Kumar IronUpdated
What a joist is
A joist is a horizontal member that carries a floor or roof and passes the load to beams or walls. Joists are usually placed close together and parallel, with the floor or roof material laid across them. A beam is usually bigger, fewer in number, and carries the joists or a wall.
Joists can be timber, RCC or steel. In north Indian building practice the word mostly means a steel I section used as a floor or roof support.
RSJ, MS joist and garder
- RSJ stands for Rolled Steel Joist: an I section rolled hot at a steel mill from mild steel.
- MS joist simply means a mild steel joist. In practice it is the same thing as an RSJ.
- Garder or girder is the local word used for the same I section, especially in bigger sizes.
- In Indian standard sizes these are listed under IS 808 as ISMB (medium weight beams) and related series, made from structural steel to IS 2062.
So when someone asks for a 6 inch joist, they usually mean an ISMB 150, which is 150 mm deep. See our I beam guide for sizes and the weight chart for kg per metre.
Where steel joists are used
- Jack arch and patthar patti roofs: joists placed every few feet with brick arches or stone slabs between them. Common in older houses across UP.
- Mezzanine floors and lofts in shops and godowns, often with chequered plate or a thin RCC deck on top.
- Lintels over wide shop shutters and openings where a slim, quick support is needed.
- Sheds and industrial structures as main beams and crane rails.
- Temporary supports and props during repair or wall removal.
Buying tips
- Size and length come from the engineer. Span, spacing and load decide the section, so do not pick one by eye.
- Ask for the weight per metre and check a piece on the scale. Joists are sold by weight.
- Check for straightness, no deep rust pitting, and clean flanges without cracks.
- Prime and paint steel joists soon after delivery, and again after fixing, especially if they will be exposed.
- Where a joist sits on a wall, the bearing should be spread with a concrete bed block so it does not crush the brickwork.
Frequently asked questions
- What is the difference between a joist and a beam?
- Both are horizontal members. Joists are usually smaller, closely spaced, and carry the floor or roof directly. Beams are larger and carry the joists, walls or slabs over a longer span.
- What is RSJ in construction?
- RSJ means Rolled Steel Joist, a hot rolled mild steel I section. In Indian sizes it is usually an ISMB section under IS 808.
- How is a steel joist priced?
- By weight, in rupees per kg. Multiply the kg per metre of the section by the length to estimate the cost.