Flat Trim & S4S

Flat Trim and S4S for Interior and Exterior Work

Flat trim is exactly what the name says: trim with a flat profile, used for the wide range of jobs where a clean, squared board is what the work calls for rather than a shaped moulding. That includes window and door casing, flat casing, corner boards, band boards, skirt boards, and building up larger built-up trim assemblies from flat stock. This category carries that flat trim in three materials, solid hardwood, cellular PVC, and urethane, along with arch pieces for curved openings and a custom option for PVC and urethane. Because the material set spans both rot-resistant synthetics and stain-grade hardwood, the same category serves exterior trim exposed to weather and finished interior work alike.

What S4S Means

S4S stands for surfaced four sides, meaning board stock that has been milled smooth on all four faces, ready to use as flat trim. It is the clean, squared trim board used for casing, corner boards, banding, skirt, and any application where flat stock is applied and cut to fit. S4S is one form of flat trim within this category, offered here in both solid hardwood and PVC. Not all flat trim is S4S, and not every flat-trim use calls for it, but where a smooth, four-sided flat board is what the job needs, the S4S lines are the place to look.

Solid Hardwood S4S in 10 Species

The hardwood S4S is solid, stain-grade lumber in FAS grade, the highest grade in the standard hardwood grading scale, selected for clean, largely defect-free faces suited to a stained finish. It is offered in ten species, red oak, white oak, walnut, cherry, maple, poplar, alder, knotty alder, sapele mahogany, and pine, plus Select-grade primed fingerjoint pine for a stable, paint-ready option. This breadth of stocked solid-hardwood flat stock is unusual: most lumberyards carry a limited species selection and special-order the rest at a premium with a long lead time. These ship from inventory, typically arriving in about a week at a far lower cost than a special order, which makes the hardwood S4S a practical way to get stain-grade flat trim in a specific species without the wait.

PVC and Urethane for Exterior and Moisture-Prone Trim

Cellular PVC and urethane are the materials to reach for outdoors and anywhere moisture is a factor. Both resist water and will not rot, warp, or feed insects, so they hold up as exterior corner boards, window and door casing, band boards, and other trim exposed to weather, as well as interior trim in bathrooms, basements, and similar damp spaces. Both take paint well for a clean, uniform painted finish. Because they will not degrade the way wood can in these conditions, PVC and urethane flat trim is often the longer-lasting choice for exterior work, where a wood board would eventually need replacement.

Custom Flat and Arch Trim in PVC and Urethane

Where a job needs a size that isn't standard, the design-your-own flat and arch trim lets you specify PVC and urethane pieces built to order rather than settling for the nearest stock dimension. This is particularly useful on trim work that has to match an unusual reveal, an oversized opening, or an existing profile that standard stock does not cover. The custom option applies to the PVC and urethane lines; the solid hardwood S4S is offered in its stocked profiles and species.

Arch Trim for Curved and Arched Openings

Straight flat trim cannot follow a curve, so arched and radius windows and doors need trim made to the arc. The signature urethane arch moulding and the design-your-own arch trim cover these openings, wrapping the curve cleanly where a straight board would leave gaps or facets. Urethane is well suited to arched trim because it holds a molded curved profile and resists the weather exposure common on the exterior arched windows where these pieces are often used. Pairing arch trim with matching flat trim on the straight runs keeps a whole opening consistent.

Matching Trim Across a Project

Keeping trim consistent across a project usually means matching material, profile, and finish from one run to the next. For a stained interior look, the hardwood S4S in a chosen species carries the natural wood through the whole job. For painted work, or anywhere moisture and weather are a factor, PVC and urethane give a uniform painted finish that holds up. When an existing profile needs to be matched or extended, the custom PVC and urethane option can hit a size that stock trim does not, and it helps to contact us with the existing profile before ordering so the new trim lines up.