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Structure of Georgian Houses

Structural engineer Brian Morton describes the typical structural configuration of many typical Georgian houses.

Recently I was asked to appraise a surveyor’s report on a property in London. The client realised that the report seemed to call for an extensive amount of unnecessary work.

I wrote to her following my own survey, describing the structure of Georgian houses associated with potential problems. She was so enthusiastic that I felt readers of Context might find the information useful.

Most Georgian buildings are constructed of comparatively soft, local stock bricks. Walls are built with lime mortar, which again is soft and, like the bricks, comparatively porous.

Floors are constructed of timber above basement level. The structural support for these floors is provided by the external walls, and generally a timber-stud partition clad with lath and plaster as a central load-bearing wall running up through the building. At ground floor level this major cross partition is generally constructed of solid brickwork. In many cases the cross partition at first floor level is in a different line to the cross partition in the ground floor level.

Timber-stud partitions clad with lath and plaster are used throughout to support floors. It is unusual to

find any masonry internally above ground-floor level, except in the case of large Georgian houses where the staircase is of stone construction. In this case, brick walls are usually carried up alongside the staircase to the underside of the staircase to first-floor level, and occasionally to second-floor level.

Roofs are constructed of timber, formed into principal trusses with secondary rafters. Often there are open internal, lead-lined gutters within the roof space to carry water across the building and out through an external (normally rear) wall. Roofs are generally clad with Welsh-type slates set on to either boarding or battens, without any underlying roofing felt.

The brick walls of the building start at basement level as probably one-and-a-half brick thick walls, sometimes two brick thick walls. As they go up the building, these walls reduce until in the upper storeys they are simply one brick thick.

The floor joists are built into the front and rear walls, restraining them. In larger houses there are generally main beams which divide up the floors, spanning the width of the building. Sometimes they run from front to back, restraining the front and rear

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walls either by secondary joists bearing on to the walls or the principal beams bearing on to those walls. The party walls are a minimum of one brick thick, strengthened by the chimneybreast to provide lateral restraint to the walls. The face of the chimneybreast is generally only half a brick thick.

Depending on the circumstances, and the sequence of building, it is not unusual to find the front and back walls butting on to an original gable end of a previously built property, and not tied in. In other cases front and rear walls simply fly past the party walls and are not significantly connected.

These buildings were constructed quickly, with little or no foundations. They were built of flexible materials. The mortar took a significant time to set and the structures tended to settle down as they were built, retaining a degree of flexibility not found in later buildings of similar size. This flexibility has undoubtedly led to the long life of these buildings.

The structure of the buildings fails not because of vertical load but because of lack of lateral restraint, and sometimes lack of bonding within the thickness of the wall. Such failures are found particularly in higher-quality Georgian houses, where the outer skin of the face of the external walls was built with very thin mortar joints, built by the experienced bricklayer, while the inside face the brickwork was built with thick joints by the apprentice. Thus the walls were never bonded together because the joints were at different levels. This form of construction is described as snapped-header construction. Even when high-quality facing bricks were not used, there was a tendency for the outer face to be built by the experienced bricklayer, backed up by the apprentice.

Basements or lower-ground floors are always potentially a problem as far as damp-proof treatment is concerned. The modern style of living does not normally suit the form of construction.

Walls of soft bricks and lime mortar work because they breathe. Rain makes the wall wet or damp, and the wind dries the external face. The internal plaster also breathes and allows a small proportion of the damp to dry out within the atmosphere of the rooms. The basements or lower ground floors were constructed with pammets (floor bricks), covered by stone slabs. Both had open joints, allowing moisture to dry up through the floors. Walls were plastered with a hair-reinforced, lime-based plaster which also allowed the walls to breathe. Even where these conditions have prevailed for perhaps 150 years, I have seen original plaster in perfect condition. Air circulated within the rooms and a damp atmosphere seemed not to worry the occupiers.

In the early days of conservation restoration, pammet floors or stone floors were taken up, and floors were concreted, generally with a damp-proof course. The result was that the moisture was driven up the walls and the plaster came off. The walls were then rendered with a waterproof render, sometimes the full height of the room but mostly about one

metre high. Amazingly, the damp within the walls then permeated the bricks up to ground-floor level. We have found ends of floor joists rotten at that level due to the damp-proof treatment in the basement.

In the early days of conservation, English Heritage and local authorities would give grant aid for the form of this form of damp treatment to the basement, until it was realised the problems that this treatment caused. Generally English Heritage will not now give consent to dealing with basements or lower-ground floors in this way. If walls are found to be damp, a properly specified lime-based plaster is used even if there is a concrete floor, generally with adequate ventilation to the room and no fixtures being built against these walls.

English Heritage and conservation authorities do not generally believe that injection damp-proof courses are necessary or, in the slightly longer term, of any real value. We have gone back to what is known to work. Views on timber treatment have also changed. It is generally not now acceptable to treat any timber element within the construction of a house built prior to 1930. It has even been suggested that to introduce timber treatment into such structures may be negligent on the part of the professional because it introduces a chemical into the atmosphere unnecessarily.

If a method used in the past with particular materials was found to work, we would certainly recommend its reuse, but we have no objection to the use of modern materials if they are going to prolong the life of a building or an element of it. Stainless steel and resin anchors, for example, can be used to tie skins of walls together. The alternative would be to take down parts of walls and rebuild them without a significantly better result.

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