Deck Board Calculator

Counting Rows of Decking

Decking is estimated by counting rows, not by dividing square footage. Every row occupies the width of the board plus the gap between it and the next one, and that combined figure is what divides into the deck.

A 5.5 in board with a 3/16 in gap occupies 5.6875 inches. On a deck 12 ft wide, that is 144 divided by 5.6875, which is 25.3, so 26 rows. The last row gets ripped narrower, or the gaps get adjusted very slightly across the deck to land on a full board at the far edge.

Turning rows into boards

Each row needs enough board length to span the deck. If the deck runs 16 ft in the board direction, one 16 ft board covers a row with no butt joint, which is the cleanest result. If the deck runs 20 ft, each row needs a 16 ft board plus a 4 ft piece, and the butt joint must land on a joist. Staggering those joints across the deck rather than lining them up avoids a visible seam.

Add 10 percent for waste on a straightforward rectangular deck and 15 percent where there are angles, a picture frame border, stairs, or curved edges. Buying boards in a length that divides evenly into the deck run saves far more than the waste percentage does.

The first dimension is the width across the boards and the second is the length the boards run. All figures assume 5.5 in boards with a 3/16 in gap and no picture frame border.

Actual Board Widths

Nominal sizes are not real sizes, and using the nominal number in the row calculation throws the count out badly.

• 5/4 x 6 pressure treated: 5.5 in actual, 1 in thick. The most common residential decking.
• 2 x 6 pressure treated: 5.5 in actual, 1.5 in thick. Used where joists are spaced wider or a stiffer feel is wanted.
• 2 x 4 pressure treated: 3.5 in actual. Needs far more rows and far more fasteners for the same area.
• Composite and PVC: most run 5.5 in, but some lines are 5.25 in or 5.4 in. Check the exact figure on the product data sheet, because a quarter inch error over 30 rows moves the edge by more than half a board.
• Hardwood such as ipe: commonly 5.25 in or 3.5 in, and often sold by the linear foot rather than by the piece.

Gap width

A 3/16 in gap is the standard for composite and for kiln dried or seasoned lumber. Wet pressure treated lumber straight off the rack is a different case: it shrinks across its width as it dries, so it goes down with a much tighter gap, often just the thickness of an 8d nail, and opens up to roughly the right spacing over the first season. Gapping wet lumber at 3/16 in leaves gaps near 3/8 in a year later, wide enough to lose keys through.

Joist Spacing

Joist spacing is set by the decking, the span and the code, and it is the one decision that cannot be corrected later.

• 16 in on center for decking laid perpendicular to the joists. This is the residential standard and it suits nearly all treated lumber and most composite decking.
• 12 in on center for decking laid at a 45 degree diagonal. A diagonal board crosses the joists at an angle, so its unsupported span between bearing points is about 1.4 times the joist spacing. At 16 in centers, a diagonal board effectively spans 22.6 in, which most composite decking is not rated for.
• 12 in on center is also required by many composite manufacturers for stair treads regardless of direction, and some lightweight composites require it for all applications.

Going to 12 in on center from 16 in adds roughly one third more joists, more hangers and more labor. Decide the board direction before framing, not after. Joist span itself, meaning how far a joist can run between beams, is governed by the species, the grade, the size and local code, and has to be checked against the local span table rather than assumed.

Fasteners

Face screwing

Face screwing puts two screws through the top of every board at every joist. Plan on about 350 screws per 100 square feet of deck, so a 12 ft by 16 ft deck at 192 square feet needs roughly 672 screws, which is two 350 count boxes. Use screws rated for the treatment chemistry in modern pressure treated lumber, which means stainless or a coated deck screw specifically listed for ACQ or MCA lumber. Plain zinc screws corrode out of treated lumber in a few years.

Hidden fasteners

Hidden fastener systems clip into a groove milled in the board edge, or screw at an angle through the edge, leaving no visible head on the surface. They cost more per square foot and take longer to install, and they set the gap automatically at a fixed width, which is a real advantage for consistency. Grooved boards are required for most clip systems, so the board order has to specify grooved edge for the field and square edge for the border and the outermost row.

Hidden fasteners also complicate a repair. Pulling one damaged board out of the middle of a clipped deck means releasing its neighbors, whereas a face screwed board comes out in two minutes.

Picture Framing

A picture frame border runs one or two boards around the perimeter, mitred at the corners, with the field boards butting into it. It hides the cut ends of the field boards and finishes the edge cleanly.

It also needs extra framing. The field boards now end mid span rather than over the rim joist, so a doubled joist or an added ladder frame is required underneath every point where a field board dies into the border. That framing has to be planned during construction, since retrofitting it means working from below. Budget one to two extra boards per side for the border, plus the extra framing lumber, and expect the mitres to consume material.

Blocking

Blocking is short pieces of joist material installed between joists. It has three jobs: it stops long joists twisting under load, it stiffens the deck against side to side sway, and it gives a bearing edge where boards change direction or where a picture frame border lands.

Install a row of blocking at mid span on any joist longer than about 8 feet, and at every point where decking butt joints occur if the joints are not staggered. Many jurisdictions require blocking or bridging at specific intervals, so check the local requirement.

Ledger Flashing

The ledger is the board bolted to the house that carries one edge of the deck, and a failed ledger connection is the most common cause of a deck collapse. Two things matter and both are code issues.

First, the fastening. Ledgers are attached with through bolts or approved structural screws in a specified pattern, never with lag screws driven at random and never with nails. The pattern depends on the joist span and the local code.

Second, the flashing. Water running down the house wall must be directed out over the face of the ledger rather than into the wall behind it. That means a metal or flexible flashing tucked under the house wrap or siding above the ledger, lapped down over the ledger face, with the siding cut back to leave a drainage gap. Skipping this rots the rim joist inside the house, which is invisible until the deck moves. Use flashing material compatible with the fasteners, since aluminum flashing corrodes in contact with treated lumber and copper based preservatives.