Depends on what your using for the side? 2x6 or 2x8 or 2x10s? IMO if you used 2x8 or bigger you could go about 6 feet I think any farther and they may start to bow on you. 2x6 I would go 4 feet. That's just me though. They may not bow right away but after time and walking on them and standing on them you will start to see it. Kinda like when you setup 2x4s for firewood. If you space your bricks that are under the 2x4s to far apart it seems ok at first but all of a sudden you look one day and your 2x4s all have a big ole dip in them now... Not worth the trouble for 2 more pipes....
RC
