Width measurement for a sliding gate:
Measure the width between the gate posts of your driveway at the top and at the bottom, take the smaller measurement of the two (mm) and this will be the available clearance width (mm).
If using the Guide Block or using the Guide Rollers mounted in upside down position with "L" Bracket to fit in to the angle channel welded to the top inside of a sliding gate or using the Guide Rollers with side mount brackets then one more post as a guiding post has to be installed inside the property to support the sliding gate. Take the available clearance width (mm) + guide post width (mm) + 400 mm (overhung required to install the sliding gate motor) = Final gate width (mm).
If using the Guide Rollers with "L" Bracket and no angle channel is being used then existing fence post can be used as a guiding post to mount the guide rollers. Take the available clearance width (mm) + 400 mm (overhung required to install the sliding gate motor) = Final gate width (mm).
Height measurement for a sliding gate:
First of all please consider the following points.
Your preferred sliding gate height.
Your existing fence height.
Council regulations on front fence or front gate height or if a permit is required.
Measure the height from your driveway up to the required ideal height of a sliding gate both at the closed gate position and open gate position. Take the smaller measurement of the two (mm) - 40 mm (need to take off this 40 mm considering the height raised by 80 mm diameter internal/ recessed gate wheels & the bottom gate track) = Final gate height (mm).