Covering Blocks

A covering block for slab (also called a cover block or spacer block) is a small support piece placed between the reinforcement bars (rebar) and the formwork in concrete slabs.

Its job is to ensure the correct clear cover—the distance between the outer concrete surface and the steel reinforcement.

Why it’s important

  • Corrosion protection – keeps rebar away from moisture and air.

  • Fire resistance – concrete cover insulates rebar from heat.

  • Structural design compliance – meets IS 456 or relevant code requirements.

  • Uniform slab thickness – prevents steel from touching formwork.

Common types

Material Pros Cons
Concrete High compressive strength, similar thermal expansion to slab Slightly heavier
PVC/Plastic Lightweight, non-absorptive Less load-bearing capacity
Fiber-reinforced Durable in aggressive environments Slightly costlier


Typical sizes for slabs

  • 20 mm or 25 mm – residential & light duty slabs

  • 40 mm – podium, parking deck, exterior slabs

  • 50 mm – heavy duty or industrial slabs

Placement guidelines

  • Place blocks at regular grid spacing (e.g., 600–900 mm apart).

  • Position under bottom reinforcement before concrete pour.

  • Use wider base blocks at slab edges and walkways to prevent tipping..