Slab reinforcement in Revit starts with a detailing question, not a software one: which layer carries what. A slab spanning between beams sags at mid-span, the bottom fibre goes into tension, so the bottom layer carries the sagging moment. Near a support the slab bends the other way, the top fibre goes into tension, so the top layer carries the hogging moment. Plenty of pages state this backwards, and backwards on paper means backwards on site.
Three familiar rules follow. The bottom layer runs across the whole panel, with the main bars along the short span. The top layer may run across the panel as well, or it may be a short length placed only over the supports and then stopped — that short length is the top reinforcement over supports. Between the two layers you need chairs, because a top layer that sags loses the effective depth the design relied on.
How far that top bar should reach has no universal figure. It comes out of the analysis and is written on the drawing. What Revit does is keep the drawing, the tags and the schedule tied to whatever figure you set.
The fifteen steps below were captured on Revit 2027.2 with the English interface. If the slab itself is not modelled yet, read structural floors in Revit first; for the whole documentation workflow, see the Revit Structure course.
Where two-layer slab reinforcement sits in the model
In Revit a bar is not a line. It is a three-dimensional element with a diameter, a shape, hooks and a host. Its position is measured from the cover face of the slab, not from the concrete face.
The image shows three 120 mm slabs at +3.570, +3.800 and +3.850, with the two directions drawn in two colours.
Rebar cover follows TCVN 5574:2018
Rebar cover is not a number you pick freely. TCVN 5574:2018, clause 10.3.1.2 and Table 19, gives the minimum by service condition, not by element name.
| Value | Service condition |
|---|---|
| 20 mm | Inside a covered space, normal or low humidity |
| 25 mm | Inside a covered space, raised humidity |
| 30 mm | Outdoors, no additional protection |
| 40 mm | In soil, no additional protection |
Three adjustments sit directly under that table: precast elements take 5 mm less, secondary bars take 5 mm less than main bars, and in every case the cover is never smaller than the bar diameter.
So the 15 mm that keeps appearing in Vietnamese slab drawings is simply 20 mm minus the 5 mm for secondary bars. Revit stores one fixed value per named cover setting and never compares it with the bar diameter, so that last rule is yours to check.
Preparing the file and the three settings to fix first
Every reinforcement tool sits on the Concrete tab. The Reinforcement panel holds seven buttons: Rebar, Path, Morphed, Area, Fabric Area, Fabric Sheet and Cover.
Three jobs must be finished before the first bar: declare the bar diameters, declare the cover list, and fix the length rounding. All three are covered in rebar settings in Revit.
This article is part of the Institute's Revit Structure series.
Slab reinforcement in Revit: placing the bottom set
Step 1 — Open a view whose cut plane passes through the slab
Reinforcement only appears in a view that cuts its host. Open the structural plan of the right storey, or a section across the slab.
Step 2 — Open Cover Settings and read the cover list
On the Concrete tab, Settings panel, click Cover Settings.
The dialog is only a list of named values. The sample model declares four rows: 15 for slabs, 40 for foundations and ground beams, 50 for foundations cast straight onto soil, and Rebar Cover 1, the 25.0 mm default that ships with Revit.
Step 3 — Assign cover face by face on the slab
Select the slab and scroll the Properties palette down to the Structural group.
| Parameter | Value read on the model |
|---|---|
| Rebar Cover - Top Face and Other Faces | Rebar Cover 1 <25 mm> |
| Rebar Cover - Bottom Face | 15 <15 mm>, the value for secondary bars |
| Estimated Reinforcement Volume | 110382.87 cm³ for the whole slab |
Cover is assigned per face, not per element, so two faces of the same slab carrying different values is perfectly normal.
Step 4 — Select the slab as the host
Click inside the slab panel. Properties must change to Floors (1) with the type name, for example S120. If clicking never selects anything, see the troubleshooting table at the end.
Step 5 — Launch Rebar from the Concrete tab
With the slab still selected, click Rebar. The ribbon switches to the contextual tab Modify | Place Rebar.
The four middle panels decide where the bar lands and which way it faces. This is the hardest part for a newcomer.
