Decking Lumber Calculator

When planning a new deck, you have to know exactly how many joists and boards you’ll need for lumber. Guessing is what most homeowners do, they end up buying either too little or to much. To prevent that mistake, count from the bottom-up. That means beginning with the frame then moving on to the top surface.

For example, how many board are needed to cover the area? How many joists is required for support? Will there be any gaps? What about waste? Every single inch of everything are included in calculation. You don’t measure in square feet; you count rows (boards). Boards each take up their own width + space for ventilation between board. Assuming a typical 5 inch wide board with a quarter inch gap, that’s around 5 and seven-eighths inches per board.

How to Count Deck Materials

So if you have a twelve foot wide deck, divide 144 inches by the number of inches covered to find that you need 26 rows. Multiply 26 x length of deck to get total linear feet of material. It feels like geometry class again, but it’s just practical multiplication.

The stiffness and cost of the structure depend on how far apart your joists is spaced. Most common materials will be fine with 16-inch on-center joists; that’s the norm because it strikes a good balance between cost and strength. Go down to 12 inches and you’re adding one-third more joists. That can come in handy if you want to lay down thinner planks, which can tends to bow, or if you’re laying boards diagonally. Going up to 24 inches loosens things, saving you some money at the price of a bouncier deck. Lighter composite materials runs this risk even more. Just go with whatever feels solid when you walk around on it.

People tend to forget about fasteners when adding on a deck, and they always goes over budget. For pressure-treated wood, that usually means two screws per intersection where it cross a joist. That means that even a basic 16-by-12 foot deck. Thirteen joists by twenty-six rows; will require almost seven-hundred screws…before you begin the stairs.

For a cleaner appearance, composite decking use hidden clips rather than face screws. They snap into grooves located along the sides of the board. It’s a prettier appearance, but there’s no interchangeability between brands. You can’t use a manufacturer’s clips with their boards, then switch manufacturers halfway through.

Plan for waste (There will be some). End pieces gets trimmed when they don’t fit on the rim joist or ledgers. Boards may also warp during storage. Additionally, triangular off-cuts from diagonal layouts won’t easily fit back into the project. Ten percent waste factor will cover most straight-lay projects nicely. Use fifteen or even twenty percent if you have a diagonal pattern, multiple levels, or picture frames. Delivery fee to bring it back costs more than buying an additional board up front.

The math doesn’t change much by material, but maintenance and timeline do. Gaps will always get bigger over time with wood deck because pressure-treated pine shrinks when dry (which it will be in its first year). With composite materials that flex according to seasonal temperatures, manufacturers recommends a little extra space to account for expansion and contraction. There is no better or worse option; both involve a trade-off between labor and maintenance. Wood decks needs to be sealed and stained periodically (every couple years?). Plastic composites can simply be washed occasionally.

Always reference local building code prior to cutting any wood. Soil conditions and footing depths varies by location. Ledger attachments and allowable joist spans also vary by location. A joist span calculator is a planning number, NOT an engineering stamp. Check your material manufacturer’s guide to be sure you are not exceeding span limits. Safety first!

With permits and a materials list in hand, the heavy lifting is easy. Frame the structure first. Do the surface second. Leave yourself some wiggle room. That’s better than guesswork. You should of planned for this sooner.