The Back of the Slab Says What Happened
Lift a cracked 900 by 600 porcelain slab off a two-winter-old terrace and the whole story is there in ten seconds. Five grey pads of mortar, each about the size of a hand, one at each corner and one in the middle. Between them, a back face still carrying the pale dust it came out of the factory with, and a crack running from one void to the next. Nothing failed under load. The slab was never held in the first place, and a hard frost in a trapped puddle finished an argument that had already been lost at laying.
Dab-fixing survives on concrete flags for a while, because a 50 mm flag is thick, forgiving in bending, and porous enough that mortar grips it whether or not anybody thought about adhesion. A 20 mm porcelain slab is none of those things: stiff, brittle, thin for its plan size, and glass-hard on both faces. Unsupported it behaves like a pane on four blocks — serviceable until something concentrates a load over a gap, and then it goes all at once with no warning cracks beforehand.
So the whole job comes down to two conditions that have nothing to do with each other and are both easy to miss. The slab must be in contact with mortar over its entire back face, and that contact must be bonded rather than merely touching. Everything below — the dig, the sub-base depth, the sand grade, the mix, the primer, the joint width — exists to deliver those two conditions and to keep delivering them after the terrace has been wet, frozen, walked on and washed for a decade.
What a mortar-bedded porcelain terrace is made of
- Jointing compound — an external brush-in or slurry-applied compound filling the full depth of every gap — it seals, it does not hold anything down, and it is bought by the manufacturer's coverage rate against joint width and slab depth
- 20 mm porcelain slabs — the wearing surface and, structurally, a stiff brittle plate that only works when it is supported over its whole footprint Paver Calculator
- Slurry bond coat — a polymer-gauged cement slurry brushed onto the back of each slab immediately before it is set, because a vitrified face will not bond to mortar by suction
- Full mortar bed — sharp sand and cement gauged by volume, laid one slab at a time and consolidated under the unit rather than screeded flat across the plot Paver Base & Sand Calculator
- Compacted sub-base — well-graded crushed aggregate placed in lifts the compactor can genuinely reach through, ordered by compacted tonnage rather than loose volume Gravel Base Layer Tonnage Calculator
- Prepared subgrade — cut to a formation that falls with the finished surface, so the sub-base above stays a constant thickness instead of thinning into a wedge Standard/Modified Proctor Compaction Percentage Calculator
Half a Percent of Water, and What Follows From It
Outdoor paving porcelain is a fully vitrified body. BS EN 14411, Ceramic tiles — Definition, classification, characteristics, assessment and verification of constancy of performance and marking, puts it in the dry-pressed group with the lowest water absorption, measured by BS EN ISO 10545-3, Determination of water absorption, apparent porosity, apparent relative density and bulk density. In round terms the body takes up half a percent of its weight in water; in trade terms nothing. Read the declared value off the sheet for the range you are laying, because a showroom rack often carries a vitrified body next to a lower-fired one.
The first consequence is the one that ruins terraces. Cement paste bonds to a porous substrate partly because the substrate drinks the water out of it. Porcelain offers nothing to drink. Mortar pressed against a bare porcelain back is in contact but not in bond, and contact without bond survives the summer, gives up in the first freeze cycle, and presents as a slab that rocks underfoot. That is why a bond coat is a specified component of this method rather than an optional refinement.
The second consequence is drainage. A sandstone or concrete flag absorbs a share of light rain and gives it back later; porcelain hands every drop straight to the falls and the joints. A terrace that would have looked merely damp in stone holds visible standing water in porcelain wherever the fall is short, and it stays wet at the surface longer — which is a slip question as much as an appearance one.
The third is frost, and it is widely misread. The slab itself is frost resistant when it has been tested to BS EN ISO 10545-12, Determination of frost resistance, and manufacturers publish that result. What is not frost resistant is a void full of water underneath a slab, or a bed sitting permanently saturated because the sub-base has nowhere to discharge. Every winter failure on a porcelain terrace is a water-management failure somewhere below the slab, not a failure of the ceramic.
Slip resistance is the last property that has to be specified rather than assumed, because a hard vitrified surface carries a water film readily. European data is usually a ramp classification to DIN 51130 or a pendulum value obtained under BS EN 16165, Determination of slip resistance of pedestrian surfaces — Methods of evaluation. North American sheets cite ANSI A326.3, whose wet threshold is written for level interior spaces rather than external paving. Take the wet figure for your chosen finish before the pallets are ordered.
