Masonry Layout and Wall Failures

Anyone can follow a line once it is up. The harder work comes before and after that: laying out a wall so openings land on whole units, keeping it in tolerance over a long day, deciding what to do when it is 28°F, and looking at a crack and knowing whether it is cosmetic or a call to the engineer.

You can already lay to the line and tool a joint. This guide adds the judgment of a mid-level mason. Numbers come from the Specification for Masonry Structures (TMS 602), the Brick Industry Association (BIA) and the Concrete Masonry and Hardscapes Association (CMHA). Your plans, specification and local code control. Reinforcement, control joints and lintels are engineering decisions. You build them. You do not change them.

What You Will Be Able to Do

• Lay out wall lengths, openings and heights on the 8-inch module.
• Check a wall against standard tolerances as it rises.
• Place control joints as drawn and explain why they differ from brick expansion joints.
• Apply hot and cold weather rules for mortar and new walls.
• Prepare and grout cells without blowouts or voids.
• Read crack patterns and know which ones go to the engineer.

Laying Out on the Module

Standard block runs on an 8-inch module, which means every length and height should work out to a multiple of 8 inches. Then each course uses full and half units with no cutting.

Lengths. A run of block has one fewer head joint than it has blocks, so it finishes 3/8 inch short of its nominal length. Six blocks are 6 x 15-5/8 inches plus 5 joints of 3/8 inch, which is 95-5/8 inches, not 96.

Openings. A masonry opening is usually sized 4 inches wider than the door or window, which leaves 2 inches a side for framing. A window opening is usually 8 inches taller than the window: 2 inches at the top and bottom, plus a 4-inch sill. Door openings are usually 2 to 4 inches taller than the door.

Worked example. A wall is 32 feet long and has a 36-inch door.

1. Length: 32 x 12 = 384 inches. 384 / 16 = 24 blocks a course. It works on the module.
2. Masonry opening: 36 + 4 = 40 inches. 40 / 8 = 5 half-block modules, so it works.
3. Place the opening so its edges land on a joint or a half block. If the plan drops it 4 inches off the module, you face cut block all the way up each side. Catch it now and ask, before the first course goes down.

Heights. Each course is 8 inches including its joint. A 10-foot 8-inch wall is 128 / 8 = 16 courses. Mark the top of each course, the sill and the lintel on a story pole.

The first course. Dry-lay it first. A footing that is not level is taken up in the first bed joint, which is commonly held to about 1/4 to 3/4 inch. If the footing is off by more than that, stop and ask. It is not something to solve by mixing thick and thin joints up the wall.

Holding Tolerance

Tolerances in TMS 602 are based on how a wall performs under load, not only on looks. These are the common ones for structural walls. Veneer and project specifications can differ.

If you cannot hold these because of earlier work, such as a footing out of place, tell the foreman. The specification expects the architect or engineer to be told, not for the mason to quietly absorb it.

Daily habits that keep you inside these numbers:

1. Check every corner with a level and a story pole, and do not trust yesterday's marks.
2. Check the rack with a straightedge across the stepped end.
3. Keep the line tight. On a long run, support it near the middle with a trig, a small piece set at the right height, so it cannot sag.
4. Do not walk the error along. Fix a course that is out before the next one goes on it.

Joints That Let a Wall Move

Concrete block shrinks as it dries. Clay brick grows with moisture and temperature. So the two need different joints.

• A control joint in block is a vertical break that lets a wall shrink and crack where you planned. It has a bond break and a sealant.
• An expansion joint in brick is a clear gap with compressible filler or sealant that leaves room to grow. It has no mortar in it.

Joint locations are shown by the designer. They typically fall at regular spacing, near corners, at returns, at changes in wall height or stiffness, and often beside openings. CMHA's empirical rule puts control joints in walls with horizontal reinforcement at the lesser of 1.5 times the wall height or 25 feet. So a 10-foot wall gets joints at no more than 15 feet, and a 20-foot wall at no more than 25 feet.

Do not move a joint to suit the scaffold, and do not fill one with mortar. A filled joint is a crack waiting to form.

