Yes, we recommend that walls greater than 3′ (1m) or with a heavy load be engineered by a qualfied civil engineer (some municipalities require even lower heights, such as 2′ (600 mm), to be designed by an engineer). Fencing and railing may also be required. Always check with local building codes.
If your concrete is in excellent condition (no cracking or deterioration), then yes you can install pavers over concrete. The existing concrete pad must slope away (min 1” of fall for 8 feet of run) from any structures to ensure drainage. Also, you must create a drainage layer (1” of bedding sand) below the pavers. For more information read our blog titled “Installing Pavers on Concrete.”
A “Paver Saver” mat (a protective pad bolted to the bottom of the tamper) between the stone and the tamper will protect the surface of your pavers.
The order in which you apply sand and sealer definitely matters. You must apply the sand first, sweeping carefully into the joints, ensuring none is left on the surface. Once the sealer is applied, it will trap anything that is left on the surface of the pavers. Make sure the joint sand is dry before sealing.
The base material we recommend is 3/4″ crusher run gravel (6″ thick for a patio and/or walkway, 10-12″ thick for a driveway). On top of this compacted gravel a 1” bedding course of coarse sand is placed prior to laying the pavers.