Water and external systems · Decks & Paving
Rigid (Bound) Paving System
This entry sets out bound paving as a stiff composite of base, bed, unit and joint, explains why cracking and debonding are behaviours of the assembly rather than faults in a material, and lists what to raise with qualified professionals.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What rigid paving is
Bound paving is stiff. A concrete or otherwise bound base spreads load by bending across a soft area instead of conforming to it, and the units above are mortared onto it so that base, bed and surface behave largely as one element. Where a poured surface is used there are no units at all, and the distinction between base and surface disappears entirely.
The nearest sibling is the flexible paving system, and the two are told apart by how each one fails. Unbound paving spreads: units drift outward, joints open, and the remedy lives at the perimeter. Bound paving cannot spread, so accumulated movement resolves itself as a fracture, and the remedy is a pattern of joints deciding in advance where that fracture is permitted. Standing on a surface, one shows open joints and rocking units; the other shows a single line running through several units and the bed under them together.
Anything stiff has to cope with the fact that it is not free. The surface changes size as it warms and cools, the base does so more slowly, and the ground beneath does not at all. Restraint at an edge, around a fixed object or against a change in construction converts that difference into stress, and the assembly relieves it either at a joint provided for the purpose or at a crack that was not.
The build-up provides no drainage path downward, so all water is expected to leave at the surface, and that changes what drainage means here. Rain stays on the surface until the shaping takes it somewhere, and any water that does get in through a crack or a failed joint has no way out and works on the bond between bed and unit from underneath.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- mortar-bedded paving
- bound paving build-up
- concrete-based paving
- rigid pavement construction
- slab-on-base paving
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Carry loading as a stiff composite so that pressure is spread by bending rather than by the confinement of separate units.
- Hold a surface in place under turning, braking and concentrated loads that would gradually displace unbound units.
- Concentrate the movement that stiff construction cannot avoid into joints chosen by the designer rather than into random cracking.
- Shed water from the surface, since the assembly deliberately offers it no route downward through the layers.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is meaningful in this system. The roles below sit in sequence, and moving one relative to another changes what the assembly does. The sequence is conceptual only: it states no thickness, no dimension, no fixing and no order of work on site.
- Layer 1 of 7. Order is meaningful.SubstrateBound or concrete base
The stiff layer beneath the bed, cast or laid as a continuous element and often reinforced. It is what actually spreads load, and its own joint pattern governs where the surface above is free to move and where it is not.
- Layer 2 of 7. Order is meaningful.AttachmentMortar bed or bonding layer
The layer that fixes each unit to the base and makes the composite real. Its continuity beneath a unit is what decides whether that unit is supported everywhere or only in patches, and the difference is invisible once the surface is complete.
- Layer 3 of 7. Order is meaningful.Finish surfacePaving units or continuous surface
The trafficked face, whether individual slabs, setts and bricks bonded to the base, or a poured surface that is the base. Once bonded, a unit is no longer an independent object: it takes its movement from what it is stuck to.
- Layer 4 of 7. Order is meaningful.Jointing and sealingRigid joint filling
The bound material closing the gaps between units. Unlike a loose filling it resists water entry and holds the units against each other, and unlike a loose filling it fails visibly, since a bound joint either holds or cracks.
- Layer 5 of 7. Order is meaningful.Jointing and sealingMovement joints and crack control
The deliberate discontinuities that decide where the assembly may move. They are only discontinuities if they pass through every bonded layer, and a joint that stops before reaching the base has separated the surface from nothing of consequence.
- Layer 6 of 7. Order is meaningful.Drainage planeSurface shedding arrangement
The shaping and the outlets that move water off the surface, together with the treatment at low points. Because there is no drainage path within the build-up, everything depends on water leaving before it finds a defect to enter through.
- Layer 7 of 7. Order is meaningful.Edge and terminationPerimeter and abutment detail
Where the bound field stops against a building, a kerb, a change of construction or nothing at all. Every one of these is a restraint condition, and the accumulated movement of the field arrives at these lines first.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The bond between bed and unit is what makes the composite, and it is also the frailest thing in it. A unit bonded across its whole underside shares load with the base; a unit bonded only in patches has voids beneath it, and every wheel crossing one flexes the unit over unsupported ground until either the unit cracks or the bond lets go. From above the two look identical until the day one of them stops looking identical.
Joints do not simply divide the surface, they decide where the assembly is permitted to move. A movement joint that appears in the surface but does not continue through the bed and the base has separated nothing: the stiff layers below remain continuous and will crack along their own line, often within sight of the joint intended to prevent it. The reverse holds too, since a joint formed in the base and not carried up through the bed and surface telegraphs a crack upward instead.
The base sits in a different relationship with the ground than an unbound pavement does. Because it can bridge, a small soft area beneath is hidden; because it is brittle, a soft area it cannot bridge produces a fracture rather than a dip. A stiff pavement therefore conceals poor ground right up to the point where it conceals it no longer, and the eventual failure is a line through several units and their bed, not the settlement of one.
Water entering a crack cannot drain away. It sits on the base, saturates the bed from below, weakens the bond and freezes wherever the climate allows it. A defect that began as a movement problem becomes a moisture problem, and the debonding that follows spreads outward from the crack as units on either side lose their support. That is why the condition of the joints, rather than the condition of the units, is the honest indicator of how a bound pavement is doing.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Substrate
Concrete and reinforcement families are commonly encountered forming the bound base beneath this kind of surface. What a base has to do, and whether it carries reinforcement at all, follows from the loading and the ground, and that determination rests with the designer.