Fixing the Fall Before the First Peg Goes In
Porcelain absorbs nothing, so the surface has one job for water and one only: move it somewhere. Set the falls first, before any depth arithmetic, because everything else is measured down from a finished plane you have already committed to. Fall away from the building, toward a channel, a gully or open ground that can take it — and check that the destination genuinely takes it, because a beautifully graded terrace draining onto a lawn that puddles has moved the problem three metres and no further.
A regulatory constraint usually decides the finished level before you get any choice in it. In England, Approved Document C to the Building Regulations, Site preparation and resistance to contaminants and moisture, requires the damp-proof course to sit clear above the finished external surface by a stated margin, capping how high paving may come against the wall. Under the International Residential Code the foundation drainage provisions require the ground to fall away from the building, capping flatness rather than height. Either way the finished level is not negotiable, so the build-up is found by digging deeper and never by raising the terrace.
Set out the slab module at the same time. A rectified 900 by 600 slab plus a nominal joint gives a repeating dimension, and where it lands decides whether the terrace ends in a full slab at the threshold or a 70 mm strip against the house. Pull the grid from the most visible edge and push the cuts outward, where they can be spread across two courses instead of appearing as one sliver in front of the doors.
Settle the paved area, the cutting allowance and the sub-base depth that the traffic class implies here, at the point where the grid and the falls are still lines on the ground and changing them costs nothing.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
Length of the paved area.
Width of the paved area.
What will use the surface.
Screeded sand layer under the units.
Allowance for cuts and breakage.
Paving area
279 ft²
Sub-base includes a 25% compaction allowance. Depths follow common guidance and should give way to a specific specification where one exists.
- Net paved area
- 253.5 ft²
- Sub-base depth
- 3.94 in
- Sub-base to order (loose)
- 3.85 yd³
- Sub-base weight
- 10,384.18 lb
- Bedding sand
- 0.98 yd³
- Area in ft²
- 278.85 ft²
They open the calculator with your figures already in it
Hardscape Area & Base Calculator: 279 ft² — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Assumes a competent subgrade. Soft or clay ground needs excavation to a firm formation and usually a geotextile separator, neither of which is calculated here.
- No allowance for edge restraint, which paving needs on every free edge or it spreads.
The Dig Is One Piece of Arithmetic, Done Once
Total build-up is the slab, the compacted bed, the sub-base, and any bound base you have decided to pour. Subtract it from the finished level and you have the formation. There is no recovery from getting it wrong: too shallow and the sub-base gets robbed to make the levels work, too deep and you buy a load of stone to fill a hole you dug for nothing. It is the one calculation worth doing twice on paper.
Cut the formation to a fall that matches the finished surface rather than digging flat. A flat formation under a falling terrace makes the sub-base a wedge — thin at the high end where it is needed and thick at the low end where it is not — and the thin end is where the first movement shows. Transfer the fall down with the same laser or line you set the surface with, not by eye.
Box out past the paving line. A rigid bed needs support under the outermost slab all the way to its outer corner, so the compacted sub-base runs beyond the finished edge and the bed is contained rather than tapering away into soil. This is not the interlock problem flexible block paving has — a mortar-bedded terrace has no interlock to lose and needs no proprietary restraint — but a perimeter slab bearing half on bed and half on loose backfill is the most predictable crack on the job.
Allow for spoil honestly: ground bulks when it comes out, so the volume disposed of exceeds the volume dug, and the difference decides whether the skip is one or two. Where a service trench or a recent footing crosses the footprint, note it now — those strips consolidate on their own schedule and are dealt with in the base, never in the bed.
Once the depths are fixed, the sub-base and bed volumes fall straight out of the area — run them together so the stone and the sand are ordered off the same set of depths rather than two remembered ones.
The length of the paved area.
The width of the paved area.
4-6 in (10-15 cm) is standard for a patio; use the higher end for driveways or heavier loads.
1 in (2.5 cm) is standard for the leveling sand layer directly under the pavers.
