Structural and base materials · Concrete & Cement
Reinforcement Spacers and Chairs
Explains the accessory that sets a dimension nobody can check once the pour is finished, and why the material a spacer is made from matters as much as its height.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What spacers and chairs is
Spacers and chairs are the small accessories that hold reinforcement where the drawing places it while wet concrete is placed around it. Spacers sit between the reinforcement and the formwork or the ground, setting the distance from the bar to the face of the element. Chairs, stools and continuous spacer bars sit under or between layers of reinforcement, holding a top layer up off a bottom one or off the deck. They are ordered by type, by height and by the material they are made from.
The dimension they set is the one that governs a great deal about how the element behaves and, once the concrete has gone in, the one nobody can measure. A bar pushed down by a boot or floated up by the pour ends up somewhere other than where the drawing put it, and there is no way to correct it and no simple way to find out afterwards. This is why a component that costs almost nothing gets an entry of its own: it decides an outcome that cannot be revisited.
The material matters because the spacer is cast in permanently. A cementitious spacer is a small piece of mortar or concrete that becomes part of the element, and the question is how closely it matches what is around it. A plastic spacer stays as plastic in the concrete, and where it reaches the surface it leaves a visible dot and a path through the cover, which is why designs for elements exposed to weather or to water often specify particular types. Wire chairs bring steel to the surface unless they are treated or tipped, and steel that reaches a face is steel that can rust at that face. None of these is a general rule — which type suits a given element is a design decision — but all of them explain why the choice is not simply a matter of height.
There is a published position in this library called the concrete cover zone, and it is a place in an element rather than a thing you order. Spacers and chairs are the ordered thing that decides where that zone ends up. Keeping the two apart is exactly the boundary the library draws between a component position and a material you can buy.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Rebar spacers
- Concrete spacers
- Bar chairs
- Cover blocks
- Continuous spacer bar
- Wheel spacers
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Beneath bottom reinforcement in slabs, rafts, ground beams and pile caps.
- Between reinforcement and vertical formwork in walls, columns and retaining structures.
- Supporting top reinforcement above bottom layers in two-way slabs and rafts.
- In precast production, holding cages within factory moulds.
- Around reinforcement cages in piles, shafts and circular sections.
- Supporting mesh and fabric reinforcement in floor slabs and toppings.
Consideration
Where it is commonly considered
- Wherever reinforcement is being placed and its position relative to the face of the element matters.
- Where the element is exposed to weather, water or ground and the designer has been specific about cover.
- Where a cage is heavy or long enough to deflect between supports during placing.
- Where concrete will be placed by pump or by skip and the impact on the cage is a consideration.
- Where the concrete face will be exposed to view and surface marking from spacers is a concern.
- Where a top layer of reinforcement has to be kept up off the deck under foot traffic during the pour.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where the spacer type has been chosen on site rather than by whoever specified the reinforcement.
- Where the element is exposed or in ground contact and the spacer material has not been considered by the designer.
- Where a wire chair would bring untreated steel to a face that is exposed to weather or water.
- Where the concrete face is a finished, visible surface and the marking left by the spacer has not been agreed.
- Where spacing between supports has been decided by convenience rather than by what will hold the cage.
- Where something improvised — a piece of brick, a stone, an offcut — is being used in place of a manufactured spacer.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Installation dependency
- Spacers are fitted as the cage is built and have to survive everything that happens afterwards: fixers walking on the deck, the cage being adjusted, and concrete arriving under pressure. Enough of them, in the right places, is the whole point.
- Substrate dependency
- What the spacer bears on matters. On soft ground or on a membrane, a small bearing area punches in and the reinforcement drops, so the type is chosen for what is underneath as well as for the height required.
- Documentation
- The specification for the reinforcement usually covers the spacers, and what was actually used is worth recording, because it is the only evidence of a dimension that can no longer be measured.
- Appearance
- Spacers show on struck concrete faces as regular dots or lines. Where the face is to be left exposed, the type, the colour and the setting-out pattern become a visual decision rather than a technical one.
- Moisture behaviour
- Anything reaching from the reinforcement to the face creates a path through the cover, which is why spacer type is raised for elements in water, in ground or in exposed positions.
- Repairability
- Nothing about a spacer can be corrected after the pour. A cage that moved stays moved, and the only remedies are structural assessments far more involved than the accessories were.
- Surface wear
- On a slab soffit or an exposed face, spacer marks are part of the surface and are affected by anything that cleans, coats or wears that face.
Exposure
Exposure and environmental conditions
- Is this element exposed to weather, water, ground or de-icing salts in service?
