Masonry, explained.
Find answers to common questions, look up masonry terms, and explore how lime changes from stone to binder and back again.
Frequently asked questions
Open a question to read the answer. Scroll inside this panel to see all the questions.
Why do brick faces flake off?
Can any new brick replace an old one?
Is pressure washing safe for brick?
Are cement and concrete the same?
Are concrete block walls waterproof?
Can cracks in a block wall just be filled?
Does every chimney use the same cap?
Will a chimney cap stop every chimney leak?
Can an unused chimney be sealed?
How are cinderblocks different from concrete blocks?
Are cinderblocks strong enough for a load-bearing wall?
Can old cinderblocks be repaired?
What is mortar?
Is stronger mortar always better?
Does masonry need maintenance?
Is every crack in masonry serious?
Should bare masonry be painted?
Why does paint peel off masonry?
Can paint be removed from masonry?
Is breathable paint a cure for damp walls?
Glossary
A
Aggregate
Mortar and ingredients
The sand, grit, or crushed stone mixed with a binder to make mortar. Aggregate makes up most of a mortar's volume, and its size, shape, and colour affect the mortar's strength, texture, and appearance.
Arch
Building elements
A curved structure that spans an opening and carries the weight above it down to the supports on each side. Masonry arches are built from wedge-shaped units called voussoirs.
Ashlar
Wall construction
Stone that has been cut and dressed to square, even faces and laid in regular courses with thin joints.
B
Batter
Wall construction
A deliberate backward slope in the face of a wall, so the wall is thicker at the base than at the top. It is common in retaining walls and dry stone walls.
Bed joint
Wall construction
The horizontal mortar joint that a brick or stone sits on.
Bedding plane
Stone types
The natural layer lines in sedimentary stone, formed as the sediment was laid down. Stone splits most easily along these planes.
Binder
Mortar and ingredients
The material in a mortar that holds the aggregate together and makes the mortar set, such as lime or cement.
Binder-to-aggregate ratio
Mortar and ingredients
The proportion of binder to aggregate in a mortar, measured by volume, such as 1 part lime to 3 parts sand. The ratio affects strength, workability, and durability.
Biological growth
Decay and defects
Moss, algae, lichen, or plants growing on or in masonry. Some growth is harmless, but roots and growth that holds moisture against a wall can damage joints and stone.
Bond
Wall construction
The pattern in which bricks or stones are laid so that the vertical joints are staggered from one course to the next. Bonding ties the wall together and spreads loads through it.
Breathability (vapour permeability)
Repair and conservation
vapour permeability
The ability of a wall or material to let water vapour pass through it, so moisture can dry out. Traditional masonry relies on this, and sealing it with impermeable materials can trap moisture inside the wall.
Bricks
Wall construction
Bricks are building units, usually made from clay shaped and fired in a kiln. Mortar holds them together and fills the joints. Brickwork can support a building or form an outer covering over another wall.
Bulging (bellying)
Decay and defects
bellying|Bowing
An outward swelling in the face of a wall. In older stone walls it often means the outer face is separating from the core or inner face, and it should be assessed by a qualified professional.
Bush hammer
Tools and stone dressing
A hammer with a grid of small pyramid-shaped points on its face, used to give stone an evenly pitted, textured surface.
Buttress
Building elements
A mass of masonry built against a wall to support it and resist sideways pressure.
C
Calcination
Lime processes and chemistry
Heating limestone in a kiln to about 900°C so that it gives off carbon dioxide and becomes quicklime. Also called lime burning.
Calcium carbonate
Lime processes and chemistry
The chemical compound CaCO₃, which makes up limestone, chalk, and marble. Fully cured lime mortar is also calcium carbonate.
Calcium hydroxide
Lime processes and chemistry
The chemical compound Ca(OH)₂, formed when quicklime reacts with water. It is the binder in slaked lime, hydrated lime, and lime putty.
Calcium oxide
Lime processes and chemistry
The chemical compound CaO, commonly called quicklime. It is made by burning limestone and reacts strongly with water.
Carbonation
Lime processes and chemistry
The slow process by which lime hardens. Calcium hydroxide absorbs carbon dioxide from the air and turns back into calcium carbonate. It needs some moisture and can take weeks to years, depending on how deep the mortar is.
Cement blocks
Wall construction
concrete block|concrete masonry unit|CMU
Cement blocks is a common name for concrete blocks, also called concrete masonry units or CMUs. Cement is the binder; concrete is the finished mixture of cement, water, sand, and larger particles such as gravel. Blocks may be hollow or solid.
