Prehistoric preservation

visit House number 7 at Skara Brae in Orkney and you won’t find a door bell, a letter box or a welcome mat. Not surprising given that it’s one of the oldest prehistoric stone dwellings in Britain and no one lives in it any more. But thanks to some cutting edge green roof technology, it’s still a contender for a 21st Century eco home.

First discovered when a wild storm hit Orkney in 1850, the ruins of Skara Brae date back more than 5,000 years and represent one of the best examples of a prehistoric village in northern Europe. When the site was first excavated in the 1930s, house 7 was identified as the best preserved of the dwellings, so archaeologists immediately resolved to cover it with a glass roof to protect the ancient stone beds, dressers, seats and carvings inside, and allow visitors to look in from above.

More than 70 years later, in 2002, Historic Scotland, the government agency which acts as legal guardian and manager of the Neolithic village, identified problems with the 1930s roof structure. Extensive monitoring revealed that while it effectively protected the interior from sand and rain scour, it was causing heat gain and drawing up groundwater from the surrounding earth.

The salt-laden water was damaging the historic masonry as the salt was crystallising between the layers of sedimentary stone, separating sedimentary layers and slowly breaking down the stones. Time lapse photography revealed that movement of the structure was a problem and after trials of a reflective film on the glass had proved insufficient to reduce heat gain, the project team resolved to replace the roof structure.

‘We needed complete darkness and a thermal buffer,’ says Historic Scotland architect Stephen Watt. ‘Obviously, when you’re dealing with a 5,500-year-old structure any intervention must be carefully considered, so we appraised various options. A sedum roof from Flatroof Design & Technology (FDT) was eventually specified because it has a lower load than a grass roof.’

Coming to this decision was not a simple process. Apart from extensive monitoring, legal applications had to be made to gain consent to make any structural modifications. It took more than five years before work began in 2007.

During construction even minor vibrations could damage the structure or the delicate stone furniture inside, so a meticulous working plan was devised.

when you’re dealing with 5,500-year-old structure any intervention must be carefully considered

Steve Watt, Heritage Scotland

‘Our stone conservators division first wrapped all the furniture inside to protect it and a crash deck was erected above to seal against debris and liquid,’ says Watt. ‘A fully enclosed scaffolding structure was then built over the house, the roof of which was loaded to prevent wind from blowing it over.’

Once this was up the glass roof was disassembled, leaving the 1930s masonry wall heads and steel ridge support beam in place. The ridge beam had minor repairs done and a lightweight timber structure of kiln-dried Douglas fir joists was fitted to create a timber base for the new roof. A plywood deck was placed on top and covered with FDT’s waterproof membrane, Rhepanol hg.

‘Rhepanol hg has a low oil content compared to PVC membranes, is root resistant and able to withstand temperatures as low as -60ºC,’ explains Watt. ‘Once that was laid, filter, drainage and growing medium layers were installed to provide a base for the sedum.’

Continued monitoring has revealed stable humidity levels, no salt water draw up and no further structural movement.

The roof is expected to protect the site for at least another 50 years and although visitors will no longer be able to see inside the house directly, a scale model has been built in the visitors’ centre.

‘We’ll continue monitoring House 7 for as long as we can, but who knows, maybe we’ll find another solution in the future,’ concludes Watt. cm