What is a hem on metal roofing?
A zoom in Dutch is the outermost edge of something, such as the edge of a forest. In building it mainly refers to the edge of a metal roof covering or sheet, for example of zinc or lead. In English this folded edge is usually called a hem or welted edge. The edge is often folded back so that no sharp cut edge remains and the sheet becomes stiffer.
Use in metal roofing
When working with zinc and lead, the way the edges are finished is important:
- Folded hem: the edge of the sheet is turned back on itself. This stiffens it, removes sharp edges and gives a neat finish.
- Hooking onto a clip or apron strip: at the eaves or verge, the hem often hooks over a continuous fixing strip or clips. This holds the sheet without through-fixings, while the metal can still expand and contract.
- Drip edge: a hem turned downwards lets water drip off freely, so it does not run back under the roofing.
Practical points
- Thermal movement: zinc expands noticeably when warm. Do not clamp a hem tight; let it slide in its fixing strip.
- No standing water: the hem must be formed so water cannot collect in the fold.
- The right tools: fold with a sheet metal folder or hand seamer, not with a hammer on a hard surface. Kinks and cracks in the metal are where leaks start.
- Specialist work: leadwork and zinc roofing require experience. If in doubt, hire a qualified roofer or plumber.
Related terms
- Zoomlat
- Folded zinc
- Sheet lead
- Standing seam clip
- Valley gutter
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.