Estimated paver base & sand needed
1.605 cubic yards (gravel base)
- Patio area
- 130 ft²
- Bedding sand needed
- 0.4 cubic yards
They open the calculator with your figures already in it
Paver Base & Sand Calculator: 1.6 cubic yards (gravel base) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The volume covers the paver footprint only — length × width at the depth you enter. A compacted base is normally carried past the paver edge on every side so the edge restraint has something to sit on, and that perimeter band is not counted here.
- Both figures are finished, in-place depths. Crushed base and bedding sand are delivered loose and lose volume as they are compacted and screeded, so the quantity to order is higher than the number shown. No waste, over-dig or shrinkage allowance is applied.
- Only the two layers you enter are counted. Geotextile separation fabric, an open-graded drainage course, edge restraint and the jointing sand that goes between the pavers all sit outside the figure — as does the paver thickness, so this is not your excavation depth.
- The calculation applies whatever depth you type; it does not test whether that depth suits the subgrade, the frost depth or the load. Soft clay, organic or filled ground, poor drainage and anything carrying vehicles are pavement design decisions made from the soil and the traffic, not from these dimensions.
- The area is treated as one rectangle at a single uniform depth. Curved, L-shaped or multi-level layouts have to be split into rectangles and run separately, and any local thickening — a deeper base under a step, a soft spot dug out and backfilled — is not in the total.
Sub-base, and the Passes Nobody Counts
The sub-base wants a well-graded crushed material with enough fines to lock — an unbound mixture under BS EN 13285, Unbound mixtures — Specifications, detailed for road pavement work in the Manual of Contract Documents for Highway Works, Volume 1: Specification for Highway Works, or its North American equivalent ASTM D2940, Standard Specification for Graded Aggregate Material For Bases or Subbases for Highways or Airports. Rounded washed stone will not do this job at any depth; it stays a heap of marbles no matter how many passes it gets.
Depth is a local determination, set by the subgrade you actually found, the traffic the terrace will carry, and the frost depth your climate imposes. A pedestrian terrace on firm granular ground and a driveway on soft clay are not the same pavement, and no published figure covers both. Ask the building control body or the highway authority where vehicles are involved, and write the answer on the job sheet rather than carrying a depth over from the last job in a different soil.
Compact in lifts the machine can reach through, at a workable moisture content, in overlapping passes both directions. A single deep dump tests firm on top and hides a lower third that never densified, and that third consolidates later under the one thing a rigid terrace cannot tolerate. Where the subgrade is fine-grained or wet, lay a separation geotextile first, well lapped and turned up the excavation faces. Then order by compacted tonnage rather than by the loose volume you measured with a tape.
Suppliers weigh crushed stone off the wagon, so convert the compacted layer you have specified into tonnes here rather than pricing a volume nobody will sell you.
The total area to be covered with gravel base.
The target compacted thickness of the base layer.
The in-place density of the base once it is compacted.
Gravel base needed
73.8 tons
Actual density varies by material gradation and compaction — confirm with your supplier's specific product density for a precise order quantity.
- Volume
- 39.81 yd³
- Equivalent in US (short) tons
- 73.82 tons
They open the calculator with your figures already in it
Gravel Base Layer Tonnage Calculator: 73.82 tons — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- This is a take-off, not a pavement design. It multiplies out whatever compacted thickness you enter; nothing here derives that depth from traffic loading, subgrade strength, drainage or frost depth, which come from a pavement design or your local road authority's standard.
- Geometry is a flat plan area at one uniform depth. Crown and cross-fall, a formation that steps between thicknesses, a dig that deepens where the subgrade was soft, and edge thickening or haunching at the perimeter all fall outside area x thickness.
- No waste, spillage or subgrade-loss allowance is applied. The figure is the exact in-place mass, so stone lost into a soft or uneven formation, over-excavation, haul and spread losses, and the tail end of a part-load all sit on top of it.
- The density field is an in-place compacted density. A supplier's loose bulk density and a weighbridge ticket carrying free moisture are different quantities, and substituting either moves the tonnage: the wetter the delivered material, the less dry stone a given delivered weight puts on the ground.
- It covers one layer of one material. A base and sub-base of different gradations, a bedding or blinding course, and the geotextile or separation membrane between stone and subgrade are not counted here.
