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The dating game

technical & practice

Dating buildings is important for survey reports: particularly for conservation appraisals, archaeological assessments, and for predicting age-related latent defects, such as Georgian 'snapped-header' walls, inter- wars 'Regent Street Disease', or post-war high-alumina cement concrete deterioration1.

When a building is original, and typical of its period, its age can usually be judged by its external appearance alone. Every era has its distinctive architectural styles, ranging from wavy roofs of the 1990s, to bow-backed Georgian terraces of the 1790s. But when a building is nondescript, atypical (a folly), has been altered, extended or overclad, we need to examine its structure.

Structural materials, components, and systems have varied through the ages. Knowing their periods of use can establish the era and evolution of a building.

Interiors are sometimes refitted and finishes renewed, but the structure beneath them is only changed if it becomes damaged, or if it is redeveloped behind a retained facade. Rarely, lost structure is replaced by second- hand earlier structure.

Building books (see below) illustrate contemporary construction, though beware obsolete examples. These books, and others, can be found in the ice or IStructE libraries, and sometimes in antiquarian bookshops.

The bar-charts on the opposite page summarise the periods of popular use (solid lines) and the tentative use (broken lines) of commonplace structural components and systems. Be prepared for more extreme examples of a particular structural component or system coming to light from time to time.

Dating buildings is not an exact science, as building gestation usually takes years, apart from notable exceptions, such as the 92,000m2 Crystal Palace which was designed and constructed in nine months for the Great Exhibition of 1851.

However, the era of a building can usually be established with confidence if several structural elements are compared.

There are many opportunities to see the structure of an occupied building without opening it up, particularly in areas that have no finishes, such as roof voids, undercrofts, cellars, pavement vaults, suspended ceilings, plant rooms, fitted cupboards, store-rooms, lift-shafts, and service holes.

Empty or derelict buildings and buildings undergoing alterations can offer more structural disclosure.

The principal structural materials are masonry, timber, concrete, iron and steel. This article deals with masonry and timber, subsequent articles will focus on the other main materials.

Masonry materials

Of all the masonry materials, stone masonry tells us the least about its construction age. The bond of the stones can broadly distinguish between medieval (rubble-cored), post medieval (brick-backed), and twentieth century (steel-framed), although any age of construction could be solidly bonded.

Unlike stone, brickwork gives many clues to its age. After use by the Romans, clay bricks were re-introduced into the uk in the 1400s, initially in the south and east, near to locations where suitable clay could be dug out and burnt in wooden clamps. With the decline of medieval timber- framed buildings and the advent of canals, railways, and better roads, bricks were transported and used throughout the country.

By the eighteenth century, brick was the most common material for houses, and many old timber-framed houses were gentrified by re-facing with bricks or mathematical tiles, particularly the latter after the first brick tax of 1784.

Since the 1400s the width of a brick has always been about 4.5 inches (114mm) - governed by the need to grasp and lay it with one hand. But the length and thickness of a brick has not always been as constant as today, being influenced by government legislation, regional variations in firing thicknesses of clay, bonding, joint thickness, and local practice.

Medieval bricks were longer and thinner than modern bricks - as at Herstmonceux Castle, East Sussex. circa 1440 which has 2 inch (51mm) thick bricks. But beware modern imitations, particularly amongst Edwardian buildings. Parliament fixed brick sizes in 1776 at 8.5 x 4 x 2.5 inches (216 x 102 x 63mm). Fighting wars is expensive. In 1784, after the American War of Independence, parliament taxed each brick used, so some bricks were made larger, up to 10 x 5 x 3ins (254 x 127 x 76mm)2. In 1803, these large bricks were further taxed, and this was avoided by reducing the size to 9 x 4.5 x 3ins (229 x 114 x 76mm). In 1850 the brick taxes were repealed, and brick sizes gradually standardised, rising four courses per foot (304mm), except in the north of England where they rose four courses per 13 inches (330mm) for much of the nineteenth century.

In 1851, machinery was designed for making pressed bricks in volume, eventually replacing handmade bricks, except for best quality work. Machine- made bricks, such as Flettons which were first made in the 1870s, are generally smoother and more regular in appearance than handmade bricks.

At the end of the First World War the Local Government Board highlighted the need to supplement traditional construction with non-traditional types. Amongst other materials and components,concrete bricks and sand-lime bricks were introduced, which like machine- made clay bricks, are smooth and regular, but with more uniform texture.

