The undisturbed subgrade pitches sharply away from the house, encouraging the water to go away from the house, and into the french drain channel. The pea gravel base drains freely and quickly, and also provides a lot of storage area for water interstitially. Pea gravel joints were used in the irregular flagstone area, and for the base material, and around the perforated pipes. Pea gravel at a depth of 6″ nearly eliminates weed growth, and does not subside in rain. Dual 3 inch diameter perforated plastic pipe was used. Since we needed only 50 ft of run and had just enough fall, we doubled it up in the french drain, maximizing the storage capacity, and minimizing cost. Sand jointing was used for the regular bluestone paver area, and the joints were kept to around 3/8″-1/4″ when possible.
Time to go put in another one!