When Stone Is Not Enough and You Pour
Some terraces need a bound base between the sub-base and the mortar bed: anywhere a vehicle will run, anywhere the ground is made or recently backfilled, anywhere the paving crosses a trench, and anywhere the design puts large-format slabs over a subgrade you could not fully prove. A concrete base spreads point loads over a much wider footprint than the bed alone can, which is precisely the demand a 20 mm slab makes on whatever it is sitting on.
Pouring one changes the failure mode as well as the capacity. A rigid pavement moves as a single body, so a crack in the base does not stay there — it telegraphs up through the bed and appears in the porcelain on the same line, often within a season. That makes joint discipline non-negotiable: every movement joint and day joint in the concrete is carried straight up through the bed and the paving, sealed, and never bridged by a slab laid across it.
For small pours, hand-mixing cement, sand and coarse aggregate by volume is normal work, and the value of doing it by parts is repeatability between batches. Gauge with a marked box, not a count of shovelfuls: a shovel of damp sand and a shovel of dry aggregate are different volumes, and the difference compounds across a morning. Anything past a few barrows is cheaper and stronger as ready-mixed.
This one is for the concrete under the bed rather than for the bed itself: it splits a three-part cement, sand and coarse aggregate ratio into component volumes and cement bags, which is the arithmetic a hand-mixed base needs and the mortar bed does not.
The volume of loose dry material the ratio divides up — not the volume of finished concrete it makes.
The cement portion of the mix ratio.
The sand portion of the mix ratio.
The gravel (coarse aggregate) portion of the mix ratio.
Cement bags needed
5 x 94 lb cement bags
These are mixing proportions, not an order quantity. The figures describe a batch of loose dry cement, sand and gravel measuring the volume entered, and mixed and placed that batch fills roughly two thirds of that volume — so batch about half again to fill the whole of it. For structural work, verify your mix design meets the required strength for its application.
- Sand needed
- 9 ft³
- Gravel needed
- 13.5 ft³
They open the calculator with your figures already in it
Concrete Mix Ratio Calculator: 5 x 94 lb cement bags — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- No bulking allowance is built in. The ratio divides the figure you type, so the three components add up to exactly it, and what comes out of the mixer is around a third smaller — the mix closes its own voids, fine material taking up the gaps in the coarse and paste taking up the rest. Scaling the input by about one and a half is the usual working allowance for hand-mixed concrete, and it is left to you on purpose: the true factor moves with the richness of the mix and with the water it takes, and water is not an input here, so a single built-in constant would run high on a rich mix and low on a lean one.
- This is a volume split, not a mix design. It does not check that the ratio you entered reaches a specified strength, exposure class or durability requirement, and it does not replace a designed mix with trial batching and cube or cylinder testing on structural, reinforced or inspected work.
- Water is not in the output. A volume ratio says nothing about the water-cement ratio, which is what actually governs strength and durability, and nothing about air entrainment, plasticiser or any other admixture dosage.
- The split ignores everything about the aggregate except its share of the ratio — grading, maximum size, the moisture already in the pile, and sand bulking, which can swell damp sand enough that a measured bucket carries noticeably less sand than a dry one. Volume batching on site is also less repeatable than weigh-batching for the same reason.
- The bag count comes from a standard loose bulk density for portland cement, not from a weigh-batch of the cement you have, and it rounds up to whole bags with no allowance for spillage or part-used bags. Mixes with no coarse aggregate — mortars, screeds, renders — are outside what this page accepts, because it requires at least one part gravel.
Gauging the Bed
The bed for 20 mm porcelain is a full, continuous layer of cement-bound sharp sand, compacted to a finished thickness in the region of 30 to 40 mm under each slab. That range is what porcelain manufacturers publish in their own installation instructions, and theirs is the instruction that governs the warranty, so read the sheet for the range on the pallets. What the bed is not is a screed you level the whole plot with. It is a bearing layer thick enough to consolidate under the slab and thin enough not to shrink while it does.
Sand grade loses more beds than proportion does. Use a sharp, well-graded concreting sand — the 0/4 material described by BS EN 12620, Aggregates for concrete, or a fine aggregate to ASTM C33, Standard Specification for Concrete Aggregates. Not soft building sand, which is graded for masonry mortar under BS EN 13139, Aggregates for mortar, or ASTM C144, Standard Specification for Aggregate for Masonry Mortar. Soft sand feels wonderful under the trowel, holds far less strength, and shrinks away from the back of the slab as it dries — a slow-motion version of the dab-fixing failure.