Step 6 — Pick a bar shape in the Rebar Shape Browser
Click Shape Browser on the Family panel. A palette opens on the right listing every bar shape in the project.
In the sample model BS_M_T1 is a closed rectangular stirrup, M_00 is a straight bar, and M_17A together with the numbered Rebar Shape entries are the bent shapes. Slab bars with a hook at each end use Rebar Shape 1.
Step 7 — Choose a placement method
| Method | Where it fits on a slab |
|---|---|
| Expand to Host | The bar stretches across the host, for reinforcement running the full panel |
| By Two Points | You click two points, for top bars and local trimmers |
Step 8 — Pick the placement plane so the bar lands in the right layer
The Placement Plane panel holds three buttons: Current Work Plane, Near Cover Reference and Far Cover Reference. Near and far are the two cover faces measured along the view direction of the open view.
Top bars take the near face, bottom bars the far face. Pick the wrong one and the bar sits in the wrong layer while the plan view looks identical.
Top reinforcement over supports and trimmers around openings
Step 9 — Set the bar orientation
Three buttons: Parallel to Work Plane, Parallel to Cover and Perpendicular to Cover. Slab bars lie flat on the slab face, so the correct choice is parallel to the cover; the perpendicular option belongs to stirrups.
Step 10 — Set the layout rule and the spacing for the whole set
Select the bar you just placed and scroll Properties down to the Rebar Set group.
| Rebar layout rule | How it works |
|---|---|
| Single | One bar, not a set |
| Fixed Number | You type the count, Revit computes the spacing |
| Maximum Spacing | You type the largest spacing allowed, Revit computes the count |
| Number With Spacing | You type both count and spacing |
The set in the image uses Maximum Spacing with a spacing of 200.0 mm. Quantity reads 16 but is greyed out, because Revit derives it from the spacing and the panel width. Widen the panel and the count grows by itself.
Step 11 — Declare the hooks at both ends
Scroll Properties back up to the Construction group.
| Parameter | Value read on the model |
|---|---|
| Partition | SB-2F, the partition name that groups bars in the schedule |
| Rebar Number, Schedule Mark | 10 and 5, the mark shown in the circle on the tag |
| Start of Bar, End of Bar | both Hook |
| Hook At Start, Hook At End | both @P21=90-200, a hook type declared by the design office |
A hook type is a named object, not a number you type. Declare it once in the office template and the schedule adds the hook length correctly every time.
Step 12 — Read back the bar length Revit has just computed
This set reads Length 1 of 4891.9 mm, with Length 2 and Length 3 both 200.0 mm — a straight run plus two 200 mm hooks. Bar Length is 5262.5 mm but the bracket shows 5260, the result of rounding to a multiple of 10 mm. Total Bar Length 84160 mm covers all 16 bars, weighing 51.87 kg.
Step 13 — Place the top bars over supports and the trimmers around openings
Over a support, use By Two Points to draw the top bar at the length the drawing calls for, then give the strip a layout rule. Around an opening the trimmers go in the same way: two diagonal bars at the corners and bars along each edge. For a long edge the Path command is quicker, because it spreads bars along a line you sketch.
A section is the only place where the layers can really be checked. The image reads three tags, 17 Ø10 a200 and 19 Ø10 a200.
Step 14 — Switch on View Unobscured to see bars through the concrete
By default a bar buried in concrete is hidden by the concrete. Select a bar, look at the top of the Properties palette and click Edit on the View Visibility States row.
The dialog has three columns: View Type, View Name and View Unobscured, with a tick box set view by view. The article on viewing rebar in Revit covers this in depth.
Step 15 — Check the result in plan, in section and in three dimensions
Three kinds of view, three jobs: the plan to count bars, the section to inspect the two layers, the three-dimensional view to see how the slab bars sit on the beams. The sample plan reads 24 Ø10 a200, 19 Ø10 a200 down to 5 Ø10 a200, and each tag is simply three parameters of the set itself.
