First of all you will have to decide on a suitable height for the dado rail, it doesn’t want to be either too high or too low, as an example, on a recent project the dado rail was fitted at a height of 37 1/2″ (950mm) to the top edge with a room ceiling height of 116 1/2″ (2450mm). When you have decided on a suitable height check the dado will look alright  from the aesthetics point of view, i.e. where it meets up to your fireplace, window ledges etc.

Mark the chosen height from the floor at approximately 3 ft (1000mm) intervals around the room and in each corner, now check these marks with a spirit level (you can always use a spirit level and a straight length of timber) and mark a pencil line around the room, hopefully the line will be as near as level, if the marks are only say 1/4″  (6mm) out this can be lost across the length of dado as long as it meets up with the next piece of dado at the corner. If the levels are out by a great margin then you can either mark a level line all the way round the room from a set starting point, eg the fireplace, or use a rotary laser level which can be purchased from your local DIY store or hired from the local tool hire shop.

At each corner you will have to cut an internal or external mitre, (see fig 1) this is best done using a mitre box as dado rail comes in assorted profiles that are virtually impossible to mark for a freehand cut. It’s worth putting a small pencil mark on the piece of dado to be cut at the angle the mitre will be as it is very easy to cut a mitre internally instead of externally or vice versa by mistake, if the wall is longer than the lengths of dado available then cut one internal and one external mitre in each length at the joint, (see fig 2) this can be held together with small nails or panel pins with the head left below the surface and finished off with filler later. 

The cuts can be made with a fine bladed wood saw or as I prefer a hacksaw again with a fine blade (22 teeth per inch) which gives a nice clean cut, the cut edges should be lightly sanded and don’t worry if the mitre isn’t perfect, if the dado is being painted it can be filled with an appropriate filler later. 

To fix the dado to the wall you have two choices, either screw and plug it to the wall countersinking the holes so the screw head is below the surface so it can be filled and therefore hidden or use a good quality adhesive such as liquid nails, in which case the wall surface should be clean (no wallpaper). The adhesive is applied to the dado rail with a silicone gun and when it is placed in position knock some panel pins underneath the bottom edge so it doesn’t drop down and out of level. Fill any small gaps in your mitres and along the top edge of the dado rail with decorators caulk using a silicone gun to give a great finish that’s ready to paint.

Tools required :- Tape measure, spirit level or laser level, wood saw, hacksaw, mitre box, hammer, silicone gun.