Mortared stone bonds natural stones into one rigid wall with lime or clay mortar, so it cannot be taken down by hand stone by stone the way a dry wall can. Lime mortar cures slowly and must be kept damp and out of frost while it hardens, wet lime burns skin and can blind, and clay mortar washes out in rain. Plan the mortar supply before starting the wall, since it is usually the limiting material.
Mortared stone masonry is the laying of stones in a bed of mortar, which fills the gaps, spreads the load and bonds the wall into a single mass.
Mortar’s Job Is To Bed The Stones, Not To Be Glue
Mortar fills the gaps between irregular stones so each bears evenly on the one below, and it hardens so the whole wall acts as one piece. A mortared wall resists being dismantled and resists the faces separating, which are the two weaknesses of dry stone (see Dry Stone Walls). It is also rigid: a mortared wall on settling ground cracks where a dry or gabion wall would flex.
What You Need
- Lime mortar. The mix in Mortar Mixing is 1 part lime putty to 2 to 3 parts sharp sand, with the slaked lime matured under water for at least two weeks before use. Lime itself comes from burning limestone; see Plaster and Lime Wash.
- Clay mortar where lime is not available: clay and sand mixed to a workable paste. It is weaker, and it dissolves in rain, so use it only for walls kept dry or rendered.
- Stone sorted by size, with the biggest at the base; see Clay, Stone & Sand Sources.
- Water, a mixing trough or a clean flat surface, trowels, buckets and a string line.
- A footing as in Stone Foundation.
Method: Lay A Mortared Rubble Wall
Step 1: Wet the stones. Dry stone pulls water out of fresh mortar and weakens the joint. Dampen the stones just before laying, not soaking.
Step 2: Bed each stone. Spread mortar, set the stone, and press it down. Large stones go in the lower courses, with their longest dimension into the wall. Do not let stone touch stone with no mortar between, since a point contact carries load badly.
Step 3: Keep the faces and the core together. Lay both faces and the core in the same course, and place tie stones across the thickness at intervals. Fill the core with small stones set in mortar, not dry rubble tipped in.
Step 4: Limit the height built in a day. Fresh mortar is squeezed out by the weight above it. Build a limited number of courses at once and let them stiffen before adding more.
Step 5: Cure damp and keep frost off. Lime hardens slowly by taking up carbon dioxide from the air. Keep the wall damp for days with wet cloth or sacking, and protect it from frost and hard sun while it is fresh. A wall that dries out fast does not harden properly.
Worked Example: Mortar For A Section Of Wall
A wall 0.5 m thick and 1.5 m high is 0.5 x 1.5 = 0.75 m³ per metre. If the stone packs to leave about 35% voids (the void fraction assumed for caged rock in Gabion Walls; a recalled assumption for loose stone, and mortared stone varies), the mortar is about 0.35 x 0.75 = 0.26 m³ per metre. Mortar volume is roughly the volume of its sand (recalled rule), so about 0.26 m³ of sand, and lime putty at 1 part to 2 to 3 parts of sand is 0.26 / 3 = 0.09 to 0.26 / 2 = 0.13 m³. For 30 m of wall that is about 8 m³ of mortar. Treat all of this as a planning estimate and lay a test metre first.
Common Mistakes
- Mortar that dries on the stone. Wet the stones and cure damp.
- Building too high in one day. The lower courses squash out.
- Clay mortar on an exposed face. Rain washes it out; render or roof the wall.
- No tie stones. The faces and core separate even with mortar.
- Treating mortar as a substitute for a good footing. The wall cracks where the ground moves.
Safety
Wet lime burns and can blind
Quicklime reacts violently with water and heat; slaked lime, lime putty and fresh mortar are strongly alkaline and burn skin and eyes. Wear eye protection and gloves, wash splashes off at once with plenty of clean water, and keep children away from the slaking and mixing area. See the hazard noted in Lime Plaster for Straw Bale Exteriors. Mortar dust, like sand dust, contains fine particles that harm the lungs: do not dry-sweep or dry-mix without covering the nose and mouth.
What’s Next
- Learn next: Gabion Baskets — a stone wall that needs no mortar
- Combine with: Mortar Mixing — preparing the mortar