Cement damage
Decay and defects
Harm caused when hard, dense Portland cement mortar or render is used on older, softer masonry. The cement traps moisture and is stronger than the brick or stone, so weathering and movement damage the masonry instead of the joints.
Chimney caps
Building elements
A chimney cap is a cover over a flue opening. It helps keep rain, animals, and debris out while allowing gases to escape. A chimney crown is different: it is the sloped top surface that sheds water away from the chimney masonry.
Chimney crown
Building elements
The sloped top surface of a chimney, usually mortar, concrete, or stone, that surrounds the flue and sheds water away from the chimney masonry.
Chimney stack
Building elements
The part of a chimney that rises above the roof. Its exposed position makes it one of the most weathered parts of a building.
Cinderblocks
Wall construction
Traditional cinderblocks are concrete masonry units made with cinders or similar lightweight material as part of the mixture. Today, people often use cinderblock to mean an ordinary concrete block, even when it contains no cinders.
Claw chisel
Tools and stone dressing
A stone chisel with a toothed cutting edge, used to level and shape a stone surface after the rough waste has been removed.
Compatibility
Repair and conservation
The principle that repair materials should be similar to the original in strength, flexibility, and vapour permeability, so they work with the existing masonry instead of damaging it.
Concrete
Mortar and ingredients
A mixture of cement, water, sand, and larger particles such as gravel that hardens into a stone-like material. Cement is one ingredient of concrete, not the same thing.
Coping
Building elements
The protective top layer of a wall or parapet, made of stone, brick, concrete, or metal. It is usually sloped or overhanging to shed water.
Corbel
Building elements
A piece of masonry, or a series of courses, that projects from the face of a wall to support a weight above it or for decoration.
Core (hearting)
Wall construction
hearting
The middle of a thick stone wall, between the inner and outer faces, traditionally filled with smaller stones and mortar.
Course
Wall construction
A single continuous horizontal row of bricks or stones in a wall.
Coursed
Wall construction
Describes masonry laid in level horizontal rows.
Cracking
Decay and defects
Breaks in masonry or mortar. Causes include settlement, moisture, frost, rusting metal inside the wall, and temperature changes. The pattern, width, and whether a crack is still moving all indicate how serious it is.
D
Delamination
Decay and defects
Separation into layers. In stone, surface layers split away along the bedding planes. In a wall, the outer face separates from the core or inner wythe.
Dimension stone
Stone types
Natural stone that has been quarried and cut to specific sizes and shapes.
Drafted margin
Tools and stone dressing
A smooth, chiselled border worked around the edges of a stone's face, often with the centre left rougher.
Dry stone
Wall construction
Stonework built without mortar. The wall's stability comes from the weight of the stones and how carefully they are fitted together.
Dutchman repair
Repair and conservation
A repair in which the damaged part of a stone is cut out and a new piece of matching stone is fitted into its place.
E
Efflorescence
Decay and defects
A white, powdery salt deposit on the surface of masonry, left behind when water moving through the wall evaporates. It is usually harmless in itself but shows that moisture is travelling through the masonry.
F
Face bedding
Stone types
Laying a sedimentary stone with its bedding planes vertical and parallel to the wall face. It is generally considered a fault because the layers tend to peel away over time.
Fieldstone
Stone types
Stone gathered from the surface of the land instead of being quarried. It is usually rounded or irregular and is used as found or roughly shaped.
Flashing
Building elements
Thin sheet material, usually metal, fitted at junctions such as where a chimney meets a roof, to direct water away from the building.
Flue
Building elements
The passage inside a chimney that carries smoke and combustion gases out of the building.
Footing
Wall construction
The widened base of a wall or foundation that spreads the weight of the structure onto the ground.
Free lime
Lime processes and chemistry
Lime in a mortar that is still calcium hydroxide and has not yet carbonated. It can dissolve and re-deposit in fine cracks, which helps lime mortar heal itself, but it can also leave white staining on the surface.
Freeze-thaw damage
Decay and defects
Damage caused when water inside masonry freezes and expands. Repeated cycles of freezing and thawing crack and flake brick, stone, and mortar.
G
Gauging
Mortar and ingredients
Adding a measured amount of another binder, such as hydraulic lime, cement, or a pozzolan, to a lime mortar to change how it sets. The word is also used for measuring out mortar ingredients by volume.
Granite
Stone types
A hard, coarse-grained igneous rock made mainly of quartz and feldspar. It is very durable but difficult to cut and dress.
