Turfing the Walls at Hailes Abbey

English Heritage has just completed a £50,000 project at Hailes Abbey in Gloucestershire to ‘turf the walltops’. This may seem like a rather strange way to use scarce resources but research over the past decade has shown conclusively that ‘soft capping’ as this process is called is a good way to protect exposed historic masonry.

It is important to remember that all of the old castles, abbeys and other historic buildings we now see as ruins in the landscape were once fully functional buildings with a distinct inside and outside and a roof over the top. They were not designed to be open to the elements from every direction and the deterioration problems which occur are a direct result of this.

The two principal causes of fabric deterioration on ruins are the ‘Freeze-Thaw Cycle’ and the ‘Wet/Dry Cycle’.

• Freeze/Thaw Cycle – when water freezes it expands in volume by 9%. If this water is in the surface layers of stone – particularly porous limestone or sandstone – this expansion can cause the surface layers of the stone to break off, a process termed ‘spalling’.  If entire blocks of stone are soaked if is possible for the whole block to shatter.

• Wet/Dry Cycle – Porous stone can hold water within its structure, especially if it is open to the elements during wet periods.  When the weather is dry again, particularly when it is warm and breezy, water evaporates from the surface of the stone and water within the stone matrix is ‘sucked’ to the surface to be lost in turn.  Some stones contain various mineral salts and these can be dissolved into solution if the stone is soaked, travel to the surface of the stone and then as the water is evaporated they come out of solution. This sometimes happens at the surface and shows up as white crusting. But it can also happen just below the surface. When salts crystallise out of solution they can exert a large pressure on the surrounding stone which can cause spalling even worse than that caused by frost.

From the turn of the C20^th (but particularly from the 1920s onwards) many historic sites were excavated and opened to the public. The belief at the time was that all vegetation was bad for the stonework (not to mention untidy!) and it was stripped off and replaced by ‘hard capping’ – walltops made from hard, cementiceous mortar designed to shed water as quickly as possible.

This work was done with the best possible intentions, but by the 1990s it was clear that fabric deterioration was not being slowed and in many cases appeared to be accelerating. Looking closely at these problems it became apparent that ‘hard capping’ was not improving the situation and actually seemed to be making it worse because;

• Hard caps always cracked, often within a few years of installation and even hairline fractures allowed water to penetrate into the core of the wall. Once in the core water took the easiest way out – this should have been the mortar joints (lime mortar allows some water movement), but along with hard caps many monuments had been re-pointed with hard cement mortar and so the water was forced out through the stones – exacerbating the problem cycles described above.

• Hard caps gave the stonework below no protection from thermal changes, so frost could penetrate from the top as well as both sides of a wall.

• Hard caps held the historic fabric below in a tight grip and expanded and contracted at a different rate to this historic stone below and in some cases have shattered the stone.

• Hard caps shed water directly down the face of the wall, in the same direction every time it rains. Not only does this increase the potential for damage by the problem cycles but in some cases it has led to a clearly defined growth of algae within the stone surface in certain areas of the wall which are repeatedly wetted and which appears, even on dry days, like a stain on the wall.

Because of these growing concerns, English Heritage commissioned Professor Heather Viles and her team at Oxford University to look into the possibility that ‘soft capping’ – the use of soil covered in grass and other herbs – on wall tops could be beneficial. They have clearly shown by both laboratory experiment and field testing that soft capping;

• Acts like a ‘thermal blanket’ on the walltop, moderating both the upper and lower limits of air temperature changes. This prevents damage from frost, but also reduces the amount of thermal movement below a soft cap – both of a hard cap and/or historic fabric.

• Sheds water in different directions each time it rains and where the soft cap is well established the longer grass stems often act like an overhang, directing water away from the wallface.

• Appears to stabilise water movement within the core of the wall – although this is much less clear because of the problems of monitoring water movement within walls

Because soft capping clearly offer protection to exposed stonework, does no harm to historic fabric and could be easily removed in the future if necessary, English Heritage believe that this option is a good way of protecting ruins from environmental damage.

The trials were carried out on small areas of walltops in various locations throughout the country, but because it is a major change in the presentation of monuments to the public English Heritage have decided to completely soft cap an entire site and over the next two years we will be monitoring the response from visitors. Hailes Abbey was chosen for this trial because it is located in a frost pocket and has suffered increasing and serious damage to the stonework over the last few years.

You can read more about the soft capping research and a copy of the research report is also available to download.