The intact stability criteria for timber deck cargo are IMO standards for ships loaded to their timber load lines, set in the 2008 IS Code. They replace parts of the general criteria because the deck stow adds buoyancy.
This guide is written for deck officers and cadets on timber and log carriers. It covers the criteria, what the stability calculation must include, the timber load line, and how deck cargo is stowed, lashed and inspected.
What Are the Intact Stability Criteria for Timber Deck Cargo?
IS Code Part A, section 3.3 requires 0.08 metre-radians of GZ area up to 40° and a maximum GZ of 0.25 m. GM must stay at 0.10 m or more all voyage.
| Criterion | Timber deck cargo (3.3.2) | General cargo ship (2.2) |
|---|---|---|
| Area under GZ curve | ≥ 0.08 m-rad up to 40° or angle of flooding | ≥ 0.055 m-rad to 30°; ≥ 0.09 m-rad to 40°; ≥ 0.03 m-rad between 30° and 40° |
| Maximum righting lever (GZ) | ≥ 0.25 m | ≥ 0.20 m at an angle of 30° or more |
| Initial metacentric height (GM) | ≥ 0.10 m at all times, after corrections | ≥ 0.15 m |
| Weather criterion | 16° steady-wind heel limit applies; 80% of deck-edge immersion may be ignored | Both limits apply |
The criteria apply to cargo ships of 24 m in length and over. The IS Code is mandatory through SOLAS Chapter II-1 and the 1988 Load Lines Protocol.
The GM limit is corrected for the free surface effect of liquids in tanks. It also allows for water absorbed by the deck cargo and any ice accretion on exposed surfaces.
Why Do Timber Ships Have Special Stability Criteria?
A tight, well-secured timber deck stow adds buoyancy and protects the deck, so ships carrying it may load deeper to a timber load line. In return, the criteria demand more GZ area and a higher maximum GZ.
When the GZ curve is calculated, the volume of the deck cargo may be counted as buoyancy, usually with a permeability of 25%. This extends the GZ curve to larger angles, which is why a lower GM of 0.10 m is accepted.
The benefit only holds if the stow stays in place. If the deck cargo shifts or is lost, both the extra buoyancy and the reduced freeboard work against the ship.
What Must the Timber Stability Calculation Include?
The calculation must show the criteria are met throughout the voyage, not just at departure. It allows for water absorption, ice, consumption of fuel and water, and free surface in tanks.
- Water absorption: timber, especially logs and unpackaged lumber, gains weight from rain and sea spray high on deck.
- Ice accretion: in winter voyages, ice on the stow and lashings adds weight high above the centre of gravity.
- Consumables: fuel and water burned from double-bottom tanks remove weight low down, raising G.
- Free surface: slack tanks reduce the effective GM.
- Trapped water: water held in gaps within a log stow adds weight on deck.
- Arrival condition: the worst case is usually arrival, with tanks run down and the cargo at its heaviest.
If these are ignored, the centre of gravity rises during the voyage. A timber ship can then develop an angle of loll, a classic sign that GM has become negative.
Why Should GM Not Be Too High on a Timber Ship?
IMO guidance recommends that the metacentric height of a ship with timber deck cargo should preferably not exceed 3% of her breadth. A higher GM makes the ship stiff, with violent, quick rolling.
Quick rolling causes large accelerations on the deck stow. They create racking stresses in the hull and high loads on the lashings, which can fail and let the cargo shift or go overboard.
The officer therefore aims for a moderate GM. It must be at least 0.10 m at the worst point of the voyage, but not so high that the ship rolls hard.
What Is a Timber Load Line?
A timber load line is a separate set of load line marks that lets a ship carrying a qualifying timber deck cargo load deeper. The marks sit aft of the Plimsoll line and are prefixed with “L” for lumber.
| Mark | Meaning |
|---|---|
| LS | Timber summer |
| LW | Timber winter |
| LWNA | Timber winter North Atlantic |
| LT | Timber tropical |
| LF | Timber fresh water |
| LTF | Timber tropical fresh water |
The ship may only use the timber marks when the deck cargo meets the stowage and securing rules in the Load Line Convention. Otherwise, she must load to her ordinary marks.
What Is the Timber Deck Cargo Code?
The Code of Safe Practice for Ships Carrying Timber Deck Cargoes is IMO’s guidance on stowing, securing and carrying timber on deck. The current version is the 2011 TDC Code, adopted by Resolution A.1048(27).
IMO first issued the code in 1972, amended it in 1978 and revised it in 1991 under Resolution A.715(17). The 2011 edition updated it for modern ships and lashing equipment.
It covers safe operational practice, stowage and securing methods, and design principles for securing systems. Shipowners use it to prepare the timber section of the ship’s cargo securing manual.
How Is Timber Deck Cargo Stowed?
The deck cargo is stowed as solidly as possible over the full length of the well between superstructures. It reaches as close to the sides as obstructions allow, and its height is limited in winter.
- Length: at least the entire available length of the well or wells between superstructures. With no superstructure aft, at least to the after end of the aftermost hatch.
