Built-up cornice incorporating pelmet and hidden picture hanging system
1. Cornice showing hanging channel
A 185mm x 18mm FJ pine trim board will form the baseboard on three walls and the pelmet on the fourth window wall. The bottom of the baseboard will be 203mm (185mm + 18mm) below the ceiling and it will be attached using masonry anchors long enough to go through the board and render and anchor to the stone in the wall. The baseboards will be rebated to fit an extruded aluminium sail track profile that will be attached to the baseboard from behind prior to installation. This will form the rail for the hidden picture hanging system. A standard cornice moulding will then be installed as shown. Finally, to add to the overall profile and to hide the masonry anchor points, a strip of panel moulding will be brad nailed without glue to the front of the baseboard below the cornice moulding as shown.
Removal (in theory)
The panel moulding strip can be pryed off to expose the masonry anchor points. The cornice is then removed using standard techniques to expose additional anchor points. The removable type anchors can then be withdrawn and the baseboards detached from the wall.
The stone is basalt "bluestone". Previous attempts to drill into it using a normal masonry bit and hammer drill failed. A wet diamond method succeeded but is very slow and potentially messy, although the mess could be managed. An SDS+ hammer drill and bit performed best of all and was very quick in test.
I want the anchors themselves to be removable, otherwise the walls could be damaged excessively if removal is ever required.
My DIY hidden hanging system
Commercial gallery hanging systems provide a wall or ceiling mountable aluminium channel from which hanging wires can be suspended. I needed a channel that could be attached to the cornice or baseboard itself rather than the wall simply to minimise drilling into the wall. There are commercial systems that would allow this, including one hidden system. The most expensive components are the hangers themselves. The wire, either steel or nylon, is less expensive to buy in bulk. The quick adjust hooks are available separately so the only tricky component is the attachment piece at the top of the wire that goes in the channel. Commercial hanger wires should work. A barrel clamp can also be used provided it fits into the channel (8mm diameter or less).
My DIY barrel clamp design is shown in Image 2 - the wire wraps around the middle and the nut is tightened to clamp it. The two larger diameter cylinders that form the clamp are 8mm dia wood dowel 10mm long with a hole drilled through the centre. Alternatively, I've found a size of pop rivet that fits perfectly into the sail track. By simply tapping out the steel rod, the hanging wire can be threaded through the hole and the pop rivet shaft crimped onto the wire. The rivet head goes into the channel and the shaft clamping the wire drops through the slot.
2. DIY hanger wire clamp
Overall with channel, drop wires and adjustable hooks, my DIY system should cost about half that of the least expensive commercial system I've found. It is about a third the cost of the only commercial hidden channel system I've found.
3. Cornice showing pelmet detail
Note that the baseboards don't extend all the way to the ceiling and the pelmet top plate doesn't go all the way to the wall. This is because the render is not a clean right angle in the corners and the boards would not sit flat if they did extend right into the corners. It also means I can get the 200mm height for the built-up cornice and pelmet using 185mm wide boards.
The drawings show a standard 90mm cove profile. The installed profile is Boral's "Sydney" profile, supplied as machine extruded paper-faced plaster cornice in 4.2m lengths.