You are here: Arches and Lintels: Part 1 - Arches

Arches and Lintels: Part 1 - Arches

A structural look at Arches and Lintels


Arches and lintels are found wherever it is necessary to support a wall etc over an opening which is usually not more than about two metres wide. They provide support in completely different ways and an understanding of this is fundamental to selecting, specifying and assessing them.
Arches occur naturally, and there are some famous examples, e.g. Durdle Dor in Dorset, and in the National Parks in the USA. However. It was many centuries before mankind understood the mechanics of the arch and devised the means to successfully construct them. There are now so many different types that a complete listing would be outside the scope of this Paper.
Arches depend for their support on the masonry surrounding them. This provides the resistance to the forces generated by the shape of the arch and the loading on it. This Paper focuses on the arches themselves and does not include problems arising in arches from external causes such as subsidence or movement of the building itself.
Lintels are much simpler, and the forces involved are completely within the lintel. They provide support by resisting the bending moments and shear forces resulting from the loads that are applied to them. The first examples were probably trees falling across a stream, and mankind quickly learnt how to make use of lintels. There are examples of stone being used as lintels as far back as prehistoric times.
This Paper is the first of two and deals with arches. The second Paper will deal with lintels.


Arches depend for their success on abutments at each end capable of providing adequate support to resist the force generated by the arch. The shape and rise of the arch are important because some arches provide much better support than others. The rise of an arch should always be at least one twelfth of the span. Some of the more common types of arch can be classified as:-

  • Circular
  • Gothic
  • Semi-circular („Roman‟)
  • Segmental (Very common in Victorian buildings)
  • Tudor (Two centred plus a straight angled centre section)
  • Indian (Four centred)
  • Arabic (With reverse curvature at the top)
  • Elliptical
  • Three centre
  • Flat‟ or Jack
  • Filled arches. (Where the space between the bottom curve of the arch (the intrados) and
  • the usually horizontal top surface of the window or door frame below is filled with non- loadbearing material such as shaped timber fillets).
  • False Arches. (Where an arch is actually only decorative and the structural support is provided by a lintel supporting the arch).
  • 'Supported' arches. (Where the arch is (usually unintentionally) dependent on support from the window or door frame below).

Arches are usually constructed from either stone, brick, tiles, or brick and tile / stone combinations. All can provide a satisfactory structural support if built correctly.  Getting a qualified Architect to produce a detailed design should always be considered.  Brick arches can be formed using standard shaped bricks with tapering masonry joints to get the curvature required. Specially made tapered bricks with constant thickness joints can be used. Standard shaped bricks can be cut to a wedge shape to form the arch, with constant thickness mortar joints. Purpose made special shaped bricks are also used. A soft, orange colour, brick known as a "red rubber‟ is often used for high quality work. These bricks are rubbed with abrasive material to form the correct shape and are formed to fit tightly together with joints often no more than 1 or 2 mm wide. The joints are filled with lime putty giving the classic white appearance to the mortar. Occasionally yellow "rubbers‟ can be seen used in buildings.
When specifying or describing arches it is useful to use a standard method. The first part of the description should be the number of rings of masonry in the arch. Many arches, especially those constructed of stone or tile usually have only one ring of masonry. Brick arches commonly have either one, two, or sometimes three rings of masonry. Where there is more than one ring it is useful to describe the lowest ring first and work up the rings in succession. In larger structures, such as railway arches, five, six or more rings of masonry are common. The separate rings act individually when loaded. This is not usually a problem with the sizes of arches found in buildings but in railway and similar arches for example the rings tend to separate from each other under load.
The next part of the description should be the height of the rings from the bottom curve of the arch (the intrados) to the top curve of the arch (the extrados). This is usually either one brick length (a nominal 9” or actual 215mm), a half brick length, or sometimes a brick laid flat (where the height is a nominal 3” or actual 65mm).
Sometimes the rings are bonded together with the occasional brick connecting them. These would be described as bonded rings, but most arches have separate, un-bonded, rings. This should form the third part of the description.
The fourth part of the description should be the type of arch.
A typical description for an arch in a domestic house would be „1.2m span single ring one brick high un-bonded segmental arch‟. Other arches might be described as „ 1.2m span two ring half brick high un-bonded Tudor arch‟ or „1.2m span two ring half brick with brick laid flat un-bonded segmental arch‟.

Specifying / assessing arches.

When specifying new arches or assessing existing ones the performance of the arch should be considered. Their performance and susceptibility to undesirable structurally significant movement depends on several factors. The span to rise ratio is particularly important. The rise is the vertical distance from a line joining the springings of the arch up to the underside of the intrados. The other important factors are the amount of masonry above each side of the arch, and the amount of masonry to each side of the opening.
The amount of masonry directly above the arch is usually the main load on the arch. As the force (T) generated in the arch by the load tends to cause the arch to spread (i.e. increase in span) the vertically downward load from the masonry each side of the arch (W) produces a compressive stress in the masonry each side of the opening. The resultant, (R), of the forces from the arch and the masonry above each side of it is distributed into the masonry at each side of the opening. Ideally this force should continue to foundation level within the masonry as shown on the left hand side of the diagram below. Sometimes there is insufficient masonry at the side of the opening and / or weight from above and the resultant (R) passes through the side of the masonry as shown on the right hand side of the diagram below. This can cause the masonry to rotate (clockwise in the right hand side example in the diagram below) or shear cracks to form along its line of action. However, in many cases (for example, bay windows in Victorian houses) there is insufficient brickwork either side of the opening and the arch depends on support from the window or door frame to keep it in position.

