Stone and Rubble Masonry Construction
Core concepts
Stone masonry is classified by the finish and coursing of the face: random rubble (uncoursed, irregular), coursed rubble (roughly levelled courses), square rubble or block-in-course, and ashlar (accurately dressed squared stones with fine joints). Dry stone walling uses no mortar at all and relies entirely on interlock.
Stone selection and preparation
Use sound, hard, durable stone free from cracks, weathered layers and clay seams. Clean it and wet it before laying, especially in hot weather, so it does not draw water from the mortar. Dress only what is needed; use a bolster and club hammer or a chisel and mallet, working with the natural bed of the stone rather than across it.
Laying rules
- Select the largest, soundest stones for the bottom courses and for the quoins at corners.
- Bed each stone fully on mortar; never prop a face stone on a dry pack with a void behind it.
- Fill the interior (hearting) with smaller stones well bedded in mortar, not loose rubble.
- Place through stones or bond stones at intervals across the full wall thickness so the two faces tie together.
- Use a mortar of about 1:4 to 1:5, and rake joints for pointing where a finished face is required.
- Keep the wall plumb, with courses levelled at intervals; wider joints at the bottom are not a substitute for sound bedding.
Traps in questions
The classic fault is a wall with two neat faces, loose hearting and no through stones: it looks right and fails. Others are un-wetted stone sucking the mortar dry, stones laid on their weak plane, and joints so wide that the mortar shrinks and cracks.
Worked rule of thumb
Allow roughly 1.25 m3 of stone and 0.3 to 0.35 m3 of mortar per cubic metre of random rubble wall, because voids and hearting consume both. Order stone with about 5 to 10 percent wastage for dressing.