Steam Bending Wood: How To Bend Solid Wood

By Simon Smith

Although hailed as a building material for the future in light of its sustainability and air-purifying properties, timber architecture typically features straight beams and panels. Unlike concrete, whose moulds can espouse any curve imaginable, timber requires a specific process to curve and hold a shape.

Trees generally grow straight due to light tropic responses. They are in the main used for straight work too; however, there are end-use applications that rely on timber in its solid form to bend.

This article will go into more depth surrounding the timber steam bending process and how this technique is used in both small-scale and architectural construction projects.

timber steam bending beams

What is Steam Bending?

Seen in the way that trees bend and sway in the wind, wood has its own elasticity, giving it the ability to adopt a new shape momentarily before returning to its original state once the pressure ceases.

Steam bending is the process of soaking a piece of wood in hot water at boiling point for a certain period of time in a steam box, softening the fibres to make it pliable and stretchier. Once the timber cools down and the fibres dry once more, it will retain its new shape.

Steam bending is an ancient and traditional craft with a complex history across many cultures of the world, it was known by Ancient Egyptians and is used even today in wooden boat building, agricultural implements, tool handles, and of course furniture. 

In the past, steam bending was particularly crucial to the production of weapons, ships and musical instruments as well as architectural beams.

Which Woods Are Best For Steam Bending?

Some types of timber bend more easily than others. Timbers such as Beech, Ash and Oak are typically used for steam bending. Shakes and knots make the grain deviate around the knot and in effect cause an abnormal slope of grain. Meanwhile, pitch pockets and other `defects’ will affect the performance of the timber.

Usually, ring-porous and diffuse-porous timbers (when pores are distributed fairly evenly across the earlywood and latewood) are more able to bend than others. Saplings, for example, contain a higher proportion of cellulose, which allows them to bend naturally in windy conditions without snapping. The cell structure changes as the lignin and hemicellulose soften and the diameter of the sapling widens.

Lignin is the cement in the cell structure that acts as a glue and keeps the cell walls from collapsing. If the lignin becomes hot from the steam, the cement thermocouples and allows the cells to move compress and stretch a little.

Steam bending wood, however, does affect the timber’s strength characteristics; the more bent the timber has been made to bend, the more the relationship between compression and tension can lead to fractures and eventually failure.

steam bending wood for decoration

Other Ways to Bend Wood Without Steam

There are a number of ways to encourage timber to bend, but the trick is to make it stay in that predetermined fixed position.

Steam/heat, chemical process, vacuum and immense compression can also lead to timber reaching a transient flexible state before it cools and re-sets itself. Strong and visually-impressive results can also be achieved using cross-laminated wood, although as timber merchants specialising in oak we prefer to emphasise the beauty and durability of pure wood, especially for boat and ship-building.

Steam Bending Wood in Boat Building

Naturally-grown crux timbers are very strong and sought-after for boat ribs, jowl posts and both wind braces in roofs and tie-beam post arrangements.

Before the hull of ships was made of steel, oak trees were trained from saplings (in areas such as the Weald of Kent, Forest of Dean, Bere, and areas around Surrey) to bend in order to make keelsons and knee braces for shipbuilding. The fact that they had grown that way meant that their strength equalled that of straight timber. Impact resistance was imperative if the ship was used for ramming. 

In HM Dockyard in Portsmouth, timber is bent in huge autoclaves (steel tubes with sealed ends) that can open where steam is pumped in and kept hot, allowing planks up 12m long and 12.5 cm wide to be steamed to soften the lignin.

In wooden boat building, huge rivets or staples are hammered home from each side to hold the plank into position, allowing it to cool and set into its new shape. Bigger planks could be in this steam environment for many hours, so handling them is tricky!

The Orlop deck on H.M.S Victory.

The Orlop deck on H.M.S Victory.

How to Bend Wood With Steam Bending

Bending needs applied force that can be mathematically calculated, key points are the fulcrum point and the lever arm, upon which force is uniformly applied.  Usually, the target bend and mould has to be exaggerated to beyond the finished size to allows for slight relaxing as the timber cools. Getting this right takes a lot of skill.

For the best steam bending, the M/C of the timber is ideally between 13-20%. Final machining needs to take place after cooling. End-trimming for some timbers is essential. Oak, for instance, is thought to be approximately 6-8% thicker before steaming to allow finishing at the desired finished size. Generally, heat and moisture can offer up to 10x the bending of room temperature timber.

As a rule of thumb, steam bending in a proper autoclave with the normal thickness (20 > 54mm as a guide) should see 1 hour of steaming per 25mm of timber thickness.

Steam bent timber can present signs of weakness as a result of the process. Outwardly visible signs of cracking will show as compression failures. These are easy to spot and can be rejected.

Once the timber is dry and down to the equilibrium moisture content, it has reached a stable, fixed state and can be finished normally.

Due to their wood grain, English Beech and Ash are superb bending timbers. W. L. West & Sons Ltd would like to further consider design applications using these two hardwoods—although, for external work, Oak’s durability cannot be beaten.

Several pictures shown feature stunning work completed by Millimetre, with whom we have collaborated on several projects involving large-scale steam bending.


WL West & Sons Ltd is a timber merchant and sawmill business with 150 years of experience. We provide a wide range of air dried and kiln dried oak timber products and supplies. We also build and install custom projects for our customers. 

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For entirely finished products, timber supplies or woodworking tools, have a look at our Retail Shop (which is temporarily closed for lockdown but will re-open closer to the New Year).


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