How to Tool Concave Mortar Joints

Job: compress and profile the fresh mortar joints in a brick or block wall — striking each one concave at exactly the right moment — so the joints densify, seal against the units, and shed rain instead of drinking it.

Time: the tooling itself is fast; a wall takes far longer to lay or repoint than to strike. The work is not the labor — it is the *waiting*, and then not missing the window. Budget the strike as a series of short returns to a wall you laid hours ago, not a task you sit down to.

Difficulty: low hand skill, high timing. If you can hold a tool to a line you can tool a joint — the entire craft is knowing the one moment the mortar is ready, and that moment moves with the weather and the hour.

The tools from this catalog:

the 3/8" & 1/2" brick jointer (Hyde 18170) — the hero: a one-piece high-carbon steel sled-runner with two polished half-round ends, 3/8" for standard modular brick and 1/2" for block and older wider joints. The gently upturned runner bridges the joint and rides its own finished work, so long bed joints come out straight instead of following every wobble of your wrist.

Everything else: the mortar is already in the wall — this job supplies none of it. A stiff bristle brush to dress the fins after. Gloves — load-bearing here, not a courtesy (the safety note in Step 5 says why). A spray bottle or hose for the cure. And, stated plainly, the tools this line does not sell: a V-jointer, a slicker, a joint raker, a grapevine or beading tool — one for each of the profiles that are *not* concave. If your wall wants a profile other than concave, the tool that makes it is one you buy elsewhere. This line strikes concave, and says so.

Mortar is roughly a sixth of a brick wall's face, and the joint is the part that fails first: it is softer than the brick, more open, and it is where water tries to get in. Tooling (striking, jointing) is the pass that closes that sixth of the wall. Run a shaped steel tool down each joint at the right moment and you compress the loose surface paste, press the shrinkage cracks shut against the brick, and leave a dense, water-shedding profile and a clean, finished line. Skip it, or botch it, and the joints stay porous — the wall looks unfinished and drinks every rain. This is a short job that protects a long one.

Before you start: read the mortar, match the tool, choose the profile

Find thumbprint-hard — the one moment this whole job waits for. Fresh-struck mortar is too wet to tool and cured mortar is too hard; the window is the interval between, and it has a name and a test. Press your thumb into the joint: when it takes a clean print without wet paste lifting onto your thumb, it is ready — stiff enough to hold the compression you are about to give it, soft enough to take it. Too early, and the tool drags water and cement fines to the surface, smears the brick arrises, and leaves a weak, pale, dusty skin that weathers off. Too late, and the tool skates on rock-hard mortar, compresses nothing, and burnishes ugly black steel-marks into the line. The window slides with the weather — an hour on a hot, dry, windy wall; half a day on a cool, damp one — and it arrives in the order the wall was built, because the first joints laid stiffen first. That single fact sets your rhythm: tool in laying (or pointing) order, checking back to where the morning's first work is coming ready while the fresh end waits its turn. The whole skill of this job is reading that one moment; everything else is just running a tool down a line.

Match the tool's working end to the joint's width. A jointer compresses by *bearing on the mortar* while the brick edges — the arrises — guide it, so the working end must match, or run a hair proud of, the joint. The 18170's 3/8" end is the standard-modular-brick width and handles most brick walls; its 1/2" end picks up concrete block and older, wider brick joints. Size matters in both directions: an end too small digs a groove down the inside of the joint instead of compressing its face; an end far too big flattens the joint instead of scooping it. Match first, then strike.

On a repair, match the wall you already have — width, profile, *and* depth. If you are tooling a repointed joint rather than fresh new work, the profile is not yours to choose: it is already set by the sound original joints around it, and your repair has to disappear into them. Read the surviving joints — their profile, their width, and how *deep* the scoop sits — and match all three. A concave strike run deeper or wider than its neighbors reads as a patch in every raking light for the life of the building. Note that on old masonry the tooling window arrives faster than on fresh laying, because dry old brick pulls water out of thin packed lifts quickly — strike as you go, section by section, not at day's end.

Choose the profile honestly — these tools make concave, which is also the right default. The profile is not decoration; it decides how the joint sheds water, and the trades settled the argument long ago. The Brick Industry Association's technical note ranks tooled concave and V joints best at resisting water penetration in exterior masonry — compressing the surface seals it, and the curved or veed face throws water off the mortar onto the harder brick — while raked and struck-flush joints leave ledges and open, uncompressed texture that let water in (BIA TN 7B; Tobermore reaches the same ranking). So on any exterior wall, unless you are matching an existing specialty profile, concave is the answer — and concave is exactly what this line's jointers strike. Here is the honest menu of what exists, when each is used, and which need a tool this store's masonry line does not stock:

Concave (a half-round scoop) — the weather default and the trade standard for exterior brick and block: compressed, water-shedding, and forgiving of slightly irregular brick. This is what the 18170 and 18182 make.

V (vee) — the other profile the BIA ranks top for weather; a crisp central vee that sheds water and casts a sharp shadow line. Wants a V-jointer (not in this line).

Weathered (struck) — the bed joint cut on a slope, pressed in at the top and left out at the bottom so it sheds water down and away; a traditional, weather-sound exterior joint struck with the trowel's edge, not scooped. Not a jointer profile.

Flush — mortar cut off flat with the trowel and left untooled; a rustic joint for interior, to-be-painted, or to-be-parged work. It is not compressed, which makes it the least weather-tight of the common joints. Struck with the trowel; no jointer needed or used.

