The mechanics
| Piece | Content |
|---|---|
| Measurement of execution | Quantities actually executed in the period, item by item, under the contract's measurement rules |
| Valued statement (relación valorada) | Cumulative quantities valued at contract prices; the period's certificate is the difference between the current cumulative statement and the previous one |
| Certificate | The document the site direction approves and which opens the right to payment, with the agreed retentions and conditions |
Certification is done cumulatively (a origen): each month the total accumulated is measured and what was certified up to the previous month is subtracted. That architecture self-corrects previous periods' errors (a quantity badly measured in March corrects itself when April's cumulative is measured) and is the reason rigorous cumulative measurement is worth more than any later reconciliation.
The golden rule and its two enemies
Really measuring means real quantities under the contract's rules. Its two usual corruptions:
Certifying in twelfths. Spreading the budget across the schedule's months, without measuring. Comfortable for everyone in the short term, disastrous at the end: the gap between certified and executed surfaces all at once in the final account, when no levers or proportionate retentions remain.
Certifying stockpiles as works. Materials on site are not executed units; if the contract admits certifying stockpiles, it does so as its own concept, with its guarantees and its limit, never disguised as executed items.
The usual friction points
Three zones concentrate the discussions and are defused in advance. Partially executed units: the valuation criterion for incomplete work (by item phases, by justified percentage) is agreed before the first certificate. Work that is hard to measure later (what will be concealed): it is measured before covering, with photographic record and both parties' agreement. And new rates and change orders in progress: they are certified only once formally approved; certifying unapproved changes is approving them de facto.
The management use
Beyond payment, the certificate series is the project's best sensor: the cumulative certification curve against the planned curve detects delays before any report, and the comparison by chapter locates where. That use feeds directly into cost control and deviations.
The essentials
Certifying means measuring cumulatively under the contract's rules, valuing at the agreed prices and documenting before covering. The two vices that ruin the instrument (twelfths and disguised stockpiles) are paid for in the final account with interest. And a series of well-made certificates is, for free, the project's most reliable dashboard.
Note: usual practices in Spain in 2026; deadlines, retentions and the stockpile regime are those each contract sets and, in public works, the applicable rules.