- Will the concrete face be visible, and does the spacer marking matter on it?
- Is the element in a water-retaining or below-ground position where the spacer material has been specified?
- What is the reinforcement bearing on — ground, blinding, a membrane, formwork?
- Will the pour be in conditions that make the cage harder to keep in position?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- Blinding, membranes and insulation beneath a slab all affect what the spacer bears on and how it behaves.
- The reinforcement drawing and schedule define the position the spacers are there to hold.
- Formwork alignment on a vertical face sets the reference the spacers work to.
- Placing method — pump, skip, chute — determines the forces the cage will take during the pour.
- Access boards and walkways for the fixing and pouring crews protect the top reinforcement from being trodden down.
- Where a spacer will be visible on a struck face, the setting-out pattern is agreed in advance with the design team.
Appearance
Appearance and finish considerations
- Spacer positions read on struck faces as a regular pattern of small marks.
- Plastic spacers can show as a different colour to the concrete and stay visible.
- Cementitious spacers are less visible but rarely match the surrounding concrete exactly.
- Wire chairs can show as small steel points unless treated or tipped, and those points can stain over time.
- On visible fair-faced concrete, the spacer layout becomes part of the design of the face.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- What spacer type has been specified for this element, and who specified it?
- How many are being used, and at what spacing, and does that suit the weight of the cage?
- What will the spacers bear on, and is that surface able to take them without punching?
- Was the reinforcement position checked before the pour, and is that recorded?
- Does the spacer material suit the exposure the element will be in?
Upkeep
Maintenance and repair considerations
- What record exists of the reinforcement position and the spacers used in this element?
- If staining or marking appeared at spacer positions, who should look at it?
- Would a coating or a repair to this face need to take spacer positions into account?
- How would anyone establish the position of the reinforcement in an existing element?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Spacers go on as the cage is assembled, not as an afterthought once the cage is complete.
- The number and the spacing are set by what will hold the reinforcement, not by what is to hand.
- Fixers and pump crews working on the deck need boards or routes so top reinforcement is not trodden down.
- A pre-pour inspection of reinforcement position is the last moment anything can be adjusted.
- Improvised substitutes — offcuts, brick, stone — behave differently from manufactured spacers and are visible later.
- Concrete placing technique influences how much the cage is disturbed as the pour proceeds.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The designer's reinforcement information, including anything stated about spacers and cover.
- The manufacturer's information for the spacer type supplied.
- Records or photographs of the reinforcement in position before the pour.
- Any pre-pour inspection record covering reinforcement position.
- Confirmation of the spacer material used where the element is in an exposed or below-ground position.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What spacer type and material has been specified for this element, and why?
- How many spacers, and at what spacing, for this cage?
- What are they bearing on, and will that surface hold them without them sinking in?
- Who checks the reinforcement position before concrete is placed, and is it recorded?
- Will the spacers be visible on any finished face, and does that matter here?
- Does the exposure of this element change what the spacer should be made from?
- How will the top reinforcement be kept in position while people work on the deck?
- What happens if a cage is found to have moved after the pour?
Blind spots
Commonly overlooked points
- A trivial-looking accessory sets a dimension that becomes unmeasurable the moment the concrete goes in.
- The spacer stays in the element permanently, so what it is made from is part of the element.
- Improvised substitutes are common on site and behave nothing like the manufactured product.
- Top reinforcement is easily walked down during a pour, which is why access boards matter.
- Spacer marks on a struck face are permanent and become part of the appearance of exposed concrete.
- A spacer that suits a slab on blinding may punch straight into soft ground or a membrane.
What this page does not do
- This page does not state what cover any element requires or what spacer suits it; both are design determinations for a qualified professional.
- No dimension, spacing or performance value is given here; that information belongs to the designer and to the manufacturer of the specific product.
- The concrete cover zone is a position in an element, published elsewhere in this library as a component; this entry is the ordered accessory that helps set it.
- Build Design Hub does not supply, test, approve or endorse spacers or any supplier of them.
Component forms
Building components commonly formed from this material
Elements this material is commonly used to make, named by the position they occupy in an assembly rather than by what they are bought as. This states common practice only — it is not a statement that this material suits any of these components in a particular building.
Related entries
Often used alongside
Entries commonly encountered together with this one. Co-occurrence only — never a claim that any combination is compatible or suitable.
Related entries
Used in
Entries this is commonly incorporated into as a constituent or layer.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Concrete & Cement Materials
Reference entries for cement-based materials — ready-mixed and precast concrete, screeds, levelling compounds, renders of the cementitious family and the admixtures that modify them.
Browse all concrete & cement entries →