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How do I Build a 12 X 15 Deck?

Building a new deck can add an attractive area for barbecues and family gatherings or just a nice place to relax. A deck can be built as a weekend project by following the traditional method used by home builders that ensures a dependable support structure and works well over flat or moderately sloped terrain. The procedure for a 12 foot by 15 foot deck is typical to all wood deck construction and can be adapted to a deck of almost any size.

Layout & Foundation

Step 1

Determine the desired deck platform elevation, typically between 2 inches and 8 inches, or one step, below the house's finished floor elevation, and mark the height on the exterior of the house to use during the construction of the deck as a reference, referred to as a "benchmark." Use a level to extend the benchmark line across the full width of the deck location.

Step 2

Define the location of both sides of the deck, then mark both points on the reference line marked in Step 1. Tack a small finish nail into the intersection of the marks on both sides where the deck will be.

Step 3

Insert a wood stake 24 inches from the house wall and approximately 24 inches beyond the proposed deck side using the small sledgehammer.

Step 4

Repeat Step 3 to insert a stake approximately 24 inches beyond the opposite side of the deck, then tie a piece of nylon string between the two stakes to establish a parallel line 24 inches from the house wall. This line defines the first row of pier blocks for the deck support structure.

Step 5

Repeat Steps 3 and 4 to establish parallel pier block lines at 6 feet and 10 feet from the house wall.

Step 6

Tie a piece of nylon string to one of the finish nails tacked into the house wall in Step 2 to define the side of the deck, then extend the string to a stake positioned perpendicular to the house and approximately 24 inches beyond the front edge of the deck (about 14 feet from the side of the house).

Step 7

Repeat Step 6 for the opposite side of the deck. At this point, you have string lines defining three rows of pure blocks parallel to the house and both side edges of the deck.

Step 8

Lay out the pier block placements along each of the parallel lines set in Steps 3, 4 and 5 using chalk marking powder as follows: Mark one pier block location at 18 inches inside both sides of the deck side edge line and one each at the center point between the two sides. This will place three piers in each row, spaced equally apart and 1 1/2 feet from the end of each row for a total of 9 pier blocks.

Step 9

Dig a footing hole for each pier block, centered at each location chalk marked in Step 8, making each footing hole approximately 12 inches by 12 inches by 6 inches deep using a shovel. Use a builder's transit to establish the required depth of the footing holes relative to the benchmark established in Step 1. Typically, the footing depth must be the pier block height plus approximately 18 inches.

Step 10

Mix concrete mix following the manufacturer's directions in a wheelbarrow, using approximately 1/2 bag for each pier block footing.

Step 11

Place the concrete mixture into each pier footing hole using a shovel, then set each pier into the wet concrete and level the pier block's top surface using a torpedo level. Make certain the center point of each pier's top wood block is centered beneath the string line set in Steps 3, 4 and 5, as well as ensuring that the top of each pier block is no less than 16 inches below the finished deck surface elevation. Note: Using the builder's transit level to set the pier blocks expedites proper placement.

Step 12

Allow the concrete mix in the pier block footings to cure a minimum of 12 to 24 hours before continuing with the subsequent deck framing.

Deck Framing

Step 1

Attach a galvanized 4 by 4 post base bracket, centered on the top block of each pier block, with bracket nails provided. Level down from the pier block row strings set in Steps 4 and 5 of Section 1 to center the brackets in line with the string lines.

Step 2

Position the builder's transit level and tripod, roughly centered just beyond the deck's perimeter, so as to have a clear view of the benchmark and all the pier blocks.

Step 3

Calculate each 4 by 4 pier post length needed by placing the end of a tape measure on the benchmark set in Step 1, then extend the tape vertically and record the measurement on the tape viewed at the horizontal cross hair of the transit level. Next, hold the tape measure vertically from the top surface of each pier block and take the transit reading on the tape, then subtract the combined total of the girder height, joist height and decking thickness (in this case: 13 inches), and the remaining dimension is the post length for that pier block. Note the dimension determined in pencil or crayon on each pier block.

