This ring was built during the summer of 2010 (published summer 2012) so we've had two years of use to work out the kinks. Issues we've run into are:
Water pooling - partially fixed by drilling holes in plywood and lining the holes with epoxy.
Delamination of treated plywood - minimized by addressing the water issues and caulking the seams between sheets.
Broken Corner post - Added angle braces.
Besides these items, the ring is holding up very well getting about 3hrs of work per week by 3-5 people... no heavyweights, though. If you're regularly going to be sparring with folks over 220lbs, you may want to consider 5/8" plywood.
Step 1: Materials List and Plans
Below is the materials list and the basic plans for the ring. Essentially, we're building a 20x20 deck with 4x4 posts every 10ft. The joists and bands break on these posts. The joists sit on ledger on the beams that cross the center of the ring and the band and these beams sit on 2x6 boards that are screwed and nailed to the posts and sit on the concrete footings that the posts are set in. Be sure that the corner posts will be high enough over the deck for the ropes.
5-6 treated 4x4x ~8' (length depends on how high off the ground the parts of the ring will be)
12 treated 2x10x10 (Band, center beam, scabs)
32 treated 2x6x10 (joists)
13 sheets of treated 1/2" 4x8 plywood
10 treated 2x4x10 (Ledger, corner braces, blocks between joists)
200ft of 3/4" twisted yellow polypropylene rope (found 300ft for $87 from Contractor's Rope, IL)
5lb box of galvanized 12d nails
5lb box 1 1/2" deck screws
10 4x1/3" lag screws and washers (to hold board to corner post that band sits on)
4 turn buckles
12 metal rings
~16' of chain
Lag hooks (to attach chain to corner posts)
1gal Thompsons water seal or something similar
Step 2: Layout, Footings, Band
First step is to layout your four corners. These need to be as close to perfect as possible! If you don't get the corners laid out right, you'll be dealing with a disaster later on. TAKE YOUR TIME HERE.
One tip: we set one side of the ring 1/2" lower than the other to facilitate water run off; however, after a couple years of use, the deck remains pretty uneven and pools water in some spots. I would recommend a 1-1.5" drop in addition to the other water control steps noted in the finishing section.
Step 3: Framing and Decking
Make sure you crown your joists and try to keep the tops of the joists and blocks as even as possible. The plywood should be staggered by 4ft; make sure you spray or roll a coat of water seal on the bottom of the decking before you lay it down.
As noted in the final corner picture below, when the ring was first built, it did not have braces on the corner posts. After one corner snapped at the deck level during a sparring match, the braces were added. The final pic also shows the repair of the corner. It was more than a year before the post broke. After several months, the braces seem to have prevented any further breaks. If you're worried about it, you may want to consider using 6x6 in place of 4x4s.
Step 4: Ropes and Finishing
1) Screw the lag hooks into the corner posts at the noted height.
2) One rope at a time, run them through the loops (we used chain patch links and carabeeners) that are attached to the chains and turn buckles.
3) Pull the rope tight and have someone check the distances from the rope to the edge of the deck, adjust chains and turn buckles accordingly. Do this with the turn buckles loose.
4) After adjustments look right, use two hose clamps to tie the ends of the rope together. Tighten the turn buckes and check your work. The bottom rope should have turn buckles on each side of the ring, the other ropes seem fine with just one.
5) When adjustments are good, put foam pipe insulation over the hose clamps and cover in duct tape. Tie small ropes vertically between the three main ropes to keep them together.
After the ropes are up, spray paint your red and blue corners. Then use a pesticide sprayer to coat the decking and band with water seal. This will need to be done once every 8-12mo. If pools of water appear on the decking, drill 5/8" holes and line the holes with epoxy. This won't completely remove the problem but will help.