Grouting
Repair and conservation
Filling hidden voids inside a wall by pouring or injecting a fluid mortar called grout. It is used to stabilise walls whose core has hollows.
H
Hair and fibre reinforcement
Mortar and ingredients
Animal hair or plant fibre mixed into lime plaster and render to hold it together and limit shrinkage cracks.
Hawk
Tools and stone dressing
A flat, square board with a handle underneath, used to hold a small amount of mortar or plaster close to the work.
Header
Wall construction
A brick or stone laid with its short end showing in the wall face, so that its length runs into the wall and helps tie it together.
Hot lime (hot-mixed mortar)
Lime materials
hot-mixed mortar
Mortar made by mixing quicklime directly with sand and water, so the lime slakes within the mix. It may be used while still warm or after it has cooled. Quicklime is caustic and the reaction releases intense heat.
Hydrated lime
Lime materials
Calcium hydroxide sold as a dry powder, made by slaking quicklime with just enough water to complete the reaction.
Hydraulic set
Lime processes and chemistry
Hardening by chemical reaction with water instead of with air. It allows a mortar to set in damp conditions or under water, as natural hydraulic lime and cement do.
I
Igneous rock
Stone types
Rock formed when molten material cools and solidifies. Granite and basalt are examples.
Indent
Repair and conservation
A new piece of stone set into a pocket cut out of a damaged stone. It is another name for a Dutchman repair.
J
Joint profiles
Repair and conservation
The shape given to the face of a finished mortar joint, such as flush, recessed, struck, or tuck. The profile affects both the wall's appearance and how well the joint sheds water.
K
Keystone
Building elements
The central wedge-shaped stone at the top of an arch. It is the last one placed and locks the other voussoirs in position.
Knocking up
Mortar and ingredients
Reworking a lime mortar that has stiffened by beating and mixing it until it is workable again, with little or no added water. It is not suitable for a hydraulic mortar that has begun to set.
L
Like-for-like repair
Repair and conservation
Repairing a building using the same materials and techniques as the original construction.
Lime burning
Lime processes and chemistry
The traditional name for heating limestone in a kiln to make quicklime. See calcination.
Lime cycle
Lime processes and chemistry
The sequence by which limestone is burned to make quicklime, slaked with water to make calcium hydroxide, and then hardens by carbonation back into calcium carbonate.
Lime kiln
Lime processes and chemistry
A furnace in which limestone is burned to make quicklime.
Lime mortar
Lime materials
Mortar that uses lime as its binder, mixed with sand and water. It is softer, more flexible, and more vapour-permeable than cement mortar, and was the standard mortar before the early twentieth century.
Lime plaster
Lime materials
An interior wall and ceiling finish made from lime and sand, often with hair added, and applied in several coats.
Lime putty
Lime materials
Calcium hydroxide in the form of a soft, wet paste, made by slaking quicklime in excess water and storing it wet. It hardens only by carbonation.
Lime render
Lime materials
A lime-based coating applied to the outside of a wall to protect it from weather while still letting it dry out.
Limestone
Lime materials
A sedimentary rock made mainly of calcium carbonate. It is both a building stone and the raw material for making lime.
Limewash
Lime materials
A traditional finish made from slaked lime thinned with water. It soaks into the surface, hardens by carbonation, and lets water vapour pass through.
Lintel
Building elements
A horizontal beam of stone, timber, steel, or concrete that spans an opening such as a door or window and carries the wall above it.
M
Mallet
Tools and stone dressing
A striking tool with a large head of wood or nylon, used with chisels for carving and dressing stone.
Marble
Stone types
A metamorphic rock formed when limestone is changed by heat and pressure. It can be polished and is widely used for decorative work.
Mash hammer
Tools and stone dressing
A short-handled, heavy steel hammer used to strike chisels and for general stone dressing. Also called a club hammer or lump hammer.
Masonry
Wall construction
Masonry is construction using individual units such as brick, stone, or concrete blocks. These units are usually joined with mortar. Some stonework is dry laid, meaning it uses no mortar.
Metamorphic rock
Stone types
Rock that has been changed from an earlier rock by heat and pressure. Marble, slate, and gneiss are examples.
Minimal intervention
Repair and conservation
The conservation principle of doing only as much work as is needed, so that as much original material as possible is kept.
Mortar
Mortar and ingredients
The mixture of binder, aggregate, and water that beds and joins masonry units and fills the joints between them. It cushions the units and helps keep water out. In traditional masonry it is meant to be softer than the brick or stone.
