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."
Unilock Authorized Contractors are required, by Unilock, to offer a minimum 2 year installation guarantee. Having said that, many Authorized Contractors offer guarantees of 5 or more years. We would highly recommend choosing a Unilock Authorized Installer. Their extensive product and installation knowledge will help you get the best design imaginable and a quality installation.
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.
We recommend using a regular jointing sand in combination with a joint sand stabilizing sealer (a one step sealer and joint sand stabilizer). Please note; we recommend waiting a minimum of 3 months after installation before applying a sealer, to allow for the natural efflorescence to dissipate. Before sealing you must clean the surface and wait until it is 100% dry.
The final part of the installation process is to sweep in sand between the joints. Depending on what type of paver you have installed you could also use a Polymeric Jointing Sand which hardens when wet and prevents weed growth, however this is not recommended for our heavily textured stones (such as Series 3000 and Il Campo).