8.4 Masonry Estimating and Cold/Hot Weather Construction
Key Takeaways
- Benchmark counts: 1.125 CMU per sq ft and 6.75 modular brick per sq ft with standard 3/8-inch joints; add a 3-10% waste factor.
- A 40x10 ft CMU wall needs 400 sq ft x 1.125 = 450 blocks, or 473 after 5% waste.
- Margin is profit over selling price; markup is over cost. A 25% margin = Cost/0.75 (33.3% markup), not Cost x 1.25.
- Cold-weather masonry procedures trigger below 40 deg F, with escalating heating/enclosure steps down to 20 deg F; never lay on frozen surfaces.
- OSHA 1926 Subpart Q requires a limited access zone of wall height + 4 ft; the general fall-protection trigger (Subpart M) is 6 feet and walls over 8 ft must be braced.
Masonry Estimating and Weather Construction
The NASCLA exam pairs masonry knowledge with the Business and Project Management reference: you must do quantity takeoffs, apply markup vs. margin, and follow cold/hot weather procedures.
Unit Counts per Square Foot
Memorize these benchmark figures with standard 3/8-inch joints:
| Item | Quantity |
|---|---|
| CMU (8x8x16) | 1.125 units per sq ft (about 112.5 per 100 sq ft) |
| Modular brick | 6.75 brick per sq ft (about 675 per 100 sq ft) |
| Standard brick | about 7 per sq ft |
| Mortar (CMU) | about 8.5 cubic ft per 100 sq ft (3 bags) |
| Mortar (brick) | about 6.5 to 7 cubic ft per 100 sq ft |
Takeoff Worked Example
Estimate a CMU wall 40 ft long x 10 ft high with no openings:
- Wall area = 40 x 10 = 400 sq ft.
- CMU needed = 400 x 1.125 = 450 blocks. Add 5% waste = 450 x 1.05 = 472.5 -> 473 blocks.
- Mortar = 400 / 100 x 8.5 = 34 cubic ft (about 12 bags of masonry cement).
Exam trap: always deduct openings AFTER computing gross area, and always add a waste/breakage factor (typically 3-10%) to units, not just to mortar.
Markup vs. Margin (Business & Project Management)
These are NOT the same and the exam exploits the confusion.
- Markup is added to cost: Price = Cost x (1 + markup%).
- Margin is profit as a percentage of selling price: Margin% = Profit / Price.
Worked example: masonry job cost = $20,000, desired 25% margin.
- Using margin: Price = Cost / (1 - margin) = 20,000 / (1 - 0.25) = 20,000 / 0.75 = $26,667.
- A 25% markup would instead give 20,000 x 1.25 = $25,000 — a different, lower number.
So a 25% margin requires a 33.3% markup. Confusing the two underprices the job.
Cold Weather Masonry (Below 40 deg F)
Follow IBC / TMS 602 cold-weather provisions. Triggered when ambient temperature falls below 40 deg F (4.4 deg C).
- 40 deg F and below: heat mixing water to produce mortar between 40 and 120 deg F; do not lay on frozen surfaces.
- 32 deg F and below: heat sand and water; protect completed work.
- 25 deg F and below: use windbreaks/enclosures; maintain heat.
- 20 deg F and below: enclose and provide auxiliary heat to keep masonry above 32 deg F for 24 to 48 hours.
Never lay masonry on a frozen base; frozen mortar has near-zero bond strength once it thaws.
Hot Weather Masonry (Above 100 deg F) and OSHA
Hot-weather provisions trigger above 100 deg F, or above 90 deg F with wind over 8 mph. Rapid water loss kills the cement hydration that develops strength.
- Keep mortar materials and mixer shaded; flush mixer with cool water.
- Use mortar within 1 hour (versus 2-1/2 hours normally) of mixing in hot conditions.
- Fog-spray and cover completed masonry to slow moisture loss.
OSHA (Occupational Safety and Health Administration) 29 CFR 1926:
- Subpart M (Fall Protection): general construction fall-protection trigger is 6 feet; masonry scaffold work falls under Subpart L. Scaffolds require guardrails/personal arrest at 10 feet (Subpart L).
- Subpart Q: a limited access zone must be established beside a masonry wall under construction, equal to the wall height plus 4 feet, before bracing — only workers building the wall enter it.
- Walls over 8 feet must be braced until permanent supports are in place.
A masonry wall is 40 feet long and 10 feet high with no openings. Using 1.125 CMU per square foot plus a 5% waste factor, how many 8x8x16 blocks should be ordered?
Under OSHA 29 CFR 1926 Subpart Q, what limited access zone must be established next to a masonry wall under construction before it is braced?
Masonry Estimating — Units and Mortar
Estimate brick and block by wall area. Rules of thumb: standard modular brick ≈ 6.75 bricks/SF (single wythe, with mortar); 8×8×16 CMU ≈ 1.125 blocks/SF (since each block covers 8×16 in = 128 in² ≈ 0.89 SF). Worked example: a 100 SF CMU wall ≈ 100 × 1.125 = 113 blocks; add ~5% waste → order ~119. Mortar: roughly 2.5–3 ft³ per 100 SF of CMU. Always apply a waste factor (~5–10%) for cuts and breakage.
Hot-Weather Masonry
Hot, dry, windy conditions flash-dry mortar before it bonds. Mitigations (per TMS 602 hot-weather provisions, roughly above 100°F or 90°F with wind): wet (but don't soak) the units if absorptive, use cool mixing water, shade materials, keep mortar workable (retemper within board life), and fog-cure completed work. The goal is to slow water loss so the mortar can hydrate and bond.
Cold-Weather Masonry
Cold weather (below ~40°F and falling) threatens the mortar with freezing before set, which destroys bond. Measures: heat mixing water (and sand), never lay on frozen units or frozen bases, use windbreaks/enclosures and heat, and protect completed masonry with insulated blankets for 24–48 hours. Admixtures with calcium chloride are prohibited near embedded metal (corrosion). Never use frozen, ice-coated units or partially set mortar.
Common Exam Traps
- Trap: ~7 CMU per SF (that's near brick). CMU ≈ 1.125 / SF.
- Trap: Skipping the waste factor on a takeoff.
- Trap: Laying masonry on a frozen base or with frozen units.
- Trap: Calcium chloride accelerator with embedded steel ties (corrosion).
Approximately how many 8x8x16 concrete masonry units are needed for 200 SF of wall (before waste)?