The "Gap Trap": Why Nominal Math Ruins Deck Estimates

There is a specific mistake that separates a first-time DIYer from a seasoned deck builder. It usually happens when you reach the very end of the deck.

You calculated that your 10-foot wide deck needed exactly 20 boards (because 10 feet ÷ 6 inches = 20). But when you lay the last board, you are short by nearly 10 inches. You have to stop work, drive back to the store, and buy two more rows of lumber.

Why did the math fail? Because a "6-inch" deck board is not 6 inches wide.

Below is the "Contractor Math" used in our Decking Calculator, ensuring your material list accounts for the reality of the lumber yard, not just the label on the shelf.

In the US and Canada, softwood lumber is sold by "Nominal Dimensions" (the size of the wood before it was dried and planed smooth).

If you calculate your coverage based on 6 inches, you will underestimate your row count by nearly 10%. On a large deck, that is a significant shortage.

Deck boards cannot be jammed together tight. They need a "gap" (typically 1/8" to 1/4") to allow for water drainage and seasonal expansion.

When calculating how many rows of boards you need, you must add this gap to the board width to determine your true "Coverage Width."

Example: For a standard 5.5" board with a 1/8" (0.125") gap:

5.5 + 0.125 = 5.625 inches of coverage per row.

It seems small, but over a 20-foot wide deck, properly accounting for the gap might save you from buying 2 extra rows of expensive composite or cedar.

Once you know your row count, you can determine the Total Linear Feet of material required. This is the "God Number" for ordering.

It is vital to distinguish between Decking (the surface you walk on) and Framing (the structure underneath).

• Decking: 5/4" or 2" thick boards. Calculated by surface area.
• Framing: Pressure-treated 2x8, 2x10, or 2x12 joists. Calculated by span tables.

Important Note: Surface calculations do not account for joists, beams, or ledger boards. For the structural frame, you must consult specific span tables provided by the American Wood Council (AWC) DCA 6 Guide. This is the industry standard for residential deck codes.

You can do the "5.5 inch" math manually, or you can use a calculator that has these industry standards built-in.

We designed the Definitive Calc Decking Tool to handle the nuances of contractor math automatically:

• Actual Width Logic: You select "6 inch nominal," and we calculate using 5.5 inches automatically.
• Gap Control: We default to the standard 1/8", but you can adjust it for composites or tight fits.
• Waste Factors: Quickly add 10-15% for bad cuts and ends.
• Board Count: We convert your linear footage into an estimated count of 12-foot boards to help you visualize the order size.

For detailed requirements on fastening these boards (screws vs. nails) and joist spacing limits (16" vs 24" on-center), always refer to the International Residential Code (IRC) - Section R507. This code dictates the maximum distance your joists can be spaced based on the decking material you choose.