Bricks were traditionally laid in lime mortar, until Portland cement became more popular in the late nineteenth century, thanks to its cheapness, faster set, and safer handling properties. Lime mortar is usually more friable than cement mortar, although laboratory analysis is the only sure way of distinguishing between them.

To improve the strength of Georgian and Victorian lime-mortar brickwork bonding, timbers were usually set in their inner faces or occasionally in the core of the walls - such as in Tittenhurst Park, Berkshire around 1800. Better quality Victorian buildings had hoop iron laid in the bed joints instead - as at Marlborough House Mews, London from around 1862 and the now demolished Stonebridge Park Power Station, London of 1906.

In early brick buildings the bond is often irregular, but English Bond became the norm by the end of the sixteenth century. Flemish Bond was introduced in the seventeenthcentury, and had largely replaced English Bond by the early eighteenth.

In the latter half of the Georgian era, snapped-header brickwork was frequently used in cheap, speculative construction. Although apparently solidly bonded, many cheap Georgian external walls only had first-quality bricks in the external face. Headers in the outer face were mostly snapped- off where they met the inner face due to misalignments in the coursing of the two faces.

This may have been the unwitting precursor of cavity (two-leaf) brickwork, or perhaps that was the nineteenth century cheap walling, Rat Trap Bond, where bricks were laid on edge to reduce consumption leaving cavities within - as at Coleshill Model Farm, near Swindon around 1854.

The earliest known true cavity wall is around 1804, recorded by bre ip16/88, although most earlyexamples are found in late Victorian buildings, not signalled by the customary Stretcher Bond, but usually by Flemish Bond with false headers.

Some early cavity walls have no ties between the leafs. Where used, ties varied from cast or wrought iron bars, to extra-long hollow glazed header bricks. By the 1920s, Stretcher Bond brickwork and mild steel cavity ties had become the norm. For better durability, ties gradually evolved after World War II through galvanised mild steel to stainless steel, and even to copper, bronze, and plastic in a few cases.

After 1945 the brick inner leaf of cavity walls was replaced by dense concrete blocks, and then in mid 1970s the 'energy crisis' arising from the 1973 Middle East War accelerated the change to insulative lightweight concrete blocks.

Some cavity walls have bed-joint reinforcement, such as Expamet, Bricktor, or stainless steel wire, the modern equivalents of Victorian hoop-iron, and lime mortar is making a comeback thanks to its greater tolerance of structural movement than cement mortar, and modern safer ways of handling the lime.

Masonry pointing is readily renewable, and therefore it isan unreliable indicator of age, although Georgian tuck-pointing, and post-war recessed pointing are seldom aped.

Timber

Hardwood framed buildings dominated the medieval period, but were in rapid decline by Georgian times3.

Native hardwood supplies were largely exhausted, and softwood was increasingly being imported from the Baltic and Scandinavia.

Brickwork became the most popular vertical load-carrying material, and timber became confined to roof carcasses, floors, and internal partitions, until after World War II when in-situ softwood timber-framed houses and latterly factory-made volumetric construction came into use.

Apart from simple collared roofs with nailed joints, King-Post and Queen- Post became commonplace in the 1700s. Weak pegged-joints evolved into stronger bolted and strapped joints, particularly for larger roof spans.

For long-span floors, timber beams were sometimes flitched with wrought- iron plates, or internally trussed with hardwood or cast-iron struts, as in Quernmore, Bromley, Kent in around 1777. Very large floors, such as the ballroom in the Carlton Club, London (1827), were trussed with parallel timber booms, wrought iron hangers, and cast iron joint-shoes.

After World War II, the shortage of large section timbers, and the availability of better glues and mechanical timber connectors enabled innovative construction with small section timbers and board.

In 1948, the Timber Development Association (tda) produced standard 'blueprints' for bolted softwood roof trusses spanning up to 60ft (18m) for residential, commercial and industrial buildings. They proved popular in a war-torn uk, short of iron and steel. Timber-shell roofs appeared in 1957, but were only popular for about 25 years4. 'Gangnail' roof trusses were introduced in 1962, initially unbraced until after the Rock Ferry School gymnasium roof collapse in 1976. 'Gangnail' superseded tda trusses by the early 1970s, at the same time as proprietary composite beams started to appear, such as 'glulam,' 'corply', and latterly 'masonite.'

Clive Richardson is a structural engineer and technical director of Cameron Taylor Bedford Consulting Engineers email clive.richardsocamerontaylor.co.uk

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