NASCLA Commercial General Building Calculations Practice

An activity has an early start of day 12, a late start of day 18 and a duration of 5 days. What is its total float?

Total float is the late start minus the early start (or late finish minus early finish). The duration does not enter the subtraction.

A trench is 8 ft deep in Type B soil and is sloped at 1:1 (given) on both sides. If the bottom is 4 ft wide, how wide is the top?

A 1:1 slope sets the wall back 1 ft horizontally for each foot of depth on each side. Counting only one side gives 12 ft.

A slab on grade is 5 inches thick. Using a joint depth of one-quarter of the slab thickness (given), how deep should the saw cut be?

One-quarter of 5 inches is 1-1/4 inches. Cuts that are too shallow let random cracks form beside the joint.

Using 1.125 standard 8 x 16 blocks per square foot of wall (given), how many blocks are needed for a wall 80 ft long and 12 ft high with no openings, before waste?

Each 8 x 16 nominal block covers 128 sq in, or 0.889 sq ft, so there are 1.125 blocks per square foot. Dividing by 1.125 gives 853.

Fourteen W12x26 beams are each 30 ft long. What is their total weight in tons (2,000 lb per ton)?

The second number of the designation is pounds per foot. Dividing by 1,000 instead of 2,000 gives 10.92.

How many board feet are in one 2x6 that is 12 ft long?

A board foot is 1 in × 12 in × 1 ft (144 cubic inches). Board feet use nominal dimensions. Forgetting to divide by 12 gives 144.

A low-slope roof measures 120 ft by 85 ft. How many squares of membrane is that, before waste and laps?

Divide square feet by 100. Adding the two sides (205) is a perimeter-style mistake.

A room has four walls, each 20 ft long and 10 ft high, with no openings. How many 4 ft x 10 ft gypsum boards are needed for one layer on the inside of the walls, before waste?

Find the total wall area, then divide by the area of one board. Counting both faces of the walls would double the count.

A building's cooling load is 540,000 Btu/h. What is the required capacity in tons?

Divide Btu per hour by 12,000. Dividing by 3,412 (the kW conversion) gives about 158.

A temporary heater draws 12.5 amperes at 240 volts. Using P = V × I, what is its power?

Power is voltage times current. Dividing 240 by 12.5 gives 19.2, which is resistance in ohms, not power.

A contractor's bid is $2,400,000 and the bid documents require a 5% bid bond. What is the bond amount?

Five percent of the bid amount. Using 0.5% gives $12,000 and 10% gives $240,000.

Activity A (4 days) and activity B (6 days) both start on day 0. Activity C (3 days) can start only when both A and B are finished. In a forward pass, what is C's early finish?

In the forward pass an activity starts at the latest early finish of its predecessors. Using the smaller value (4) gives day 7; adding all durations gives day 13.

A basement excavation measures 60 ft by 40 ft by 6 ft deep in bank measure. How many bank cubic yards (BCY) is that?

There are 27 cubic feet in a cubic yard. Dividing by 9 (square feet to square yards) by mistake gives 1,600.

A slab is 40 ft by 30 ft by 4 inches thick. How many cubic yards of concrete does it need, before waste?

Convert the thickness to feet, multiply, then divide by 27. Dividing by 9 instead of 27 gives 44.44.

A CMU wall is 60 ft long and 10 ft high and has two openings of 3 ft by 7 ft and one of 6 ft by 4 ft. Using 1.125 blocks per square foot (given), how many blocks are needed before waste?

Subtract the openings from the gross area before multiplying, and round a partial block up. Ignoring the openings gives 675 blocks; dividing by 1.125 gives 475.

A steel plate is 1/2 inch thick, 24 inches wide and 10 ft long. Steel weighs about 490 lb per cubic foot (given). What does the plate weigh?

Convert every dimension to feet before multiplying. A shortcut: 1/2-inch plate weighs about 20.4 lb per square foot.

An order is 80 pieces of 2x10 joists, each 16 ft long. How many board feet is that?

Find board feet per piece, then multiply by the count (or do 2 × 10 × 16 × 80 ÷ 12 in one step).

A roof rises 6 inches for every 12 inches of horizontal run. What is its slope expressed as a percent?

Rise over run, times 100. A 6:12 slope is also called a 1/4 pitch (rise over the full span of 24 inches).

A paint covers 350 sq ft per gallon per coat (given). How many gallons are needed for two coats on 2,800 sq ft of wall?

Coverage is per coat; multiply by the number of coats.

A 3-inch building drain runs 80 ft at 1/4 inch per foot. How much total fall does it have?

Multiply the slope per foot by the length. 1/8 inch per foot would give 10 inches.