What is open time?
Open time is the maximum time between spreading an adhesive or mortar on the substrate and placing the material onto it. Within that time the adhesive still bonds fully; after it, a skin forms on the surface and the bond is largely lost.
With tile adhesive this is easiest to see: a thin film forms on the ridges left by the notched trowel. Press a tile into it later and it appears to stick, but there is barely any contact between tile and adhesive. The result is a hollow-sounding tile that comes loose a year later.
Three terms that are often confused
- Open time — the time after spreading the adhesive on the substrate within which the tile or material must be placed. For common tile adhesives this is in the order of 20 to 30 minutes; always follow the manufacturer’s technical data sheet.
- Pot life (working time) — how long the mixed adhesive stays usable in the bucket. It is generally longer than the open time.
- Adjustment time — how long a placed tile can still be moved. It is shorter and only starts once the tile is set.
What shortens open time
- High temperature and low humidity — summer conditions, or underfloor heating already switched on, can easily halve the open time.
- Draught and direct sun — an open door or a work lamp on the surface dries out the adhesive bed.
- Absorbent substrates — aerated concrete and old plasterboard pull the water straight out of the adhesive. Priming solves this.
Practical tips
- Work in small areas. Do not spread more adhesive than you can tile within the open time — in warm weather sometimes no more than half a square metre at a time.
- Do the finger test. Touch the ridge: if adhesive sticks to your finger, it is still open. If it feels dry or skinned, scrape it off and reapply.
- Never spray or wet an adhesive bed to rescue a dried surface; that does not restore the bond.
- Check the transfer by lifting a freshly placed tile now and then. The adhesive should have transferred evenly across the back — full coverage is required externally and for large formats.
Related terms
- Bonding
- Wood glue
- Epoxy adhesive
- Thin-bed mortar
- Primer
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.