On any wall I`ve seen plaster never goes to floor.
About an inch or two or such at bottom left unplastered.
Well below skirting and more than a good half of skirting depth to make a base to rest it against whilst still leaving plenty gap at bottom.
Agree with many of the posts regarding the use of a long SDS masonry drill bit.
I will normally use a 16mm drill bit and drill into the wall at the top of the skirting and gradually change the angle allowing the drill bit to drill down the back of the skirting. This method will not damage the top of the skirting.
I either use a 14mm meter lond SDS bit, or my patented behind skiting board chasing tool (an 18" bit of M10 threaded rod with a bit of a bend in the end)
Solid walls:
Tradition skirting fixing (Victorian & earlier) = timber wedges driven into brickwork perks (the narrow vertical gaps) then skirts nailed with cut clasp nails into the timber wedges ... these wedges approx 1 yard apart. Often additional cut clasp nails were toenailed through the skirt base into the floorboards ... this was done mainly on very tall +12" Victorian skirts. The plaster drops about 1" or so below top of skirting level.
Post Victorian fixing was often timber grounds (thickness equal to that of the plaster) cut clasp nailed directly to the brickwork then the skirts cut clasp nailed onto the grounds.
More modern fixing of skirts tends to be AFTER the plaster has been applied often to or near to the floorboards. The skirts will be masonary nailed or plug & screwed and/or glued with some form of building adhesive.
Stud and plasterboard walls:
Skirt fixing usually oval nailed or screwed to studwork post skimming. Often building adhesive is used.
So clearly there is no hard and fast rule for fixing skirts and plaster depths.
A SDS cranked chasing tool is perfect in older houses where the plaster is generally quite thickly applied and the bricks softer but as other have said a very long masonary drill works well in most situations even if you slightly damage the top edge of your box cut-out when attempting to get a really shallow angle (fix with filler once the boxes are in place). Tip: maybe start with a smaller dia, say a 10mm, which will easily find its way behind the skirt and even through the floorboards then enlarge to suit.
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