Polyurethane Corner Blocks: Simplify Your Crown Molding Project

A crown molding project has two challenges that take most of the install time: the miters at the corners, and the alignment of the cornice along the wall. Corner blocks solve the first challenge entirely.

The miters at the outside corners of a crown molding are the most visible joints in the install. They're also the most likely to open up over time, because the building moves seasonally and the miter is the weakest point. A corner block replaces the miter with two butt joints, which are stronger and don't open with seasonal movement.

A typical room has four outside corners, which means four miters. With corner blocks, the four miters become eight butt joints (two per corner), all of which are square cuts. The install time on the corners drops dramatically, and the joints are tighter and more durable.

The corner blocks also serve as the alignment reference for the cornice. Once the blocks are installed plumb and square at the corners, the cornice runs between them, aligned to the top edge of the blocks. The blocks hold the alignment, and the cornice follows.

The most common PU corner block for crown molding is a simple square block, the same thickness as the crown, the same width as the crown's wall leg, and the same height as the crown's drop. The block is invisible as a feature, but it makes the install faster and more reliable.

A decorative corner block is a more traditional option. The block has a small raised panel, a rosette, an acanthus leaf, or a similar classical detail on its face. The block is a feature in its own right, and the design intent is for the block to be seen. Used in traditional and transitional interiors, where the corner detail is part of the room's vocabulary.

For sourcing, corner blocks are usually sold in matched sets with the crown profile. The factory offers a corner block for each crown profile in the catalog, in the same material, the same finish, and the same pre-primed surface. The corner block is the right size for the crown, so the selection is automatic.

A practical note: the corner block can also be used at the inside corners of a crown molding, though this is less common. An inside corner block is typically smaller and is used to terminate the crown cleanly at an inside corner without a cope joint.

A practical install note: the corner blocks should be the first pieces installed in a crown molding project, before any of the cornice goes up. The blocks are plumbed and squared at each corner, then the cornice is measured and cut to fit between them. This sequence makes the install faster and the result more accurate.

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