Determine how to position the deck around the tree. You preserve the most usable deck space with the tree in a corner. The tree serves as a focal point if it grows through the center of the deck, but unless the deck is very large, your only seating options may be a bench around the perimeter of the deck or a bench around the perimeter of the tree.
Choose deck piers with slots to hold the boards in place. For an 8-foot-by-10-foot deck, lay out the piers in three vertical columns of five horizontal rows, working around the tree. Position the columns approximately 4 feet apart, measuring from the centers of the piers. Position the rows up to 2 feet apart, measuring from the centers of the piers. Note that the tree trunk needs approximately 4 inches of space around the entire perimeter to allow for growth.
Framing the Deck
Joists are the support boards that form the foundation of the deck. They should extend approximately 1 foot over the first and last rows of piers. Set a 2-foot-by-6-foot joist in the slots of each row of piers that is not interrupted by the tree, so that the piers are centered under the joists. Then cut a section of each joist that accommodates the tree, so these joists end 6 inches before the tree and continue 6 inches from the other side of the tree. The hole will be rectangular, so cut the joists on each side to the same length. Finish the deck frame by fastening end boards to the outside, cut ends of the joists.
Framing Around the Tree
To frame around the tree, measure the span between the two uncut joists closest to the tree. Cut two beams to span that distance and create the frame. Butt the back face of the first beam against the cut ends of the joists on one side of the tree. Fasten the beam to the cut ends and to the uncut joists on either side using deck screws. Repeat this process with the other beam, working on the other side of the tree, to complete the frame. When you’re finished, the frame will be composed of uncut joists on two sides and beams on two sides.
Laying the Floor Boards
Lay out the floor boards across -- perpendicular to -- the support boards. Begin at one end and work in. Fasten the boards that won't be interrupted by the tree by driving deck screws through the floor board and into the joists and end boards. Measure the needed lengths of the floor boards that terminate at the tree frame. Fasten these boards to the end boards, the joists and to the tree frame with deck screws.
Use a level at each state of building your deck to make sure the piers and the boards are level. In addition, use a square to make sure the corners are 90-degree angles. After construction is finished, protect the deck with a coat or two of waterproofing stain.