A house frame uses several bar sizes at once, each doing a different job. Knowing which goes where helps you read your drawings, check the steel order and spot a substitution on site before it goes into the concrete.
What Sizes Are Available?
TMT bars are rolled in standard diameters. For a house, four sizes do almost all the work.
| Diameter | kg per metre | Main use in a house |
|---|---|---|
| 8 mm | 0.395 | Stirrups, column ties, slab distribution bars |
| 10 mm | 0.617 | Main slab bars, lintels, sunshades |
| 12 mm | 0.888 | Beams, columns and footings in G+1 / G+2 |
| 16 mm | 1.58 | Heavier columns and main beams, larger footings |
| 20 mm | 2.47 | Heavily loaded columns, long-span beams |
| 25 mm | 3.85 | Multi-storey columns, transfer beams |
| 32 mm | 6.31 | Large structures; rare in houses |
Which Bars Go in Each Member?
Footings
Footings spread the column load into the ground. They use a mesh of bars at the bottom, typically 10 mm or 12 mm at around 150 mm spacing both ways, sized by the load and the soil's bearing capacity. Hyderabad's sheet rock and murrum soils often allow smaller footings than black cotton soil does, which is one reason a soil test matters.
Columns
Columns carry the building's weight to the footings. A house column usually has four to six main vertical bars of 12 mm or 16 mm, held together by 8 mm ties (rings) at regular spacing — closer near the beam and footing junctions. Main bars continue from the footing up through each floor with laps at the floor levels.
Beams
Beams carry slabs and walls to columns. They have main bars at the bottom (where tension is greatest in the middle of the span), bars at the top (where tension is greatest near the supports), and 8 mm stirrups wrapped around them at intervals. Typical main bars are 12 mm or 16 mm.
Slabs
A slab is a thin, wide member reinforced with a mesh. The main bars run across the shorter span, usually 8 mm or 10 mm at 100 to 150 mm spacing, and distribution bars of 8 mm run the other way. Extra bars are placed at the top near supports.
Lintels and Sunshades
Lintels over doors and windows typically use 10 mm or 12 mm main bars with 8 mm stirrups. Sunshades (chajjas) are cantilevers, so their main steel is at the top, tied back into the lintel.
How Much Cover Should the Bars Have?
Cover is the concrete between the bar and the outer surface. It protects steel from rust and fire. IS 456 sets nominal cover by exposure; typical values for a house in mild to moderate exposure are:
| Member | Typical nominal cover |
|---|---|
| Slab | 20 mm (moderate exposure: 30 mm) |
| Beam | 25 – 30 mm |
| Column | 40 mm (not less than the bar diameter) |
| Footing | 50 mm |
Use cover blocks, not stones or brick bats, to hold the steel at the right distance.
Why You Must Not Swap Sizes
A bar's strength depends on its cross-sectional area, which rises with the square of the diameter. That makes small changes in size large changes in capacity.
| Swap | Change in steel area |
|---|---|
| 12 mm replaced by 10 mm | about −31% |
| 16 mm replaced by 12 mm | about −44% |
| 10 mm replaced by 8 mm | about −36% |
Substitution is sometimes possible — for example, more bars of a smaller size giving the same total area — but only with the engineer's recalculation. Swapping on site to use leftover steel or save money is how a member ends up under-reinforced.
How Is Steel Split Across Sizes in a House?
For a typical G+1 house, steel by weight often splits roughly as below. Use your bar bending schedule for the real figures.
| Size | Typical share by weight |
|---|---|
| 8 mm | 25 – 35% |
| 10 mm | 15 – 25% |
| 12 mm | 25 – 35% |
| 16 mm | 10 – 25% |
Ordering all sizes from one brand keeps each member's bars consistent.
The Short Version
8 mm ties and distributes, 10 mm carries slabs and lintels, 12 mm and 16 mm carry beams, columns and footings, and anything larger is for bigger buildings. Follow the drawing, keep the specified cover, and never swap a size on site.