Cement is a common cement to BS EN 197-1, Cement — Composition, specifications and conformity criteria for common cements, or a portland cement to ASTM C150, Standard Specification for Portland Cement — and the plasticiser stays in the van, because the air an admixture introduces is bubble area that is not bond area. Proportion by volume in the band manufacturers publish, roughly four parts sand to one of cement up to about six to one, gauged with the same bucket every time. A lean bed crumbles under a point load at the slab corners and a rich one shrinks and drags the bond with it, but consistency from the first batch of the morning to the last before lunch matters more than which end of the band you pick.
Water content decides whether the method works at all. The mix should be moist rather than wet: squeezed in a gloved hand it holds its shape and marks the glove, and it does not bleed water or slump when released. Too wet and the slab sinks under its own weight, keeps sinking, and cannot be brought back up. Too dry and it will neither consolidate under the slab nor accept the slurry, and you get a bed that looks perfect and is bonded to nothing.
| Variable | What actually sets it | Where to get the figure |
|---|---|---|
| Compacted bed depth | Slab format and thickness, plus the tolerance the bed has to absorb | The paving manufacturer's published installation instructions for that range |
| Sand grade | Bond strength and shrinkage, not workability | Sharp concreting sand to BS EN 12620 or ASTM C33 — never masonry sand |
| Cement type | Availability and setting time in the ambient conditions | BS EN 197-1 common cements or ASTM C150 portland cement |
| Sand to cement proportion | Crushing resistance at the slab corners versus drying shrinkage | The band in the paving manufacturer's instructions, gauged by volume |
| Water content | Whether the bed consolidates under the slab and holds level | Hand test on site, re-checked each batch as the sand stock dries out |
The bed is priced as sand plus cement, and sharp sand is sold by weight — turn the bed volume you just fixed into a tonnage before the order goes in, because a bed that runs out mid-terrace sets a cold joint through the middle of the job.
The volume of sand required, in cubic yards.
Approximate weight
13.5 short tons
About 1.35 short tons per cubic yard for dry building sand; damp sand is heavier and bulks up in volume. This is a planning figure, not a specification. Confirm the density with your supplier before ordering by weight.
- Conversion factor applied
- 1.35 short tons per cu yd
They open the calculator with your figures already in it
Sand Cubic Yards to Tons Calculator: 13.5 short tons — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Weight is the honest unit for sand, with one qualification: part of what a weighbridge records is water. Sand kept in the open carries whatever it has taken up, so a tonnage bought after a wet spell is less sand than the same tonnage bought dry, and nothing on the ticket separates the two.
- The density is for loose-tipped sand, not for sand in the ground. Undisturbed sandy ground is usually packed tighter than the heap it makes once it is dug, so a volume measured in the excavation converts to more tons than this returns. Do not borrow the correction from an earthworks take-off — sand loosens far less on digging than clay or mixed soil, and a swell factor lifted off those overstates it.
- A tonnage is what the sand weighs, not a check on what is carrying it. The sand for one small area already runs to tons, which is enough to matter on a trailer, on a scaffold or a suspended floor, and at the edge of an open excavation where the heap surcharges the face. Those limits come from the vehicle's rating, the structure's design or the excavation's support, not from this figure.
Sand is bought by the yard for bedding and screeding and sold by the ton at most aggregate yards, so this conversion sits between the take-off and the order. Sand has a peculiarity the other aggregates do not share: damp sand bulks, occupying noticeably more volume than the same sand dry or fully saturated, because surface water holds the grains apart. That means a volume measured from a damp stockpile overstates what is actually there, while the weight does not lie. Where accuracy matters, buy by weight and convert, rather than trusting a measured heap.
The Slurry Is the Bond; the Bed Is Only the Support
The bond coat is a thin slurry of cement gauged with an SBR or acrylic polymer, or alternatively a cementitious tile adhesive classified as improved under BS EN 12004-1, Adhesives for ceramic tiles — Requirements, assessment and verification of constancy of performance, classification and marking, used as a bond bridge. Either way it is mixed to the primer manufacturer's own ratio, and that ratio is not a matter of taste: too thin and it is coloured water, too thick and it will not wet the surface properly.
