A site diary is the daily record of what actually happened on a construction site: weather, labour and plant on site, work done, deliveries, delays and their causes, instructions received, visitors and health and safety events.
Its value is evidential. Because entries are made on the day, a site diary is contemporaneous evidence, which tribunals and adjudicators give far more weight than accounts reconstructed later from memory. Extensions of time, loss and expense claims and defect disputes are routinely decided by whose daily records are more complete and credible.
A good diary is specific, dated, attributable to its author and supported by photographs. The most common failure is not poor writing but non-existence: diaries kept thinly, in arrears or not at all, because they depend on the busiest person on site finding time at the end of the day.