Building Conservation Team
The Building Conservation team at the National Park have a vested interest in keeping the area’s traditional buildings in good repair so as to maintain and secure this particular element of the North York Moors’ special qualities. We have a new Advice Note on using lime mortars, the rudiments of which are explained below.
Most buildings in the North York Moors were traditionally constructed using stone and lime mortars (to bind the stonework together), often with earth and rubble filled cores. Lime was used for bedding and pointing the stonework and for rendering. Rendering with mortar was used to cover poorer-quality masonry underneath or to make a building more weather proof, for example on the coast. Most historic buildings were constructed from materials found locally and this contributes to what is distinctive about the ‘local vernacular’.
When it comes to repairing a surviving traditional building it is important to understand how they work. These buildings are usually of simple construction and built using breathable materials. Bricks and stone are bonded with flexible and permeable mortars made of lime and sand which allow the building to ‘breathe’. When it rains moisture is absorbed into the external surface but is then able to evaporate through the more porous pointing or render. Using a lime based mortar or render for repairs nowadays means that this process can continue.
In contrast modern cement mortars and renders along with plastic paints, waterproof sealants and damp-proof courses all act as barriers to a traditional wall’s natural ability to breathe. The trapping of moisture within permeable materials like stone can exacerbate the very problems these products are trying to resolve. The use of cement-based mortars can have a significant negative visual (photo below left) and physical impact on traditional historic buildings. The photo below (right) shows the extent of stone decay caused by the use of a cement mortar. Because the cement is much harder than the stone, moisture cannot evaporate through the joints and instead evaporates through the stone causing it to ‘weather away’ through premature erosion.
You can usually tell what type of mortar has been used most recently; cement based mortars tend to be dark grey and hard in appearance and texture whereas lime based mortars are generally lighter and softer in appearance and texture. Because they allow the surrounding masonry to dry out the colour of the stone will also lighten.
There are a number of different types of lime mortars/renders. Replacing like for like is important.
Non-hydraulic lime is the raw material produced when limestone (calcium carbonate) is fired, often called ‘quicklime’ (calcium oxide). It is sold in a slaked form (with water) as lime putty which is then mixed with an aggregate (e.g. sand) to produce mortars and plasters. These putty limes possess good breathability and flexibility and are ideal for use with soft porous materials allowing the maximum permeability.
Hydraulic lime comes in powdered form and will start to set as soon as it comes into contact with water. It is ideal for use in wet or very exposed situations or where there is a need for a higher compressive strength or a quick set. Hydraulic limes come in a variety of strengths e.g. NHL2, NHL3.5 and NHL5 – the higher the number the less flexibility and breathability the mortar will have.
Hydrated or bagged lime is a form of non-hydraulic lime which is sold as a powder. It is sold by builders’ merchants as an additive for cement mixes in order to give modern cement mixes more plasticity and workability. It is generally considered to be inferior to lime putty, not least because an unknown proportion will have reacted with carbon dioxide and set by the time it reaches the site.
Hot-mixed lime is made when quicklime is mixed with water and aggregate simultaneously. The vast majority of historic lime mortars were probably hot-mixed. There are several benefits of using a hot-lime mortar: it can produce cleaner work as there is less leaching, it has a easily-workable elasticity which produces solid and full joints, it appears to be more breathable and therefore more compatible with stonework, and it has potential for use in colder weather notwithstanding the requirement for protection from freezing during the curing (setting) process.
A standard pointing mix consists of a lime mortar mix of 1:2½ lime:sand (sand mix of 50% sharp sand and 50% builders sand) for a slightly recessed bagged finish. However there may be times when a bespoke mix is required such as when different colour sand is needed to match existing historic mortar.
Repointing is only needed where mortar has become loose, decayed or eroded to an extent that water has started to penetrate the joints. If the mortar is firm or so hard that it needs to be chiselled out then it is best to leave it in place as removal could damage the masonry. The repointing of delicate ashlar joints (made out of worked stone) is not generally advisable as the joints are so fine that getting old pointing out can lead to irreversible damage of the masonry.
If repointing is necessary joints should be carefully raked out manually or by non-electric tools (no angle grinders) to a depth equal to one and half times the width of the joint and never less than 35 mm. Great care must be taken to keep the edges of the stone intact and joints should never be widened. Work should not be carried out when there is a danger of frost or heavy rain (this is less important when using a hot lime mix). Mortar must be protected from drying out too quickly from wind, rain and frost by protecting the area. Rain must never be allowed to strike the mortar and stonework until the setting process is complete.
The mortar should be stippled as the initial set takes place, with a stiff bristle brush, to produce a textured appearance capable of shedding water and slightly set back from the stone outer surface to ensure the full arris (edge) of each stone shows clearly in relief.
The photo below (left) illustrates a good lime mortar mix and pointing method. The aggregate used in the mix has been exposed by brushing back the pointing to a recessed finish which allows the stone to be the dominant feature. The other photo below (right) shows historic pointing on an old outbuilding where roof tiles were used to fill in larger gaps between the stones, adding to its particular character and appearance.
If you need further information, or advice on sources of materials, or any clarification regarding the need for listed building consent or planning permission before re-pointing or rendering a traditional building, get in touch with our Building Conservation Team.