French Drain pipe under paver patio, how to prevent sagging
I am going to dig two french drain trenches in my back yard. I will build a patio across one of the french drain. I will compact the gravel after I bury the 4" pipe. But I still worry that in the future when I lay the patio the portion directly above the pipe will sag.
If you use a standard sewer & drain pvc pipe or the black corrugated pipe, you may get some flex on the patio if the pipe is really shallow. In that case, you might look into sdr-35 pipe (available perforated), which has a thicker wall and will be less likely to flex. Either way, the gravel around the sides and on top the pipe, if compacted properly, will distribute the load better than non compacted gravel.
This pic shows solid sdr-35 pipe running under a soon-to-be patio. In this case we had to set the pipe very high, just screeding sand over top of the pipe to set pavers. No flex whatsoever in the surface of the patio for over 4 years now.