Mortar analysis
Mortar and ingredients
Testing a sample of existing mortar to identify its binder, aggregate, and proportions, so that a repair mortar can be matched to it.
N
Natural bed
Stone types
Laying a sedimentary stone with its bedding planes horizontal, the way it formed in the ground. This is the strongest position for most stones in a wall.
Natural cement
Lime materials
A binder made by burning limestone that naturally contains clay, then grinding it to a powder. It sets quickly with water and was widely used in the nineteenth century, before Portland cement became common.
Natural hydraulic lime (NHL)
Lime materials
NHL
Lime made from limestone that naturally contains clay or silica. It sets partly by reacting with water and partly by carbonation, and is graded by strength as NHL 2, NHL 3.5, or NHL 5.
Non-hydraulic lime (air lime)
Lime materials
air lime
Lime that hardens only by carbonation, by absorbing carbon dioxide from the air. It will not set under water. Lime putty and hydrated lime are both non-hydraulic limes.
O
P
Paint
Repair and conservation
Paint forms a coating on masonry. Some coatings allow more water vapour to pass through than others. Painting changes both the appearance and the way the surface dries.
Parapet
Building elements
The part of a wall that rises above the roof line. Because it is exposed on both sides and on top, it is especially vulnerable to weather.
Perpend (head joint)
Wall construction
head joint
The vertical mortar joint between two bricks or stones in the same course.
Pigment
Mortar and ingredients
Finely ground colouring material, usually a mineral oxide, added to mortar, render, or limewash. Pigments used with lime must be able to withstand its alkalinity.
Pinning
Repair and conservation
Small stones pushed into the wider joints of rubble walling to wedge the larger stones and reduce the amount of mortar needed. The word is also used for securing loose or cracked stone with metal pins.
Pitching tool
Tools and stone dressing
A wide chisel with a thick, blunt edge, used to break waste stone off along a line and leave a straight edge or a rock-faced finish.
Plugging chisel
Tools and stone dressing
A narrow chisel with a tapered blade, used to cut old mortar out of joints without damaging the edges of the brick or stone.
Point
Tools and stone dressing
A stone chisel drawn to a point, used for roughing out and removing waste stone.
Pointing
Repair and conservation
Filling and finishing the outer part of mortar joints. The word also refers to the visible mortar finish itself.
Pointing iron
Tools and stone dressing
A narrow, flat steel tool used to press mortar firmly into joints and finish them.
Portland cement
Lime materials
A modern manufactured binder made by firing limestone and clay at very high temperatures and grinding the result to a powder. It sets quickly and hard, and is much less vapour-permeable than lime. It became the common binder in buildings from the early twentieth century.
Pozzolan
Lime materials
A material containing reactive silica, such as volcanic ash or brick dust, that reacts with lime and water to give the mortar a hydraulic set.
Punch
Tools and stone dressing
A stone chisel with a very narrow cutting edge, slightly wider than a point, used for removing waste and roughly levelling a surface. In some regions punch and point mean the same tool.
Q
Quicklime
Lime materials
quick lime
Calcium oxide, made by burning limestone in a kiln. It reacts vigorously with water, releases intense heat, and is caustic.
Quoin
Wall construction
A stone or brick that forms the outside corner of a building, often larger than or dressed differently from the rest of the wall.
R
Raking out
Repair and conservation
Removing old or failed mortar from joints to a set depth before repointing, usually about twice the width of the joint or until sound mortar is reached.
Random
Wall construction
Describes masonry laid without continuous level courses, as in random rubble walling.
Rebuilding
Repair and conservation
Taking down a failed section of masonry and laying it again, reusing the original units wherever possible. It is used when repair in place is not possible.
Repointing
Repair and conservation
Removing deteriorated mortar from the outer part of the joints and replacing it with new mortar.
Reversibility
Repair and conservation
The conservation principle that a repair should be removable in the future without damaging the original material.
Rising damp
Decay and defects
Ground moisture drawn up into the base of a wall by capillary action. It is often misdiagnosed, because poor drainage, raised ground levels, leaks, and impermeable coatings cause similar symptoms.
Rock-faced finish
Tools and stone dressing
A stone finish in which the face is left rough and projecting, like natural rock, while the edges are trimmed straight.
Rubble
Wall construction
Walling built from rough, irregular stones that are undressed or only roughly dressed. It may be coursed or random.
