CIBSE TM31 Explained: The Building Log Book
The building log book is required for new and refurbished buildings, routinely produced to discharge a Building Regulations obligation, and then almost never opened again. This guide explains what TM31 actually asks for, and why the log book is a genuinely different document from the O&M manual it is often confused with.
In short
CIBSE TM31 is the Building Log Book Toolkit — guidance setting out the scope, structure, and contents of a building log book. The log book is a concise summary of how a building is designed to operate and perform, intended for whoever manages it day to day, and supports the Building Regulations requirement to provide building operating information.
What a log book is for
The log book exists because the O&M manual is too large to answer a simple question. Someone taking over a building needs to understand, quickly, what the systems are, how they are meant to be operated, what the design assumptions were, and what the building should be consuming. That is a summary document measured in tens of pages, not thousands.
TM31 sets out that structure and provides templates, including a means of recording energy use over time. Recording consumption alongside the design intent is what makes the log book useful beyond handover: it turns a static description into a baseline against which drift can be spotted. A building consuming materially more than its design figure has a problem, and without a recorded baseline nobody can see it.
- A concise summary aimed at the building manager, not the maintenance engineer
- Covers design intent, how systems operate, and expected performance
- Includes a means of recording actual energy use against that intent
- Measured in tens of pages, against an O&M manual's thousands
What it should contain
A TM31-based log book covers the building and its construction in summary, the mechanical and electrical systems and how they are meant to be controlled, the design conditions and occupancy assumptions the systems were sized for, and the metering strategy with a record of consumption. It should also record changes: alterations to plant, controls, or occupancy that move the building away from its original basis.
That last point is the one most often skipped and the one that decides whether the log book stays useful. A log book describing a building as it was at handover in 2018, in a building substantially altered since, is worse than none — it gives confident answers that are quietly wrong. Keeping it current is a small ongoing task that depends on whoever makes a change knowing the document exists.
- Summary of the building, its construction, and its services
- Control strategies and how systems are intended to be operated
- Design conditions and occupancy assumptions
- Metering strategy and a running record of energy consumption
- A change log covering alterations to plant, controls, or use
Log book, O&M manual, and health and safety file
The log book draws on the O&M manual and the health and safety file but is distinct from both. The O&M manual is the detailed technical reference for maintaining and repairing systems. The health and safety file, required under CDM, carries the information needed to carry out future work safely. The log book is the operational summary: what this building is, how it should run, and how it is actually performing.
For an incoming FM team the log book is the fastest route into a building — which is exactly why its absence is felt most at mobilisation, when it would have saved weeks. In practice the three documents are handed over together and then separate: the O&M pack goes to a server, the H&S file to a compliance folder, the log book to a shelf. Keeping them retrievable together is what preserves the relationship between them.
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How this applies by sector
Further reading
Sources
The regulatory positions on this page come from the following primary sources. Where guidance and legislation differ, the legislation governs — always confirm the requirements that apply to your own site and jurisdiction.
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