The trim piece labeled A. is the fascia. To learn more about designing and building eaves, see “Working Out Eaves, Details.”

All apply. A closed eaves with a level soffit sheds water from the roof beyond the wall plane, and creates a modest shadow line. The plumb fascia offers a better surface for hanging gutter than a “square cut” eaves that runs perpendicular to the slope of the roof. The level soffit often holds a continuous vent strip that can provide more roof ventilation than the usual round vents used in open eaves.

For more on design eaves, see “Where Eaves Meet Rake.”

Flashing using flexible, peel-and-stick membranes is critical to keeping water out of walls. This needs to be done on all windows units, whether in new construction of for replacement windows when the exterior trim is being replaced.

To learn how to efficiently flash and trim windows, see “Window Trim and Flashing That Works.”

If you are selecting a highly rot-resistant wood such as black locust or teak, or using a synthetic material with a long life span, choose fasteners that will last at least as long as that material. Bronze or stainless-steel nails, pins, bolts, or screws are suitable to hold joints together with these materials.

For more on durable trim details, see “Exterior Trim That Lasts.”

Horizontal trim boards, such as the bottom rail of window trim or skirt boards should have a Z-flashing along the top edge, or at the very least be sloped, or canted, so the top edge does not hold water. Even if the trim material is impervious to water, this water can get wicked up into a less durable siding.

For more on durable trim details, see “Exterior Trim That Lasts.”

Poly-ash composite trim does not have the same expansion coefficient as PVC and will not move in any direction very much. However, it still needs to be fastened according to the manufacturer’s instructions (typically every 16 inches placed within 2 inches of the edges).

To learn more about detailing trim to avoid gaps, read “Preventing Gaps in Exterior Trim.”

PVC trim expands and contracts along its length with changes in temperature. An 18-foot-long PVC board can change seasonally by as much as 3/16 inch — easily enough to create the large gaps you describe.

To learn more about detailing trim to avoid gaps, read “Preventing Gaps in Exterior Trim.”

Wood trim expands and contracts across its width with changes in moisture content.

To learn more about detailing trim to avoid gaps, read “Preventing Gaps in Exterior Trim.”

Classical head trim is very much like a classical entablature. The head is assembled with an architrave bead or molding at the bottom, a frieze (what we think of as the head), and then a cap or crown molding wrapping the top. The end of the head is usually plumb with the outside of the casing.

For more on trim design, see “A Look a Traditional Trim Design.”

“Lookouts” are the blocking that project over the gable-end wall, spanning the distance from the last common rafter to the rake subfascia, and running perpendicular to the gable-end wall. Typically, framers will drop the gable-end wall the thickness of the lookouts, and then infill with blocking on top for the gable wall to provide nailing for the roof sheathing.

For more on proving a strong base for exterior trim, see “Framing Eaves and Rakes”

For a classical porch, a good rule of thumb is the entablature height is 2 time the diameter of the columns. Shorter columns of the same diameter would have a shorter entablature height.

For more on design details for porches, see “Columns, Posts, Piers: What's Best for the Porch?”

Inspired by classical architecture, porch columns have a base at the bottom, and long shaft, and a capital at top.

For more on design details for porches, see “Columns, Posts, Piers: What's Best for the Porch?”