This writeup focusses on blocking for crown molding. The term 'blocking' is a general carpentry term and, as such, may at times apply to load bearing applications.
The interior wooden framework built to support crown molding. It may be used on top of cabinetry, at the join of wall and ceiling, in mantel and valance buildups, and in other applications.
Blocking is usually necessary with stacked crown molding, and always necessary when applying even a single layer of crown molding to frameless cabinets or full overlay framed cabinets if the molding in question does not have a flat bottom side by which it can be attached.
The size and shape of the blocking necessary depends on the intricacy of the crown and on the skill of the installer; it might be a small 1" x 1" (2.54cm x 2.54cm) rail, an elaborate infrastructure made of several pieces, or anything in between. It does not have to be pretty because you'll never see it. Typically, blocking for moldings bears no loads other than the weight of the moldings.
Some finish carpenters include the cost of blocking in their basic price for crown installation. Others may charge extra to account for the additional time required and the cost of the material used for the blocking.
Side view of the top of a wall cabinet, showing the cabinet box, door, crown molding and blocking in cross section
Key:
∏ - blocking
c - body of the cabinet
d - cabinet door
_
|/ \
/ξ | ← crown
∏|/ /
ccccd
cd
cd
cd
cd
cd
Skill and experience are required to install crown as shown above. Even small errors can result in gaps between the crown and the top of the cabinet through which light can be seen. These gaps can be wildly obvious, especially if you are backlighting the crown onto the ceiling. I've had a few contractors tell me they take out some extra insurance by applying color matched caulk on the inside of the framework.
A two piece stacked crown molding is easier in some ways:
_
|/ \
/ξ | ← crown
∏|/ /
===ℑ /
ccccd
cd
cd
cd
cd
cd
Here, the '===ℑ' represents a second piece of molding which is flat and has a decorative front edge. The crown, the blocking, and the flat piece are attached together squarely and then mitered as a whole. It is easier to attach the crown stack to the top of the cabinet due to the flat piece, and since the flat piece is... flat, there is far less chance of gaposis.
This is only the most general overview of blocking for crown molding. If you are not extremely comfortable working with crown molding and using a compound miter saw, I strongly urge you to hire a finish carpenter. If you want to try it yourself, practice first! Solid wood crown molding can cost from $50 to way over $500 US per 8 foot (~2.5 m) length, and once you start stacking it, the price shoots up from there. There is always waste when cutting crown, but at those prices, you will want to minimize it. For practice, buy several long pieces of the cheapest plastic molding you can find – more like $20-$30 apiece – and be prepared to waste it all.
I've had several people tell me that installing complicated crown molding is an art, not a science. One person once said to me, "I could tell you exactly what to do, and you'd still screw it up." I rely on my contractors to tell me whether the crown systems I devise will work as I expect, and I rely on their craftsmanship to ensure that the installation goes smoothly and the result is beautiful.