Riverlands Cob Cottage, a heritage listed historic earth building on State Highway 1 just south of Blenheim (New Zealand), has a new lease on life since undergoing it's second major restoration.
The 150 year old building has been plagued by rising damp issues and was also affected by several earthquakes.
In 1960 the Marlborough Historical Society and Historic Places Trust started the first major restoration, using techniques that at the time were considered best practice. The deteriorating material at the base of the walls was replaced with two to three courses of concrete blocks, and the whole cottage was re-plastered with a cement stabilized earth render. Unfortunately no damp proof membrane was installed at the time and so the root cause of the damage was not mitigated, but the repairs certainly strengthened the building and were crucial for its survival to this day. However, it was apparent that the building was in dire need of love and that there was still salt damp damage affecting all surfaces. Solid Earth Ltd from Nelson were contracted to carry out structural repair work and give the cottage a total makeover, working to the guidelines of historic places trust and under the supervision of structural engineer Gary Hodder of Nelson.
The main aim was to insert a damp proof course, which was no small feat given the work had to happen at the base of approximately 70 tonnes of earthen wall. The repair team removed small sections of concrete blocks at a time, inserting DPC and rebuilding the base of the walls with earthen materials. We had to leap frog the sections, working from the outside and inside in different locations, as to defy gravity and carry out the repairs without the walls collapsing. During this work, the walls were heavily propped according to the engineer's specifications.
A total of around 300 concrete blocks and a whole skip of very dusty old render and loose material were removed from the building, and were replaced with around 600 mud bricks and 4cum of pre-mixed cob, that had been brought over from Nelson. The idea was to minimize the amount of shrinkage in the repairs, but the whole building still settled a few millimeters, which caused the doors to stick.
While opening up the walls, it became apparent just how much salt was present, especially closer to the wall surfaces where moisture had been trapped behind the less permeable cement stabilized render. The salt turned the earthen material flakey.
After a bit of research on salt damage (also called salt damp or salt attack) we decided to go for lime plasters on all surfaces, because lime can handle salt better, in actual fact it can draw it out of the walls and safely store it in its open pore structure.
Before plastering could start however, the repairs had to thoroughly dry out. We gave it all summer, as it wasn't a good idea to carry out the lime plastering in the windy and hot Marlborough weather.
The exterior surfaces will eventually be painted with two coats of Silicate Paint (Keim Granital), to cut down water penetration while ensuring maximum breathability. We are allowing around two months before carrying out this last step of the job, so that the lime plasters can carbonate adequately.