close, because in an hour it is under a slab forever.
There is normally an independent pre-pour or mandatory inspection by a certifier or the engineer at this point, and that inspection is the one that legally matters. You are not replacing it. You are making sure it happened, and giving yourself a set of photos and a marked-up drawing if anyone ever argues about what is under your floor.
Can I walk the slab with the engineering drawing at pre-pour, and see the certifier's inspection record?
The pages on BuildPilot this guide leans on.
It is the structural set, stamped by an engineer, that shows the slab and footings, the steel inside them, the hold-down bolts and any piers. Along with the soil report it decides what is poured under your house, and it is the only record of it once the concrete has set.
The thick bands on a footing plan are the beams, the deep concrete ribs that stiffen the slab so it rides over moving ground. An edge beam runs around the perimeter and internal beams grid across the middle. The more reactive the soil class, the deeper the beams and the denser the grid.
Cover is the distance the reinforcing steel sits from the surface of the concrete, usually around 30 to 40mm. Plastic bar chairs hold the mesh and bars at that height so they end up in the middle of the pour. Steel lying in the dirt has no cover and will rust, so it is a reason to stop a pour.
At the pre-pour or steel stage, once the beams are dug, the mesh and bars are placed on chairs and the vapour barrier is down, but before the concrete is ordered. That is the last point anything can be changed. Walk it with the footing drawing and photograph everything.
Yes. On reactive or problem sites the plan shows each pier position and its depth. Pier depth should be checked and signed off before the pour, so ask for that record: it is the proof the holes were bored as deep as the drawing required.
Sizes quoted are typical for Australian residential work and are given to help you read drawings and ask better questions. They are not a specification. Your engineer, designer, builder and the National Construction Code decide what applies to your house.
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