12 ft vs. 16 ft vs. 20 ft deck boards

The best stock length is not automatically the longest board or the cheapest board on the shelf. The right choice depends on your board-run length, seam support, reusable offcuts, transport, handling, and the local price for each available size.

Why the board-run direction matters more than deck area

For this example, boards run along the 20 ft side of a 16 × 20 ft deck. With 5.5 in boards and 1/4 in gaps, the deck needs 34 board rows across its 16 ft width. Each row must span 20 ft.

One 20 ft board can cover a full row. A 16 ft board needs an additional 4 ft piece. A 12 ft board needs an additional 8 ft piece. Those joins become butt seams, and repeat pieces can sometimes be recovered from other stock boards.

12 ft vs. 16 ft vs. 20 ft: worked comparison

These results use the actual DeckCalculator board-planning example: 16 × 20 ft deck, 5.5 in board width, 1/4 in board gap, 16 in joist spacing, 1/8 in saw kerf, no per-board end trim, boards parallel to the 20 ft run, and 10% material contingency.

All three options install the same 680 linear ft of deck-board rows: 34 rows × 20 ft per row. The difference is the purchased stock required to create those rows, including contingency and reusable offcut assumptions.

What the purchase-overage percentages mean

DeckCalculator reports purchase overage as the unused share of purchased stock:

For the 20 ft option, the purchase plan contains 38 × 20 = 760 linear ft. With 680 linear ft installed, the difference is 80 linear ft. Dividing 80 by 760 gives approximately 10.5% purchase overage.

Other estimators define waste relative to installed material instead. Under that different denominator, 80 ÷ 680 would be about 11.8%. Neither calculation changes the actual purchase quantity; they are different ways to describe the same material difference. Always check which denominator a tool uses before comparing percentages.

When 20 ft boards make sense

A 20 ft board is attractive when the board-run dimension is close to 20 ft and the selected product is actually available in that length. In our example, full-length boards eliminate the row butt seams and reduce the amount of stock purchased.

That does not make every 20 ft board the right choice. Longer boards can be harder to transport, carry, store, and place without damage. Availability and acceptable overhang also depend on the product profile.

Check the actual product specification. Trex lists certain standard-width boards in 12, 16, and 20 ft lengths, while other board profiles and product lines can have different availability.

When 16 ft boards can be the better purchase

A 16 ft stock board naturally fits a 16 ft run without a field seam, which may make it preferable if you change the board direction or if your deck is 16 ft long. On a 20 ft run, the remaining 4 ft pieces can sometimes be recovered from repeat offcuts, so the purchase quantity can still be materially lower than a naive two-boards-per-row estimate.

For our specific 20 ft run, the current cut planner estimates 51 × 16 ft boards rather than 68 boards. That difference reflects reusable pieces across rows. Actual results change with saw kerf, board-end trimming, required seam-piece length, and the available joist locations.

When 12 ft boards make sense

Twelve-foot stock is easier to handle for many projects and can be a natural fit for a 12 ft board run. It may also be the only length stocked locally for a selected color or profile. On a 20 ft run, however, each row still needs another piece and creates a butt seam.

The 75-board result above belongs to the specific 16 × 20 ft example. It does not mean a 12 ft board is always wasteful; change the run to 12 ft and the comparison can reverse.

How to compare price without guessing retailer costs

Multiply the actual local price per stock board by the quantity required for that stock length:

For the example quantities, the 20 ft plan costs less than the 16 ft plan when the price of one 20 ft board is less than approximately 1.34 times the price of one 16 ft board: 38 × price₂₀ < 51 × price₁₆.

Against the 12 ft plan, the break-even point is approximately 1.97 times the 12 ft board price: 38 × price₂₀ < 75 × price₁₂. These are comparison thresholds, not retailer prices or guarantees.

The Deck Cost Calculator uses only prices you enter. It can also add whole fastener packs, sales tax, and known project costs without pretending to know current prices in your area.

Seams require more than an attractive pattern

A butt seam has to land on the support arrangement required by the selected product, and both board ends need the prescribed fastening. TimberTech's composite installation guide addresses doubled joists and fastening at butt seams; other products and hidden-fastener systems can use different approved details.

Our cut planner avoids impractically short seam pieces, but that guardrail is a planning assumption rather than a universal code requirement. The calculator does not engineer joists, design blocking, certify structural spans, determine local building code compliance, or verify permits.

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