Diagram showing the main forces in arches

The problem of supported arches is often not recognised within the building industry and especially by window replacement companies. Removing the original, usually timber, window frame deprives the arch of its support and carries with it the danger of collapse, either at the time of replacement or later because the replacement windows are typically at least 5mm smaller all round than the original ones. This is necessary for the installation of the replacement windows. The original windows were often built into the brickwork at the time of construction and therefore could provide the unintentional support to the arch which was not available from the small amounts of surrounding brickwork in bay windows.
Arches where the masonry is visible are relatively easy to assess. However, there are some buildings where the external surface has been rendered and the arch construction is not visible. In these situations a probabilistic approach is required, unless physical investigation can be undertaken.

Shapes of arches.

Arches come in many shapes. The circular arch is the most stable because, being a full circle, the forces are balanced around the arch. There are very rarely problems with this type of arch.
Semi-circular arches comprise the top half of a circular arch. Again, there are very rarely problems with this type of arch except where they are used externally over a side gate next to a building. In this situation the forces on the building side of the arch are adequately resisted by the building but the other side often has a slender brick pier rising from ground level. This has insufficient strength to resist the forces at this end of the arch, and the top of the pier can move outwards away from the building causing the arch to loose support, crack, and sometimes fail.
As the span to rise ratio increases the arches exert more force at each end. There are many segmental arches in Victorian buildings and where they are built into a length of wall in the same plane as the arch they are rarely problematical. Arches with Tudor, Indian, Arabic. or Gothic shapes rarely give problems as they have good span to rise ratios.
Elliptical arches are expensive to construct due to their complex shape and are only usually found on high quality buildings. They are usually constructed with red rubbers in lime putty. A similar but simpler and cheaper arch is the three centred one. This approximates in shape to an elliptical arch but is formed of three radius curves, one at each end and one forming the centre. The span to rise ratio for both types of arch is usually sufficient to prevent problems occurring.
'Flat' or 'Jack' arches have a large span to rise ratio, and frequently show signs of structural movement. They were typically constructed with a slope of between 1:96 and 1:100 along the intrados from each springing “to allow for settling”. The slightest movement, including poorly fitting joints, drying shrinkage, or structural movement of the building is very likely to result in undesirable structurally significant movement. It is not uncommon for bricks to fall out of these arches.
There are various unusual shapes of arches. One fairly common one comprises a semi- circular arch between two "Flat" or "Jack" arches, one either side, above a three bay timber window. This type of arch is not structurally stable and is wholly dependent on the support from the window frame below. Being a three section window there is a timber jamb under each end of the semi-circular arch and the mullions at the tops of the side windows provide support to the "Flat" or "Jack" arches. Work on this type of arch / window combination requires special consideration.
The area between the intrados of the arch and the top of the window or door frame is often filled with a non-structural filler such as shaped timber fillets. These filled arches rarely give problems because the span to rise ratio of the arch is usually adequate to provide the structural support.
In more modern building the arches can be "false" (i.e. only decorative) and the structural support provided by lintels with curved front sections. In cavity walls the lintels usually support both leaves of masonry. The arch should be ignored and the lintel examined when making assessments of the adequacy of the structural support over the opening.
If an arch has a longer span (typically over about 1.5m), or has a large span to rise ratio, or has little masonry above it at each side of the arch, or has little masonry either side of the opening it may be a „supported‟ arch deriving support from the window or door frame below. Specialist advice should be sought when dealing with supported arches.

Assessment table.

A table to assist with the assessment of arches is shown as Table 1.


Type: Circular
Span/ rise ratio: 1
Stability: Very Good

Type: Gothic
Span/rise ration: Usually between 1.0 and 1.2
Stability: Very good

Type: Arabic
Span/ rise ratio: About 1.3
Stability: Usually satisfactory

Type: Semi-circular
Span/ rise ratio: 2
Stability Very good in walls
Comments: Beware of isolated semi-circular arches over side gates.

Type: Elliptical
Span/ rise ratio: From about  2.8 to 5
Stability: Usually satisfactory
Comments: Beware of large spans and flat elliptical curves

Type: Three centre
Span/ rise ratio: From about  2.8 to 5
Stability: Usually satisfactory
Comments: Beware of large central radius

Type: Indian
Span/ rise ratio: About 3 to 4
Stability: Usually good
Comments: Not commonly found

Type: Tudor
Span/ rise ratio: Usually about 3 to 4
Stability: Usually good
Comments: Beware of large radii at the centre of the arch which can be a weakness

Type: Segmental
Span/ rise ratio: Usually between 10 and 20
Stability: Usually good
Comments: Widely used in Victorian buildings, with spans up to about 1.2m

Type: ‘Flat’ or ‘Jack’
Span/ rise ratio: Greater than 90
Stability: Prone to movement from internal and external causes
Comments: Beware of loose bricks and displacements in the brickwork

Type: ‘Supported’
Span/ rise ratio: Varies
Stability: Dependent on window or door frame below
Comments: Seek specialist advice before carrying out any work.

Type: Unusual shaped
Span/ rise ratio: Varies
Stability: Very variable, depending on the shape
Comments: Seek specialist advice before carrying out any work

Note: Spans over about 1.5m need special consideration due to the larger thrusts generated by the arch.


Arches provide a useful method of providing support over openings. There are many different types, and some are more structurally stable than others. All types depend to a greater or lesser extent on the surrounding masonry for support. In specifying or assessing arches the implications of the type and condition of the arches should be carefully considered and where necessary, e.g. for a house purchase report, brought to the attention of the present or future owner of the building.  A Building Surveyor will always be able to give good advice relating to the condition of an arch but in complex situations a Structural Inspection from a Structrual Engineer may be more appropriate.

Chris Shaw CEng FICE MIStructE MCMI FIIExE
15 Mareschal Road Guildford GU2 4JF
Tel: / Ans: / Fax: 01483 536577