That last image is the one worth checking hardest: the slab bars run over the edge beam, reading 19 Ø10 a150 and 15 Ø8 a200, while beam DXM.X1 at 220×400 carries 3Ø16a with 17 Ø6 a150 stirrups.
Area reinforcement or Rebar: which tool to use
The Area button on the Reinforcement panel is a drop-down holding two commands.
Autodesk describes the first one plainly: sketch the boundary of a region and populate it with rebar. The result is one region object carrying up to four evenly spaced layers, that is two directions on top plus two directions at the bottom. See the Area Reinforcement page from Autodesk for more.
Pick the tool that matches the shape of the steel: Area for a large panel reinforced evenly in all four layers, Path for a strip that follows a line such as a slab edge, Rebar for individual bars and sets. The sample model uses Rebar throughout and its schedules still come out complete.
Editing one bar inside a set
Select a bar and the ribbon shows the contextual tab Modify | Structural Rebar.
| Panel | What it does |
|---|---|
| Presentation | Show only the first and last bar, or every bar in the set |
| Host | Propagate Rebar copies the whole set to identical panels |
| Customization | Edit Bars breaks the set into individual bars |
| Symbol | Bending Detail produces the bending schedule figure on a sheet |
Propagate Rebar and Edit Bars are the two worth remembering. The first repeats a finished set across typical panels. The second is the only way to edit one bar without touching the rest; the price is that the broken set no longer updates itself.
Splitting the top and bottom layers onto two sheets has nothing to do with the bars themselves. Vietnamese offices do it with view filters, declared once and then switched on the Filters tab of the Visibility/Graphic Overrides dialog.
When it does not work
| Symptom | Cause and fix |
|---|---|
| Clicking inside the panel never selects the slab | The Select Elements by Face toggle at the right of the status bar is off. Switch it on |
| All seven Reinforcement buttons are greyed out | You are on a sheet. Open a plan or a section |
| No click selects anything at all | A reinforcement command is still running. Click the red cross on the Mode panel |
| Bars are placed but the view stays empty | The view does not cut the slab, or View Unobscured is not ticked for that view |
| Editing one bar changes the whole set | A set is one object. Use Edit Bars to break it apart first |
| Typing a count into Quantity does nothing | The layout rule is Maximum Spacing, so Quantity is greyed. Change the spacing, or switch to Fixed Number |
What people often get wrong
| Common belief | What is actually true |
|---|---|
| The bottom layer carries the hogging moment | The other way round. The bottom layer carries sagging at mid-span, the top layer hogging at supports |
| 15 mm cover is a separate value in the standard | It is 20 mm less the 5 mm allowed for secondary bars, under TCVN 5574:2018 |
| Rebar in Revit is just linework, as in CAD | A bar is a three-dimensional element. Revit computes its length including hooks, and its weight |
| Detail Level must be Fine before bars appear | No. The two real switches are View Unobscured and the visual style |
Frequently asked questions
Which layer should be placed first in Revit?
Either. Each set is attached to its own cover face. Placing the bottom layer first is more convenient, because it gives you a reference for setting out the top bars.
How long should the top bars over a support be?
There is no universal figure. It comes out of the analysis of that particular panel and is stated on the drawing. Revit only keeps the geometry and the schedule matching the figure you enter.
How many layers can one area reinforcement region hold?
Up to four evenly spaced layers, two directions on top and two at the bottom, exactly as Autodesk describes the command.
Can one bar be edited without breaking the whole set?
Yes, but at a cost. Edit Bars splits the set into individual bars, and the set then stops updating when the slab boundary changes.
Do slab bars reach the schedule automatically?
They do. Set the Partition parameter for each group and the schedule collects them by group. In the sample model partition SB-2F totals 1,340.2 metres and 825.88 kilograms of Ø10 bar.
Related articles
- Rebar settings in Revit — bar diameters, cover and bend radius
- Beam reinforcement in Revit — main bars, stirrups and anchorage
- Column and foundation reinforcement in Revit — vertical bars, ties and starter bars
- Rebar schedules in Revit — bar schedules, lengths and weights
- Structural floors in Revit — modelling the slab before reinforcing it