I sell lime mortar out of a small yard, which means I spend a lot of time talking people out of buying it for the wrong job and into buying it for the right one. The job it is most often wrong for is a modern block wall. The job it is almost always right for is a rubble stone wall like the ones holding up half the older houses in Prospect Hill.
Rubble is not brickwork with lumps
A rubble wall is built from stone of irregular shape and size, laid with whatever bedding gets each piece stable, and packed with smaller stones — pinnings, or galleting — in the gaps. The mortar’s job is to fill voids and stop the small stuff migrating. It is not carrying much load and it is not what makes the wall stand.
That has a direct consequence for pointing. In brickwork you cut a joint to a consistent depth and refill it. In rubble the joint width varies from a hairline to three inches within the same square foot, and treating it like brickwork produces something that looks like a stone wall drawn by someone who has only read about them.
The two ways it goes wrong
Cement. A Portland-rich mortar in a rubble wall is harder than the weakest stones, impermeable, and rigid. Water that used to leave through the joints goes into the stone and the wall behind instead. On a foundation wall, that moisture ends up in the sill plate. On a garden wall it freezes and pushes stones out. And cement pointing cannot be removed later without damaging the stone it is stuck to, so it forecloses the correct repair permanently.
Ribbon pointing. This is the aesthetic failure, and it is everywhere. Mortar is smeared proud of the stone faces in wide bands, sometimes with a tooled edge, so the wall reads as mortar with stones set into it. It also creates ledges that hold water against the stone. The fix is to keep the mortar recessed slightly behind the stone faces so the stone reads and water runs off.
Doing it properly
- Rake out loose and cement mortar by hand to sound material — depth varies, and in a rubble wall it can be several inches in places. Never a disc grinder.
- Flush out the dust and dampen the joints. Dry stone drinks the mix water and you get a powder joint.
- Pack deep voids with mortar and pinnings in layers, letting each layer stiffen. Trying to fill a four-inch void in one go gives you a shrinkage crack down the middle.
- Finish slightly recessed, and brush the surface back when it has stiffened to expose the aggregate. That texture is what makes new pointing look like it has been there a century.
- Protect it with damp burlap for several days. This is the step people skip and it is the one that decides durability.
My default mix for exterior rubble around here is NHL 3.5 with a well-graded sharp sand at about 1:2.5 by volume, going to NHL 5 at the base of a wall or anywhere permanently damp. The reasoning behind those grades is set out in the rundown of lime types, and the National Park Service preservation briefs, available via the National Park Service, are the reference I hand to anyone who wants the detail.
Rates: hand-raking and pointing rubble runs $35 to $80 per square foot of wall face, and it is slow — a mason might do 30 to 60 square feet in a day on a difficult wall. Removing old cement pointing adds $10 to $25 per square foot and is the part that blows up budgets.
Frequently asked questions
Can I point a rubble wall myself? Physically, yes, and plenty of owners do good work on garden walls and out-of-sight foundation walls. The learning curve is in judging when the mortar has stiffened enough to brush back and in resisting the urge to fill everything flush. Practice somewhere invisible first.
What time of year should this be done? Roughly May through September in this climate. Lime needs temperatures above 40 degrees and no frost for several weeks after application. Late autumn work can be done under tenting and heat, but it costs more and the risk of a bad batch rises.
How do I remove old cement pointing without damaging the stone? Slowly, with hand chisels and a light hammer, working from the middle of the joint outward so you are never levering against a stone arris. Small pneumatic chisels help on large areas. If the cement is bonded harder than the stone in places, it may be right to leave those spots rather than tear the face off.