01 Apr Soft Capping
English Heritage look after more than 400 of the nation’s most important historic monuments, many of which are now ruinous. These ruins are often comprised of exposed masonry which was originally built with the intention of being sheltered. Without the protection of a roof or similar, many are now open to the elements and remain vulnerable.
A large number of these wall tops (which are often hundreds if not thousands of years old), lost their roofs during the Dissolution of the Monasteries in the mid-16th Century, or some, like Bishops Waltham Palace (read more here), are the result of the Civil War in the 17th Century.
The Roman and medieval masonry that we have been working on recently for English Heritage was often constructed in the role of defensive walling, and these wall tops play now an important role in protecting England’s oldest and arguably most significant masonry.
‘Soft capping’ can be defined as the use of grass and other soft planting in soil, laid to cover horizontal masonry surfaces to protect the wall below. This differs from what we know as ‘Hard capping’ which generally refers to the use of stone and mortars to do the same. In the early 20th Century, the Office of Works began to encourage the stripping of all organic growth from ruined sites and capped the wall heads with masonry and the newly popularised material cement (see previous works completed at Pevensey Castle). This model was followed until a greater understanding of materials was widespread and the lime revival started began in the late 1970’s.
In contrast, ruinous walls that had remained untouched for decades slowly build up a natural ‘soft’ cap. Study of this masonry when the capping has been removed during past consolidation works, has shown that very little deterioration has occurred to the masonry beneath.
Masonry wall tops to abbeys, priories and castles have all been consolidated with soft capping at a variety of ruinous sites since the 1980’s, but it wasn’t until data in the form of a research paper (produced by EH in 2018) was published, that the process has been more widely championed.
Soft capping has a number of proclaimed benefits, in that it protects masonry from deterioration by freeze/thaw, it reduces potential damage to wall heads from thermal movement and retains a level of moisture during rainfall, reducing the extent of water running down the face of masonry – helping to mitigate the cycles of wetting and drying which may be harmful to both brick and stone surfaces.
The process involves first consolidating the wall top (flat, stable surfaces are most suited to this approach) and then applying a rolled turf which encapsulates the soil within it. The turf is applied in 2 parts with a gap in the centre and rolled so that the growing face of the turf (generally grass, but can be seeded to include wild flowers etc) covers the exposed surface fully and doesn’t leach soil down the wall face. These rolls are then turned back over to meet one another and are held into place using short bamboo staves which pin the turf though to itself (see diagram below).
The overall aesthetic is one of controlled fecundity, which on the proviso the masonry is solid and watertight below and the plant choice is appropriate, should help maintain the structure.
Author: Spencer Hall ACR IHBC MCIOB PGDip HBC
Conservation Consultant | PAYE