How To Calculate How Much Wood You Will Need To Build A Room
Things Needed

Sketch pad or architectural plans

Calculator
Tip
Online calculators can help with estimates, especially difficult calculations such as roofing and foundation materials. Always round up on estimates. Another option, to check your numbers against or to obtain direction, involves visiting a lumberyard. Given building plans or dimensions, a lumberyard can provide what is known as a material takeoff, which lists the materials needed.
Warning
Check your area's building code to verify the spacing and size of wood components needed.
The average cost to build a house in the early 21st century is about $100 per square foot, and the amount is unlikely to go down in the future. You can save a lot of money simply by doing your own construction. Building a room, either as an addition or within an existing area of your home, is a good beginning point. Estimating the amount of lumber you will need is a step in the right direction to keep your costs down.
Step 1
Sketch a plan showing the room you wish to build. Draw in doors and windows. Label the dimensions for length, width and height along with door and window openings. Alternatively, use architectural plans to determine size specifications.
Step 2
Create a tally sheet. Divide it into room component sections with counts and sizes for each section. Alternatively, make a long running list categorized only by the type of lumber and count. Generally, walls are at least 8 feet high; framing materials are also the same dimension. Adjust your estimates to allow for longer boards or more material if the room's ceiling will be higher.
Step 3
Find the number of 2by4inch wood boards you will use for wall studs. Each wall requires a stud every 16 inches; so calculate the wall lengths, together or separately, and divide the amount by 16. Increase the estimate with 2 additional studs for each wall section built for double stud beginnings and endings.
For example, a house addition with three walls — the fourth wall is already part of your home — each measuring 12 feet in length, translates to 36 feet total or 432 inches. Divided by 16 inches per stud results in 27 studs. Because each wall has two wall sections, add four extra studs per wall, which equals 12; that brings the total to 39 studs. Add a minimum of 10 percent — four more studs — for a total of 43 studs.
Step 4
Factor in additional 2by4 wood boards for each wall section: two for each top plate, one for each bottom plate and one more board for midwall bracing between the studs — a total of four additional boards per wall section.
Thus, three 12foot long walls need 144 feet of plates and bracing combined. Broken into components, this means 36 feet of wall bracing, divided by 8 feet — the standard length of 2by4 boards — equals five boards for bracing, rounded up to six boards for leeway. Three walls totaling 36 feet, with double top plates and a single bottom plate, translates to 108 feet of plates, or 14 boards, rounded up to 16 for leeway. So 22 boards are necessary for plates and bracing for a 12by12foot, threewall room addition.
Step 5
Allow for a double stud for framing on each side of windows and doors. An addition with one entrance and two windows would equal an extra six 2by4 studs with one more for leeway — a total of seven.
The total lumber calculation at this point is sufficient for building a room inside a house. Proceed for exterior room addition estimates.
Step 6
Determine the length of the header board above each door and window opening. Add 9 inches extra to each figure, and double it to find the length of 2by12 boards required. Each header is composed of a double layer of 2by12s with plywood in between.
For example, a single window measuring 2 feet (24 inches) in width results in 33 inches of singlelayered header board. Doubling for thickness translates to 66 inches of 2by12 board needed. Adding a door 36 inches wide adds another 90 inches of 2by12 board needed. This results in 156 inches of 2by12 necessary for the window and door header board. Divided into 8foot (96inch) lengths, a minimum of two boards that are 2 inches by 12 inches by 8 feet are needed.
Step 7
Find the room perimeter to calculate the treated sill plates required. They attach to the concrete foundation and may consist of any size from 2 by 4 to 2 by 12. A 12by12foot room requires 36 board feet of sill plates, or five boards 8 feet long. Add an extra board for leeway.
Step 8
Tally the number of sheets of plywood the exterior wall frame requires if you build a room as a house addition. For 8feethigh walls, this is simple: Each sheet of plywood is 4 by 8 feet; so divide the length of the wall by 4 feet.
Thus, a room addition with 12foot long walls requires three sheets of plywood per wall. Three walls would need a total of nine plywood sheets with one added as leeway for a total of 10.
Step 9
Calculate the number of sheets of plywood required for the subfloor of the room. Because the floor area is not guaranteed to be variations of 8 feet, figure the area by multiplying length by width.
Thus, a room 12 feet long and 12 feet wide is 144 square feet. A sheet of plywood has 32 square feet. So 4 1/2 sheets of plywood would be needed. Round up to at least six sheets of plywood for leeway.
Step 10
Allow for both floor and ceiling joists, often made out of 2by6 lumber, spaced either 16 inches or 2 feet apart.
In the 12by12foot room example, the 12 feet in width would be divided by the desired joist spacing, either 16 inches or 2 feet, to determine the number of joists needed. If the joists are spaced 2 feet apart, you would need six floor joists and six ceiling joists; add two joists for leeway for a total of 14 joists. If the joists are spaces 16 inches apart, though, you would need nine floor joists and nine ceiling joists; add two joists for leeway for a total of 20 joists.
Step 11
Consult rafter specifications for your location's building code as well as the specific rafter involved. Rafters may be spaced from 2 feet apart and up. Due to the size of the room and rafter spacing variations, the amount needed is highly variable. In addition, plywood required for roofing over the rafters varies due to the roof slope involved. The amount needed will be greater than the amount needed for flooring; a rough guess would allow for half again as much.