Raked (mortar raked back square to a set depth) — a deep, dramatic shadow line loved on interior feature walls and modern design work, but the square ledge it leaves holds water, so it is a poor choice on a freeze-thaw exterior. Wants a joint raker / skate wheel (not in this line).

Grapevine and beaded — historic and colonial-revival profiles (a grooved bead run down the joint, or a raised convex bead) chosen to match period buildings; each wants its own specialty jointer (not in this line).

The rule under all of it: on a repair, match the wall (Step 3); on new exterior work, concave unless you have a reason; and if the profile you need is not concave, buy the tool that makes it — this line will not pretend a half-round strikes a vee.

Striking the joints

Glove up, then strike the short vertical head joints first. Wet and stiffening mortar is strongly alkaline — pH 12 to 13 — and it burns skin caustically with no pain to warn you; a national survey found cement burns in 35% of apprentice masons, and tooling rides your hand along wet mortar joint after joint (eLCOSH, Poison Control, NPCA). Gloves before the first joint. Heads-first is not arbitrary — the bed-joint pass you make next runs *through* these verticals and erases the little stop-marks the tool leaves where it lifts out at each intersection.

Run the long horizontal bed joints through the heads. With the heads struck, lay the jointer into a bed joint and draw it along the whole run in one steady pass, letting the upturned sled ride the finished channel behind it so the line comes out straight. The bed pass wipes out the head-joint stop-marks and leaves the horizontal lines continuous — and the horizontals are the lines a finished wall reads by, the ones raking evening light travels straight across, so their continuity is most of what "a tooled wall" looks like. Refeed a slightly hungry spot with light downward pressure; but a joint that runs *truly* starved of mortar is a filling problem, not a tooling one — pack it (the pointing trowel's job) and come back, rather than dragging steel over an empty joint.

Compress — do not carve — and let the burnish tell you when to stop. The half-round is not cutting a groove; it is compressing the joint's face — densifying the surface paste, closing the shrinkage micro-cracks against both brick arrises, and leaving a concave meniscus whose curve sheds water. Bear on the mortar with firm, even pressure, the barrel riding on the mortar and steered by the arrises on either side, and know that one smooth pass beats three fussy ones — every extra pass over drying mortar risks the black burnish-marks of tooling too late. The right result is a joint that is faintly sheened and tight, uniformly curved, sitting a hair recessed from the brick face. If the tool is *tearing* or *chattering* instead of pressing a clean curve, the mortar has gone past the window — forcing it only marks the joint, so stop and let the fix be next time's timing, not more pressure now.

After the strike

Let the fins dry crumbly, then dress them off — never wet. Tooling squeezes thin ridges of mortar — fins — out along both edges of every joint. Leave them. Wiping or brushing them while they are wet is the classic ruin: it drags cement paste across the brick and burns in a gray haze that no rinse fully forgives, and on porous or textured brick it is close to permanent. Wait until the fins have dried crumbly — an hour to a few, depending on weather — then knock them off with a stiff bristle brush stroked diagonally across the joints (diagonal, so the bristles skim the fins off without digging the fresh profile out), and lightly brush or sponge the faces clean only then.

Cure it slow — mist and shade. A struck joint is finished but not yet strong: mortar, and lime-rich mortar most of all, cures by chemistry that needs *water and time*, and mortar that flash-dries in sun and wind cures weak and powdery no matter how well it was tooled. Keep fresh joints damp — a periodic light mist, or draped damp burlap on a repair — and out of direct sun and hard wind for the first days. The tooling gave the joint its density and its shape; a slow, moist cure is what turns that shape into something that lasts. A wall tooled beautifully and cured fast still fails.

What goes wrong (and the fix)

"The brick faces are smeared gray around my joints." You tooled too wet, or brushed the fins wet — both drag cement paste onto the brick. Strike only at thumbprint-hard, and dress fins only after they dry crumbly. Cement haze is far easier to never make than to remove.

"The joint is pale, dusty, and crumbly on the surface." You tooled too early: the tool pulled water and fines up and left a weak skin. There is no polishing it back — the surface has to weather off, or be cut out and repointed. Wait for the clean thumbprint next time.

"The tool left black shiny streaks and pressed in nothing." Too late — the mortar went hard and you burnished it instead of compressing it. Nothing fixes a marked joint but cut-and-repoint; the lesson is to catch the window as it moves along the wall in laying order.

"The joints are deep here and shallow there / the profile wanders." Inconsistent pressure, or the wrong-width end for the joint. Match the end to the joint (Step 2), keep even pressure, and let the arrises — not your wrist — steer the depth.

"My repair joints don't match the old wall." Wrong profile or wrong depth against the surviving originals. Match width, profile, *and* scoop depth (Step 3) before you commit an elevation — a concave strike in a flush or grapevine wall is a permanent scar. And if the wall's profile isn't concave, this line's jointers are simply the wrong tool: buy the one that matches.

On irregular stone the sled bridges shorter spans, so keep the pressure even and let the barrel find its own bearing on the mortar.

The history footnote (because these pages should be worth reading)

The concave joint did not win on looks — it won by outlasting everything beside it. Masons watched walls weather for generations and kept the profile whose compressed curve stayed tight and dry; only later did the Brick Industry Association's technical notes write the observation down as doctrine — tooled concave and V joints best resist water penetration, raked and flush joints invite it (BIA TN 7B). The sled-runner jointer is the anonymous trade pattern built to lay that curve down at production speed: no inventor's name on it, just the shape the work demanded. The deeper lineage — the two brick-trowel patterns, the finishing trades, the doctrine layer — lives in the line document these guides are cut from.