Attachment
Bedding mortar and adhesive families are commonly encountered bonding units to a base. Whether a given product belongs in an externally exposed, trafficked position is entirely a matter for the manufacturer's documentation and for the professional specifying it.
Finish surface
Unit and poured surface families of this kind are commonly encountered as the trafficked face of bound construction. Exposure, slip behaviour and dimensional movement differ sharply between them, and those characteristics belong with the product documentation and the designer.
Jointing and sealing
Sealant and joint profile families are commonly encountered where a movement joint has to remain open and still resist water entry. What a joint has to accommodate is a design output, and the product documentation governs what any sealant is used for.
Protection
Impregnating treatments are commonly encountered on porous units in bound external surfaces. What they change about staining, appearance and the way a surface dries varies by product and by stone, and the manufacturer's documentation governs their use.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Bound base onto the foundation
The foundation here is asked for uniformity rather than raw strength, since the base responds to a local weakness by fracturing. Ground that would merely settle beneath unbound paving is ground that cracks a bound one, and the difference is not visible until it happens.
Read about Pavement Foundation →Outlets, channels and low points
With no drainage route inside the build-up, the collection arrangement is the assembly's only defence. Channels and gullies also introduce fixed frames into a field that expands and contracts, so they frequently become the line along which the surface first cracks.
Read about Surface Water Collection →Bound surface against the building
A stiff surface abutting a building is restrained along that line, and the movement it cannot take up elsewhere arrives there. The junction has to allow the surface to move relative to the wall while still keeping water travelling away from the threshold rather than towards it.
Read about Door Threshold Interface →Bound paving across a rooting volume
Bound construction cannot absorb root growth by lifting a unit at a time; it fractures along the line of least resistance instead. Where a stiff surface passes over or beside a rooting volume, the position of joints and of any root guidance is a shared decision rather than two separate ones.
Read about Tree Pit System →Fixed frames, covers and upstands
Any rigid object cast into the field, from an access cover to a light base, becomes a point the surface cannot move past. Movement joints are placed in relation to these objects rather than on a convenient grid, because corners of fixed frames are where cracks start.
Anchorage penetrating a bound surface
Guarding at a level change is fixed back into structure, and its anchorages pass through both the surface and the bed. Each penetration is a discontinuity in the bond and a route for water into a construction that cannot drain, so the two systems are detailed together.
Read about Guarding System →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Movement
- Everything bonded moves together or fractures. The joint layout is the single most consequential decision in this assembly, and it is decided by the geometry of the field, the fixed objects within it and the restraints around it rather than by appearance.
- Thermal
- The surface warms and cools faster than the base and far faster than the ground, so the layers try to change size at different rates while bonded together. This entry states no thermal value; what the difference amounts to on a given surface belongs with a professional.
- Moisture
- Water that finds a way in cannot find a way out. Trapped moisture works on the bond from beneath, and where freezing occurs it works on it faster, so the integrity of the joints is a moisture question rather than a tidiness question.
- Durability
- The units may outlast the assembly. Debonded areas, cracked joints and a fractured base can all coexist with a surface that still looks respectable from a standing height, which is why an inspection is done by sound and underfoot response as well as by eye.
- Maintenance and access
- Opening a bound surface for a buried service means breaking it, and the repair is a new bound patch bonded to an old one. Where services are known to run, deciding whether the surface above should be bound at all is a question worth asking early.
- Buildability
- Bond continuity beneath every unit is the property the assembly depends on and the one that cannot be checked after the fact. What is agreed about achieving it, and what evidence is kept, matters more here than in constructions that can be lifted and reworked.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Where are the movement joints, and do they pass through the surface, the bed and the base rather than stopping at the top?
- What fixed objects sit within the field, and has the joint layout been set out in relation to them?
- How does water leave the surface, and what happens at the low points if an outlet becomes blocked?
- Is the ground beneath uniform enough for a construction that fractures rather than settles when it is not?
- What is intended where the surface meets the building, and is the surface free to move relative to that wall?
- Do buried services run under this area, and is anyone prepared for what opening a bound surface involves?
- Is the choice of a bound construction driven by loading, by appearance, or by an assumption that it needs less attention?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Cracking in a stiff pavement is usually the assembly relieving movement it was given nowhere else to put, rather than poor material.
- A surface joint drawn on a plan is not a movement joint unless it continues through every bonded layer beneath it.
- Hollow-sounding units are reporting a loss of bond and a void beneath, which is a structural condition rather than a surface blemish.
- Bound construction is often assumed to need no upkeep, when in practice its joints and its outlets need attention precisely because it cannot drain internally.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- How has the joint layout been arrived at, and what movement is each joint expected to accommodate?
- What is the base intended to do, and does its design assume uniform support across the whole area?
- How will full bond beneath the units be achieved and evidenced before the surface is closed?
- What happens to water that enters through a cracked joint, and is there any route for it to leave?
- Where the paving abuts the building, how is the surface separated from the wall while keeping water moving away?
- If part of this surface has to be opened in future, what does a satisfactory repair look like?
What this page does not do
- No dimension, joint spacing, base thickness or mix is stated or implied here; each is a design output that depends on loading, ground and exposure.
- Slip behaviour of an external bound surface depends on the product, the finish and the conditions it is used in, and is assessed by the professionals specifying it.
- Laying a new bound surface over an existing one carries the condition of everything beneath it forward, and whether that is acceptable is established on site rather than assumed from appearance.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Decks, Paving and Hard Landscape Systems
External surfaces people stand on, plus the substructure, edge, support and guarding that make one usable, each decided by the layer beneath and the route its water takes.
Browse all decks & paving entries →