Stonemasonry

Stonemasonry is one of the oldest building arts, and structures built of stone are amongst the most ancient surviving monuments in the world. There were not too many dwellings built with solid stone in New Zealand, but you do encounter the odd one to two. It is a beautiful and simple construction technique, where the walls are typically laid 600 - 900 mm thick. Inside and outside faces are formed, with rubble filling the centre. ‘Through’ stones are placed periodically, to tie the two faces together. Lime or mud mortar was historically used to fill all the gaps, and make the walls relatively air-tight. The interiors were usually whitewashed. Such walls are incredibly long lasting, and will survive centuries unless an earthquake or some other force tears them down early.

This art form has largely been lost to the building industry, but there are a few dry stone walling masons who carry on the tradition. Dry stone masons don’t use mortar. They stack stone in a structurally sound manner. Nowadays, if stone is to be used, it is often laid on its edge, in a rather haphazard way. Fast setting cement is then relied upon to give the stone its structural stability. Dry stone wallers don’t use cement, they take care to lay the right stone in the right place to ensure stability. Stone is also sometimes cut into tiles and fixed to a metal frame that is in turn fixed back to the structural wall. But this method is also not traditional.

Nowadays, if you want a stone building or addition and you want it to look traditional, use only natural stone, 250 - 300 mm thick, and lay it as a veneer on the flat i.e. with the biggest face laying down. You will need to tie the stone back to the structural wall with stainless steel brick ties, ensuring you leave a drainage and ventilation cavity between the stone and the structural wall. You will also need weep holes, as is the practice with brick veneer. This type of installation overcomes some of the limitations of solid stonemasonry, namely, allowing for insulation and services in the wall. It is also essential to build in this way in earthquake prone lands. Solid stone walls will not withstand severe shaking.

Shoe-box sized rocks are the best to work with, although you do want a range of sizes in the wall. The best stone work uses variation in the composition i.e. different colours, types of stone, or shapes etc. Good stonework always has a horizontal pattern, as each course tries to maintain a level band from side to side. Stone is easier to work if it’s fresh from the ground, before it has dried. Ensure you wear thick leather gloves, to protect your hands.

Coloured mortar usually looks best. Ideally use an earthen colour, and this will normally compliment the stone. Always wet the surface you're putting mortar on, and keep the mortar moist for up to 6 days to properly cure. You want to cover it with plastic sheeting or wet burlap bags, wetting it a couple of times a day. Don't use mortar in freezing weather.

Try to lay no more than three vertical causes of stone a day, and wait at least two days before doing another 3 courses. The day after laying stone, rake the joints with a pointing tool, then wire brush them to make the stone stand out. Mortar joints should be raked back to a consistent line. If stones protrude, rebate the mortar joint further back, to match the rest of the wall.

Always lay stone in a stretcher bond, however don't beat yourself up if you break this rule every so often. You always want to be bridging a gap, rather than maintaining a vertical running joint. A vertical running joint of two stones is acceptable, but never allow it to extend to three stones. A running joint of two stones is often required to bring two smaller stones up to course with a bigger stone. Stones that are reasonably flat on the top and bottom are the easiest to lay. Use a plum bob, level and string guide often to stay true. Interior walls, buttresses and chimneys should all be interlocked back into the main stone wall for structural stability.

Use the biggest and the best stones for the corners, with 90 degree edges. The corners should alternate in each direction as you move up the wall, in a zig zag pattern i.e. a header should always overlap a stretcher. This is more structurally stable and it also looks better. When laying stone, you start at the corners and then infill in between. Historically, there was often a ceremony when the first corner stone was placed, as it signified the start of the construction phase of the project. Dates and other important facts may be carved into this rock, to commemorate the occasion.

Lintels can be formed from single big stones (for small openings, using the strongest stone such as granite or limestone), durable timber members (if they are well protected from the weather), reinforced concrete, or alternatively, a beautiful stone arch may bridge the opening instead. Always ensures stone walls have more mass than openings however.

Keep in mind that stonework on the second storey costs considerably more than stonework on the ground floor. It is also not the best idea in a country with earthquakes to have heavy material up high, above the ground.

When constructing stone retaining walls, always put a lean on them, with the mass leaning back into the cut at the rear. This resists pressure from any freeze thaw cycles, which may push the wall over in time. Stone fences are best with both faces tapering. Both stone fences and retaining walls may be dry laid and they will usually be built out of solid stone, rather than a veneer. Do not use thin flat stones to cap a wall or fence, as they tend to get pushed off. Use thick flat stones laid on the flat instead.

For gates in stone walls or pillars, set hinge pintles into the mortar at the correct height as you build the wall up. Heat them (to red hot) and wack the top of the pintel (with support beneath), to create a mushroom bulge rivet. This will stop the wrought-iron gates from being lifted and stolen.

Stone steps are a very durable and attractive option, except be aware that they may ice over and become slippery in cold climates. With stone steps bigger than a single piece of stone, use a single slab for the riser and one or more flagstones for the tread.

Flagstone floors are a beautiful option, and you can put hydronic heating pipes beneath them to take the chill off your feet. For indoor flagstone floors, set the stones on sand, ensuring you have insulation and a vapour barrier beneath, and mortar them together. There is no need to install a concrete slab under the flagstone, and this was never done historically. Flagstones can be all different thicknesses, but they need to be at least 25 mm to ensure they are strong enough. With a flagstone floor, you don't strike the mortar joints, they should be finished as flush as possible with the top face. Keep in mind that furniture has to be pretty flexible to stand on a flagstone floor, three legged objects are the best.

Stonemasonry

Further reading