Brush it onto the entire back of the slab, corner to corner and out to the arris — half-coating the back is the dabbing mistake arriving from a different direction. Work it in with a brush rather than pouring and spreading, because the point is to wet a closed face and a poured film sits on top of whatever dust it finds.
Timing is what most crews get wrong. The slurry has to be genuinely wet when the slab goes down, into a bed that is also fresh. Skinned or tacky, it has stopped being an adhesive and started being a release agent, and laying through it gives a slab that seems perfectly bedded and is bonded to a film. Prime one slab ahead at most, and re-coat anything left standing.
Two checks catch a bad method early. Brush the back of every slab before priming, because porcelain arrives with press and kiln dust on the reverse and slurry bonded to dust is bonded to nothing. Then set the first slab, lift it straight back up and look: full grey transfer across the whole back face, not four smears and a dry middle. Partial transfer means bed moisture, slurry consistency or timing, and it is worth solving before slab two rather than slab two hundred.
One Slab, One Bed, No Second Chance
Lay onto a fresh bed made for each slab rather than screeding a field and working across it. A bed left open stiffens, and a stiffened bed will not consolidate under the slab — it holds it up on high spots and leaves voids around them, which is exactly the condition the whole method exists to avoid. It feels slower than screeding and it is not, because nothing gets relaid.
Consolidate positively. Set the slab down, then bring it to the line with a rubber mallet, tapping across the whole face rather than at the corners so the bed compresses uniformly instead of tipping. Work to a string line or a laser held on the falls and check each slab against its neighbours both ways. A slab that has gone low cannot be tapped back up, and packing under a corner is not a repair — it comes out, the bed is rebuilt, and it is laid again.
Keep off new work. Kneel on boards, run barrows on boards, and do not stand on a slab set within the hour — a slab moved after the bed has begun to set is a slab bonded to a broken bed. Boards also stop grit being ground into a wet joint, which is a finish problem you will be scrubbing at later.
Cut wet, and cut on the ground. A continuous rim diamond blade with water gives a clean arris and controls the respirable crystalline silica that dry cutting porcelain generates in quantity — the hazard covered by HSE guidance on silica in construction and by OSHA 29 CFR 1926.1153, Respirable Crystalline Silica. Holes take a diamond core with water and a slow start on the tilt. Keep cutting slurry off laid slabs; it dries into the surface texture.
Blend across pallets as you go. Porcelain is a fired batch product with real shade variation between runs, and drawing from three or four pallets at once distributes it invisibly; working pallet by pallet lays it out in blocks. Large-format 20 mm slabs are also heavy, awkward and easy to chip, which makes manual handling a planned part of the day under the Manual Handling Operations Regulations 1992.
- Brush the slab back clean of press and kiln dust.
- Lay a fresh bed slightly proud of the finished depth, sized for that slab only.
- Brush the bond slurry over the whole back face, out to every edge.
- Set the slab while both slurry and bed are wet, positioning off the string line.
- Tap down across the full face, checking against both adjacent slabs.
- Set the joint with spacers to the minimum width, clearing squeeze-out before it stiffens.
- Move the boards and repeat, never working further ahead than the next slab.
Fix the slab count and the cutting allowance before the first pallet is broken open, because a short order returns as a different firing batch and a visible tonal break across the terrace.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The length of the area to be paved.
The width of the area to be paved.
Any part of the rectangle that is not paved — a pool, a planting bed, a tree pit. Leave it at 0 for a plain rectangle.
The length of a single paver.
The width of a single paver.
Extra pavers for edge cuts, curves, and breakage.
Estimated paver needed
143 pavers
The pavers are the visible tenth of the job. What determines whether they stay flat is the base beneath them and the edge restraint around them, and both are larger quantities than the surface.
- Patio area
- 130 sq ft
- Coverage per paver
- 1 sq ft
They open the calculator with your figures already in it
Paver Calculator: 143 pavers — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Paver setting-out grid — drawn from your figures
A 3.96 x 3.05 m patio set out in 0.3 x 0.3 m pavers, stack bond.
A DXF plan in metres. Opens in AutoCAD, LibreCAD, QCAD and most jobsite viewers.
What this drawing does not show (3)
- The bond. This is drawn as a stack bond because the calculator counts by AREA and assumes no pattern — running bond, herringbone and basketweave all cover the same area and change only the cuts.
