An edge purlin (Dutch: kantgording) is the lowest purlin in a roof structure, which at the same time acts as the inner fascia (the finishing edge on the inside of the eaves). In many structures the wall plate fulfils this role. A purlin is a horizontal beam running parallel to the ridge that supports the rafters or roof boarding; the edge purlin is the lowest of these, at the edge of the roof plane.
Function
The edge purlin sits at a critical junction in the roof: this is where different materials and systems come together, such as the roof, the wall, and the floor or floor boarding. This detail therefore calls for careful execution.
Points to note during execution
At the position of the edge purlin, timber comes into contact with other building elements and with air. Two things matter here:
- Airtightness — the sides of the edge purlin that are in contact with air or adjacent elements must be finished sufficiently airtight. Gaps at this point cause draughts, heat loss, and the risk of condensation
- Movement of timber — timber shrinks and swells with changing moisture. By using a sufficiently elastic (flexible) insulation or sealing material, this movement is absorbed and the junctions stay sealed
A well-detailed edge purlin thus contributes to a draught-free and moisture-free eaves.
Related terms
- Purlin
- Wall plate
- Fascia
- Eaves
- Airtight construction
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.