As promised during episode 26 of our podcast, I have added some notes on estimating material. The information below consists of the most common types of materials that construction managers are asked to order for their trade partners. It is essential that construction managers have a basic understanding of how to estimate materials so that they do not get over order material and so that they can evaluate requests from their trade partners from a place of understanding and not blind trust. There is nothing worse than too much or too little of the material you need to complete a job. When subcontractors come to you and ask for additional material you must be able to decide if their request is valid and if the material they are requesting is the correct quantity. Far too often our trade partners come to us and declare that they need 200 2x4s to complete the interior framing of this or that, so we order it trusting that there request is based on proper estimation of the material they need. After the material is ordered and arrives there are usually 1 of 2 things that happens, first, the trade partner comes to you and says that they need more material or, they have a ton of leftover material and now your job site looks like a lumber yard, at least until the material is stolen. Below are some commonly used mathematical equations used on the job site to determine the size and volume of areas as well as for estimating material on the job site. I hope this helps you better manage your jobs and your trade partners.
Calculating square footage = Length x Width
Example: 30’x20’= 600 square feet
Calculating Volume = Length x Width x height
Example: 30’x20’x10’= 6,000 cubic feet of volume
Calculating yards of concrete and dirt = Length x width x depth, then divide by 27 (the number of cubic feet in a yard)
Example: 10’x10’x .35’ (4 inches) = 35, now divide by 27 = 1.3 cubic yards of concrete
Converting yards to tons (Hauling and importing dirt)
Typically a 1 cubic yard of dirt is equal to 2,000 lbs or 1 ton of dirt. Dump trucks are measured by how many cubic yards they can hold. A standard dump truck would most likely be used for your project, and they can typically carry 10 to 12 cubic yards of dirt in a truckload or 10 to 12 tons of dirt.
Example: Your cut sheet from your engineer says you have to cut 2’ of dirt from your building pad that measures 40’x40’.
40’x40’x2’= 3200 Square feet of dirt. Divided by 27 = 118 cubic yards of dirt or 118 tons. This equates to 10 to 12 truck loads.
Lumber take offs for typical 2×4 and 2×6 framed walls.
Review your construction drawings and determine the on center stud locations for your wall, typical is 16” on center for exterior walls and 24” on center for interior non load bearing walls. “On-center” spacing of the wall studs: the distance between the centers of neighboring studs. Also, note any corners in the wall or places where the wall intersects with other walls. Standard frames for load-bearing walls include a single bottom plate and two top plates. To calculate the linear feet needed for the plates, multiply the total length of the wall times 3, then add 5 to 10 percent for waste.Determine the number of studs needed by starting with a quick calculation for the general studs and adding studs for specific elements:
Headers for standard-size doors and windows often are built with two pieces of 2×12 lumber sandwiched over a piece of 1/2-inch-thick plywood cut to the same size as the header. The total thickness of the header is 3 1/2 inches to match the width, or depth, of a 2×4 wall frame.
To estimate the framing materials for each window and door opening, use the total width of the opening plus 7 inches. For example, for a 36-inch wide door, you need two pieces of 2×12 at 43 inches and one piece of plywood at 11 1/4 inches (the actual width of a 2×12) by 43 inches.
Interior and exterior wall sheathing (Drywall or plywood) based on 4×8 sheets of material
To calculate the number of 4-by-8-foot sheets of sheathing needed for a wall frame, determine the area of the wall, then convert that value to the number of sheets:
Example: The wall is 8 feet tall and 20 feet long, it has a total area of 160 square feet. If there is a 4-by-5-foot window opening, the total area to cover is 160 – 20 = 140. Divide 140 by 32 to get 4.375. Round up to 5. This wall requires five 4-by-8-foot sheets of sheathing.