- Joint width and edge restraint.
- Where the cuts fall, which depends on which corner you start from.
It is a setting-out aid drawn from the figures you entered, not a construction drawing, and nobody has checked it against a design.
What this calculation does not cover
- Excludes the sub-base, bedding layer and jointing sand, each of which has its own depth and its own compaction allowance.
- Excludes the edge restraint, without which the field spreads laterally under traffic and the joints open.
- Does not model cutting waste on a curved or angled layout, which is far higher than on a rectangular field.
Joints Wide Enough to Move
Rectified porcelain has machined edges and tempts everybody toward a hairline joint. Resist it. Manufacturers publish a minimum external joint width for this product — commonly around 5 mm, but take the figure from your own supplier — and it has to be that wide for two reasons: enough section for the compound to be a body of material rather than a smear, and enough room for a rigid pavement to expand in July without slabs meeting edge to edge and spalling their arrises.
Fill joints with a compound made for external porcelain: a resin or polymer-modified cement, brushed in or slurry-applied, taken to the full depth of the slab. Kiln-dried jointing sand belongs to flexible block paving, where it transfers shear between units relying on interlock. On a bonded rigid bed it transfers nothing and washes out. Read the compound's limits on temperature, surface dryness and rain within the curing window before opening the tub; residue cured onto a porcelain face is difficult to remove.
Movement joints are a separate item from the jointing compound and are the ones most often left out. Run one round the perimeter where the paving abuts the house or any rigid structure, run one over every joint in a concrete base beneath, and break large areas into bays at the spacing the base design implies. These get a compressible backing and a sealant, not the joint fill. A rigid terrace with no perimeter joint pushing against a house wall through one hot summer is a specific, avoidable and very visible failure.
The First Winter Is the Test
Keep traffic off while the bed gains strength; the timing comes from the cement and the conditions, and cold, damp weather stretches it considerably. Protect fresh work from heavy rain, which washes an uncured joint out and can float a bed mixed a shade too wet. If frost is forecast inside the curing window, cover the work or do not lay it — mortar that freezes before it sets does not recover by warming up again.
Expect some efflorescence and know what it is telling you. Salts carried by moisture moving through the bed and up the joints crystallise on the surface as a white bloom, usually in the first months, usually weathering off. What is not normal is a bloom that keeps returning year after year, because that means the bed is permanently wet and the water has no way out. The answer is at the sub-base and the discharge point, not on the surface with a bottle of cleaner.
Do not seal porcelain. There is no open porosity for a sealer to enter, so it sits on the surface as a film, alters the slip characteristic you specified, and wears off in traffic patterns. If anything is treated it is the jointing compound, and only where its maker says so.
When something does go wrong, read it backwards. A slab that rocks means a void, and therefore a bond or bed-moisture failure on that unit. A clean crack across one slab means the same void plus a load that found it. A crack running straight through several means the base moved, usually a joint bridged instead of carried through. Standing water means the fall was lost during laying; white bloom at the joints means a moisture path. In every case the diagnosis sits one layer below the symptom — the recurring lesson of a surface that cannot absorb, cannot flex and cannot forgive.
Ordering a mortar-bedded porcelain terrace
Quantify downward from the finished level, because the finished level is the one dimension the regulations have already fixed for you and everything else is found by digging.
- Paved area with cutting allowance — Measure to the finished edge, then add for perimeter cuts against the slab module — large-format porcelain generates more waste per cut than a small unit does.
- Total build-up, measured off the damp-proof course — Slab plus compacted bed plus sub-base plus any bound base; the finished level is capped by the adopted building document, so the depth is found downward.
- Sub-base tonnage at compacted thickness — Well-graded crushed unbound material, split into lifts the compactor on site can genuinely densify, priced as tonnes off the wagon rather than as loose volume.
- Bed sand and cement, gauged by volume — Sharp concreting sand rather than soft building sand, plus cement at the proportion the paving manufacturer's instructions publish — no plasticiser.
- Bond slurry by back-face area — Every slab gets its whole back coated, so the coverage runs against total slab area and not against a perimeter or a dab pattern.
- Jointing compound and movement joint materials — Compound to the joint width and full slab depth, plus backer and sealant for the perimeter joint and every joint carried up from the base below.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.