Slab Bolster
A continuous wire support that holds slab rebar at the right height.
Quick Answer
A slab bolster, often just called a bolster, is a continuous wire support that holds slab reinforcing bars at the required height above forms or the ground. Takeoff is normally in linear feet by support height, since bolsters are sold in set lengths, and the quantity depends on row spacing across the slab.
The Full Picture
A slab bolster is a formed wire rail with legs that holds rebar above the formwork or subgrade so it ends up inside the concrete rather than on the bottom. Without supports, bars get pushed down during the pour and the required cover is lost. Bar supports hold reinforcing in place to achieve the required concrete cover, and bolsters are one type alongside individual chairs and blocks.
Bolsters come in standard heights, commonly measured in fractions of an inch to a few inches, with versions for bottom-of-slab use, and upper versions that hold top steel off the bottom mat. They come with plain or protective finishes such as plastic-coated legs or all-plastic versions, and the structural drawings and the Division 03 specifications state the cover and any finish requirements for exposed surfaces or slabs on grade.
In a takeoff, the base unit is LF of bolster, grouped by height. The count comes from the number of support rows across the slab times row length. Row spacing varies by the engineer, the placing drawings and the supplier's recommendations, so the estimator should read the notes rather than assume. Supports are commonly sold in set lengths such as 5 ft, which the estimator can use to convert LF to pieces, and overlapping or extra pieces for ends are generally not needed because bolsters are laid end to end.
Pricing pitfalls: leaving supports out because they look small, using a single height for a job with several cover requirements, and pricing steel chairs in places where the spec requires plastic, such as slabs on grade over a vapor retarder or surfaces exposed to view. Post-tensioned slabs and mats also use different support systems, so the support type is not always a bolster.
Real Examples
Common Misconceptions
People assume: Bolsters and chairs are the same item.
Actually: A bolster is a continuous rail supporting several bars, while a chair is an individual support. They are priced and counted differently, by LF and by EA.
People assume: Bar supports are included in the rebar tonnage.
Actually: Supports are accessories usually carried separately from the weight of reinforcing steel, though some suppliers include them in a furnished-and-placed price.
Does MeltPlan Solve This?
Partially — adjacentMeasuring support lengths and heights from structural plans is a takeoff task. MeltPlan's takeoff service combines AI that pulls quantities from plans with US-based experienced estimators who review and adjust them, so the output is a verified, ready-to-use takeoff. It works from PDF plans, not BIM models.
Get rebar support quantities measured from the plans →Frequently Asked Questions
How are slab bolsters measured?
In linear feet, grouped by height, from the number of support rows and the row length. Suppliers often sell them in fixed lengths, so LF can be converted to pieces.
What is the difference between a bolster and a chair?
A bolster is a continuous support under multiple bars. A chair is an individual support under a bar or at a single point. Both set the height of reinforcement.
What is an upper bolster?
An upper bolster supports the top layer of rebar, holding it above the bottom mat. It is carried as its own height and quantity.
Where are bar supports specified?
In the structural general notes, reinforcing details and the Division 03 concrete reinforcement specification, with cover requirements on the structural sheets.
Why does bolster material matter?
Surface-exposed concrete and slabs over membranes may call for plastic or protected supports so that rust does not stain the surface or the membrane is not damaged.