S
Sacrificial mortar
Mortar and ingredients
A mortar deliberately made softer and more permeable than the masonry around it, so that moisture, salts, and movement wear away the mortar instead of the brick or stone. Mortar is far easier to renew than masonry.
Salt damage
Decay and defects
Decay caused when salts dissolved in water crystallise in or on masonry as the water evaporates. The growing crystals break up the surface. Salts can come from the ground, de-icing salt, or cement.
Sandstone
Stone types
A sedimentary rock made of sand grains bound together by a natural cement such as silica, calcite, clay, or iron oxide. Its durability depends largely on that natural cement.
Sedimentary rock
Stone types
Rock formed from layers of sediment that were compacted and cemented together over time. Limestone and sandstone are examples.
Sharp sand
Mortar and ingredients
Coarse sand with angular grains in a range of sizes. It gives mortar strength and reduces shrinkage.
Sill
Building elements
The horizontal member at the bottom of a window or door opening, usually sloped and projecting to shed water away from the wall below.
Slaked lime
Lime materials
Calcium hydroxide, made by adding water to quicklime. It is a general term that covers both lime putty and dry hydrated lime.
Slaking
Lime processes and chemistry
Adding water to quicklime to produce calcium hydroxide. The reaction releases intense heat and is hazardous.
Spalling
Decay and defects
The flaking, chipping, or breaking away of the face of a brick or stone. Common causes are freeze-thaw cycles, salts, and hard cement mortar trapping moisture.
Stone consolidation
Repair and conservation
Treating weak, crumbling stone with a liquid that soaks in and strengthens it. It is specialist work and difficult to reverse, so it is used with caution.
Stretcher
Wall construction
A brick or stone laid with its long side showing along the face of the wall.
String course
Building elements
A continuous horizontal band of brick or stone across a wall face, usually projecting slightly. It is decorative and helps shed water.
Subflorescence
Decay and defects
Salts that crystallise just beneath the surface of masonry instead of on it. It is more damaging than efflorescence because the crystals push the surface off.
Sulphate attack
Decay and defects
A chemical reaction between sulphate salts and Portland cement that forms expanding crystals, causing mortar to crack and crumble. The sulphates can come from some bricks, from soil, or from flue gases in chimneys.
T
Through stone
Wall construction
A long stone that runs the full thickness of a wall and ties its two faces together.
Tooling
Tools and stone dressing
The marks or texture left on a stone surface by dressing tools. The word is also used for compressing and shaping the face of a mortar joint.
Trowel
Tools and stone dressing
A hand tool with a flat steel blade, used for lifting, spreading, and shaping mortar. Different shapes suit laying, pointing, and finishing.
U
V
Voussoir
Building elements
One of the wedge-shaped stones or bricks that make up an arch.
W
Weep hole
Building elements
A small opening left in a wall to let trapped water drain out.
Workability
Mortar and ingredients
How easily a fresh mortar can be spread, placed, and pressed into joints. Lime mortars are known for good workability.
Wythe
Wall construction
A single vertical layer of masonry, one unit thick. A wall may be built of one or more wythes.
X
Y
Z
The lime cycle
Heat changes limestone into quicklime. Water changes quicklime into slaked lime. As lime mortar cures, it takes carbon dioxide from the air and gradually becomes calcium carbonate again.
Limestone
Calcium carbonate · CaCO₃
Quicklime
Calcium oxide · CaO
Slaked lime
Calcium hydroxide · Ca(OH)₂
Calcination · about 900°C. CO₂ leaves.
Slaking · H₂O enters. Heat is released.
Carbonation · CO₂ from the air re-enters as lime cures.
Lime putty · Ca(OH)₂
Calcium hydroxide made by slaking quicklime with excess water, then storing it wet. Mixed with sand, it can become mortar. It hardens by carbonation after use, not while kept under water.
Hot lime · CaO to Ca(OH)₂
Quicklime is slaked together with sand and water to make hot-mixed mortar, which may be used while still warm. The lime binder becomes calcium hydroxide; the mortar is a mixture, not a separate chemical compound.
Lime putty and hot-mixed mortar are preparation routes, not extra chemical steps in the cycle. Both return toward CaCO₃ through carbonation. Quicklime is caustic, and slaking releases intense heat: this is an explanation, not a DIY mixing guide.
I’m adding to this collection over time as people ask me questions about masonry. The information is general guidance, not a diagnosis of your building. Structural movement, falling masonry, and chimney or venting concerns need an on-site assessment by a qualified professional.