Step 4

Repeat Step 3 for each pier block, then cut a 4 by 4 post block for each pier post dimension determined and install the block in the post base bracket with the bracket nails provided.

Step 5

Position two 4 by 6 girder beams, end to end with the 6 inch dimension vertical, on top of the pier posts installed in Step 4, with the ends together on the middle pier post. This will cantilever the outside ends beyond the last pier block and the side edge string lines on each end.

Step 6

Attach the girder beams to the 4 by 4 pier posts with 8d box nails driven diagonally through each side of the girders into the top of each pier post.

Step 7

Verify that the side edge string lines set in Step 6 are square to the house using a framing square. Adjust the outer stakes as necessary to adjust the string line until it's square.

Step 8

Determine the girder end cuts by leveling down from the side edge string lines to the top of the girders using a level, then move the mark away from the end of the girder 1 1/2 inches to define the girder end cut line. Use a combination square or framing square to extend the cut mark across the top and down both sides of the girder. Repeat at all the girder ends on both sides of the deck.

Step 9

Cut the girders at the end cut lines using a circular saw.

Step 10

Define the horizontal house wall ledger position by marking the point on the house wall level with the top surface of the girders at both ends using a level, then connecting the two points on the house wall using a chalk line.

Step 11

Position a 2 by 6 board horizontally against the house wall, aligning the bottom edge on the ledger line chalked in Step 10, then secure the ledger to the house's exterior wall. For homes with wood siding and framing, use two 16d galvanized box nails, vertically spaced at 16 inches apart, into the wall. For homes with exterior stucco or a concrete foundation wall at the deck level, use wall anchors with 1/2 inch by 4 inch bolts at 24 inch spacing to secure the ledger board.

Step 12

Lay out for deck joist spacing on the outermost girder. Measure from one end of the girder and make a mark at 15 1/4 inches, then in each subsequent joist put layout mark at 16 inch increments. Use a hand square to extend each layout mark across the top of the girder.

Step 13

Transfer the same layout marked in Step 12 to the top surface of the girder closest to the house, then use a chalk line from the two laid out girders to mark the layout on the girder row between.

Step 14

Install a 2 by 6 by 12 foot board on edge at each of the layout lines marked in Steps 12 and 13, plus one at the ends of the girders on both sides of the deck. Keep all of the joists to the same side of the layout lines and secure each joist with a 16d box nail diagonally through the side of the joist into the girder. Make certain the joists are the same length, that the joist ends are square and that they are butted tightly against the ledger board installed in Step 11.

Step 15

Secure the ends of the deck joists to the wall ledger with one 2 by 6 joist hanger at each joist using joist hanger nails.

Step 16

Install a 2 by 6 rim joist to the outer end of the deck joists (parallel to the wall ledger) and attach with two 16d galvanized box nails vertically spaced at the end of each joist. Check to make sure you maintain the layout spacing to keep the joists straight.

Step 17

Install the 2 by 6 wood or composite decking boards, starting at the outer edge of the deck and perpendicular to the deck joists. Align the first row of deck boards flush on both ends and with the rim joist, then secure with two 2 1/2 inch deck screws at each joist. Note: Position any end joints necessary centered over a joist for support.

Step 18

Repeat Step 1 for all the remaining deck boards to complete the deck. It is generally recommended that a consistent minimum space of 1/4 inch be maintained between the deck boards. Use a screwdriver shaft, flat carpenter's pencil or 1/4-inch plywood block as a spacer during installation.

Warning

  • National, state and local building codes contain specific requirements for exterior deck construction, minimum beam sizes and the spacing of support piers. Consult your local building department to ensure proper permitting and code compliance before undertaking this project.

About the Author

Paul Massey has been writing since 2009, drawing on a 35-year career in the construction industry. His experience includes 15 years as a general building contractor specializing in architectural design, custom homes, commercial development and historic renovations.