- Breadth: as close as possible to the ship’s sides, with allowance for obstructions such as guard rails and bulwark stays.
- Height: at least the standard height of a superstructure, other than a raised quarterdeck, to qualify for timber freeboard.
- Winter limit: in a seasonal winter zone in winter, no higher than one-third of the ship’s extreme breadth above the weather deck.
- Logs: stowed compactly fore and aft, with any voids filled by logs stowed athwartships.
- Bridge view: the stow must not obstruct the view from the bridge or the ship’s safe navigation and work.
Openings in the weather deck must be closed and secured. Vents, air pipes, sounding pipes, fire hydrants and valve controls under or near the stow must stay protected, clearly marked and reachable.
How Is Timber Deck Cargo Lashed?
Timber deck cargo is held by uprights at the sides and by independent top-over lashings tightened across the stow. Each lashing has a tightening device and a quick-release hook so the cargo can be adjusted or jettisoned.
- Uprights: fitted where the nature or height of the cargo requires, set in deck sockets or supported by strong bulwarks. Spacing not more than 3 m; they may be omitted for packaged timber only.
- Hog wires: transverse wires at about three-quarters of the stow height, joining port and starboard uprights through pad eyes.
- Top-over lashings: independent lashings over the whole stow, spaced not more than 3 m apart, with lashings close to each end of each continuous stow.
- Lashing material: close-link chain of at least 19 mm, or wire rope of equal strength, with no permanent deformation.
- Tightening devices: a turnbuckle or similar on every lashing, with at least half its tightening capacity left after initial tightening.
- Slip hooks: pelican or slip hooks allow quick adjustment and emergency jettisoning; pelican hooks are moused.
Lashings slacken as the stow settles and the ship rolls. They are checked and re-tightened regularly during the voyage, and every check is logged.
What Safe Access Is Required Over Timber Deck Cargo?
Where there is no safe passage on or below deck, a walkway at least 600 mm wide is fitted on the cargo. Guard lines, no more than 330 mm apart, rise at least 1 m above it.
- Walkway at or near the centreline, fastened securely to the top of the stow.
- Guard lines or rails on each side, supported by stanchions not more than 3 m apart and tightened with tightening devices.
- A wire lifeline along the centreline, supported by stanchions fixed to the cargo.
- Properly built ladders, steps or ramps with guard lines or handrails for access onto the stow.
What Do Inspectors Check on Timber Deck Cargo Ships?
Port state control inspectors and class surveyors check the stability calculation against the timber criteria. They also check that the cargo is secured to the approved cargo securing manual, and inspect lashings, walkways and access.
- An approved cargo securing manual with a timber section, as required by SOLAS Chapter VI, Regulation 5.
- Stability calculation for departure and arrival, including water absorption, ice and free surface.
- GM at or above 0.10 m at the worst point, and timber load line marks not submerged.
- Lashings, uprights, hooks and tightening devices in good condition, with certificates for the lashing equipment.
- Records of lashing checks and re-tightening during the voyage.
- Walkways, guard lines and lifelines rigged and tight; ladders safe.
- Sounding pipes, vents, hydrants and valve controls marked and reachable.
Common findings are stability calculations that ignore water absorption, slack or missing lashings, and missing guard lines. A ship with dangerously low GM or an unsecured deck cargo can be detained.
Related Topics
Frequently Asked Questions
What are the intact stability criteria for timber deck cargo?
Under IS Code Part A 3.3, GZ area must be at least 0.08 m-rad up to 40° and maximum GZ at least 0.25 m. GM must stay at 0.10 m or more throughout the voyage.
Why is a lower GM allowed for timber ships?
The deck cargo volume may be counted as buoyancy in the GZ curve, which extends the range of stability. Higher GZ area and maximum GZ are required in return.
What is the maximum recommended GM for a timber ship?
IMO guidance recommends GM should preferably not exceed 3% of the ship’s breadth. A higher GM causes violent rolling and overloads the lashings.
What is the timber deck cargo code?
It is the IMO Code of Safe Practice for Ships Carrying Timber Deck Cargoes. The current version is the 2011 TDC Code, Resolution A.1048(27).
How high can timber deck cargo be stowed?
In a seasonal winter zone in winter, no higher than one-third of the extreme breadth above the weather deck. It must never obstruct the view from the bridge.
How far apart are timber deck cargo lashings?
Independent top-over lashings are spaced not more than 3 m apart, with lashings close to each end of each continuous stow.
Why does a timber ship develop a list during the voyage?
Water absorption, ice and the use of low fuel tanks raise the centre of gravity. If GM falls to zero or below, the ship lolls to one side.
What is a timber load line?
A set of load line marks, prefixed “L”, that lets a ship with a qualifying timber deck cargo load deeper. They are placed aft of the ordinary load line marks.
- Great Lakes Maritime Academy: Programs, Cost and Admission – October 14, 2026
- Bunker Delivery Note (BDN): Contents, MARPOL Rules and Checks – October 11, 2026
- Oil Record Book: Part I and Part II Entries, Rules and PSC Checks – October 11, 2026
