Shiplap Joint

A shiplap joint is the profile formed at the meeting rails between adjacent sections of a sectional garage door. The top edge of the lower section has a raised rabbet, or step, that overlaps the face of the bottom edge of the section above it. When the door is closed, this overlap creates a physical barrier that prevents rain from flowing directly into the joint and that blocks a line-of-sight path for wind and air infiltration.

The shiplap profile works the same way as shiplap siding on a house: each board (or section) overlaps the top of the one below, and gravity plus the overhang prevents water from entering the joint. No additional seal material is strictly required, though many manufacturers add a compressible foam or rubber bead to the meeting rail to improve the air seal.

Shiplap vs. tongue-and-groove:

The two most common section joint profiles are shiplap and tongue-and-groove. Shiplap is simpler and cheaper to roll-form: one meeting rail is flat, the other has a single step. Tongue-and-groove is more elaborate: a projecting tongue on one rail fits into a matching groove on the adjacent rail. The tongue-and-groove profile provides a more positive interlock and a longer infiltration path, giving it a slight edge in air sealing.

In practice, both joint types perform well in residential applications when combined with a perimeter weatherstrip package. The joint type becomes more important on low-insulation or single-skin doors where the section joint is the primary air-sealing line.

Where you see shiplap:

Most steel residential doors from the major manufacturers use a shiplap or modified shiplap profile at the meeting rail. When inspecting a door, you can identify shiplap by running a finger along the section joint from outside: you will feel the stepped overlap rather than a flush surface or an inset groove. A clean shiplap joint with no visible gaps between sections is an indicator that the sections are still properly aligned and that the track has not been knocked out of plumb.