fillet

A fillet is a narrow strip of wood, typically between 12 and 50 millimeters wide and 6 to 25 millimeters thick, employed to bridge gaps, conceal joints, or provide a finished edge where larger pieces meet. In carpentry, it functions as both a practical spacer and a visual transition element, sitting somewhere between trim and structural member.

The term encompasses several distinct applications. A fillet may be inserted between wall studs and a beam to adjust the bearing surface; placed behind baseboard to compensate for uneven plaster; or used to close the triangular void where a sloped ceiling meets a vertical wall. Quarter-round fillets, shaped in cross-section like a quarter circle, are particularly common for softening interior angles and concealing gaps at skirting level. Fillets are also worked into structural connections, such as between rafters and plates, to distribute load across a wider bearing area.

Material selection varies by intended visibility and duty. Pine and other softwoods are standard for concealed applications, while hardwoods like oak or mahogany may be used where the fillet will be stained and exposed. Pre-moulded fillets, often sold with a decorative profile, are common in finished carpentry; straight-sawn fillets are more economical for rough framing. Width and thickness are typically matched to the existing trim stock and the size of the gap being closed.

Installation is straightforward: the fillet is glued, nailed, or screwed in place depending on whether it must remain removable and the load it will carry. Where a fillet bridges a true structural gap rather than merely concealing one, its effective bearing length on each side of the joint matters. Poorly fitted fillets allow movement and can open gaps again as timber shrinks; tight fitting ensures they remain effective across seasonal changes.

The distinction between a fillet and a batten lies chiefly in dimension and role. A batten is broader and often carries structural or weatherproofing duties, whereas a fillet is narrow, decorative, or gap-filling in primary function. Both terms reflect traditional timber-frame vocabulary in which thin stock is sized by eye and named by purpose rather than by metric classification.