The interlock of a roof tile is the way in which roof tiles grip into one another, so that wind and rain cannot penetrate beneath the tiles. Each roof tile has special edges, grooves or nibs along its sides and at its top and bottom that fit into the adjacent tile. This interlocking system determines the watertightness of the roof.
Types of interlock
| Interlock | How it works | Example |
|---|---|---|
| **Single interlock** | Simple overlap on the side | Traditional Dutch pan tile, trough tile |
| **Double interlock** | Double groove on the side | Many modern improved Dutch tiles |
| **Triple interlock** | Groove on side, head and foot | High-performance storm-resistant tiles |
| **Head lock** | Extra groove at the top where tiles overlap | Prevents uplift by wind |
Why is the interlock important?
- Watertightness — a good interlock prevents rainwater (including driving rain) from getting beneath the tiles
- Storm resistance — the more interlocks, the better the tile stays in place during high winds
- Roof ventilation — the interlock must seal sufficiently, yet still allow air through for ventilation beneath the tiles
Interlock and roof pitch
The minimum roof pitch depends on the type of interlock:
| Interlock | Minimum roof pitch |
|---|---|
| Single interlock | Approx. 35° |
| Double interlock | Approx. 25-30° |
| Triple interlock | Approx. 15-25° |
At a low roof pitch, a tile with multiple interlocks is necessary to remain watertight.
Inspection and maintenance
- Check annually that tiles still sit properly in their interlocks
- Reposition displaced tiles, replace broken ones
- Watch for moss and debris in the interlock grooves — this can compromise the seal
Related terms
- Roof tile
- Roof pitch
- Roof decking
- Ridge tile
- Tile batten