Hot and Cold Weather

TMS 602 treats 40 to 100°F as normal. Outside that, set rules apply. Preparation and work rules go by the air temperature. Protection rules go by the mean daily temperature, which is the day's high and low averaged.

In the coldest bands, grouted masonry needs protection for 48 hours, unless the only cement in the grout is Type III. Never heat water or sand over 140°F. Antifreeze admixtures are not recommended in mortar and are not allowed in grout.

Rain. Plastic mortar washes out. Cover the top of the wall at the end of the day and when rain is likely. Keep water out of open cells, and cover stored units and bags, because wet units do not bond well.

Grouting Without Trouble

Grout carries the steel in reinforced walls, so a hidden void matters more than a visible one.

1. Check first. Steel is in place and tied, and cells line up. Remove mortar fins of more than 1/2 inch and droppings from cells.
2. Use the right mix. Conventional grout has a slump of 8 to 11 inches. It is meant to be that wet, because the block pulls water out of it. Do not stiffen it.
3. Know your lift. A low-lift pour is commonly limited to about 5 feet 4 inches. High-lift pours can reach 24 feet, but they need cleanouts at the bottom of the reinforced cells, inspection and a plan. Lifts over 5 feet 4 inches need the conditions the specification sets.
4. Vibrate. Over 12 inches of pour, use a small pencil vibrator. Keep it short, a few seconds in each cell, and pull it out slowly.
5. Reconsolidate. Go back 3 to 10 minutes later, after the block has drawn off water, but before the grout loses its plasticity.
6. Mind the clock. Place grout within about 1-1/2 hours of adding water.

Too much vibration or a pour on a green wall can push out a blowout, where the face shell bursts from the pressure. Let new masonry set up first, and stop the pour if the wall moves. If a pour stops at a cold joint, stop the grout about an inch below the top of the course so the next lift can key into it.

Reading Cracks and Failures

Before you judge a crack, ask where it is, what it follows, how wide it is, and whether it is growing. A pattern is a clue, not a diagnosis.

Water Problems

Walls in Wind

A new wall has almost no strength against wind until the mortar cures and the floor or roof ties in. OSHA requires a limited access zone on the unscaffolded side before the wall starts, as wide as the wall will be tall plus 4 feet. Masonry walls over 8 feet tall must be braced until permanent supports are in place, unless they are already held up well enough.

Warning: Stop work and clear the zone when wind reaches the limit in your bracing plan. Do not stand on the unbraced side to check a wall. These collapses happen in seconds and leave no time to move.

Check Your Understanding

1. A plan shows a 40-foot wall with a 44-inch window. Does the opening work on the module?

A window 44 inches wide needs a 48-inch opening. 48 / 8 = 6, so yes. Check that its location also lands on the module. 40 feet is 480 inches, and 480 / 16 = 30 blocks a course.

2. It is 95°F with a steady 12 mph wind. What must you do differently?

That is hot weather under TMS 602. Keep mortar under 120°F, keep sand damp, flush tools with cool water, use mortar within 2 hours, and fog spray the wall at least three times a day until it is 3 days old.

3. A coworker wants to stiffen the grout so it stops running out of a gap at the wall base. Is that the right fix?

No. Grout needs an 8 to 11-inch slump to fill cells around steel. Seal the leak instead.

4. You find a long horizontal crack in a block basement wall, and it bows inward at mid-height. The backfill just went in. What now?

Stop work there, keep people clear, and report it. That pattern points to soil pressure, and it is an engineer's call.

5. Your supervisor wants the control joint moved 3 feet so it falls behind a scaffold post. What do you do?

Ask the designer. Joint locations are set in the drawings for a reason, and scaffold convenience is not one.

Key Takeaways

• Plan lengths, openings and heights on the 8-inch module before the first course goes down.
• Hold plumb, level and line to 1/4 inch in 10 feet, and tell someone when earlier work makes that impossible.
• Block shrinks and needs control joints. Brick grows and needs expansion joints.
• Follow weather rules outside 40 to 100°F, and cover new work.
• Grout at 8 to 11-inch slump, vibrate, then reconsolidate.
• Stair-step cracks, bowing and rust stains go to the engineer, not to the patch bucket.