Timber movement in panelling and veneered walls

At a glance

AWS is the Architectural Woodwork Standards, 1st edition (2009, US). WH is the US Forest Products Laboratory Wood Handbook (FPL-GTR-282, 2021). Both are US documents: no UK moisture table was read. Movement figures are our arithmetic from the Wood Handbook. The partner workshop's own limits take precedence.

How much solid timber moves

Wood takes up moisture from the air and swells, and gives it up and shrinks. It moves most in the direction of the growth rings (tangentially), about half as much across them (radially), and hardly at all along the grain: for white oak the US Forest Products Laboratory gives 10.5% tangential and 5.6% radial shrinkage between green and oven-dry (Wood Handbook, Table 4-3). So a plain sawn board moves most across its width, and a quarter sawn board about half as much (see timber cuts explained).

The humidity of the room decides how much. The same handbook gives the moisture content wood settles at for each relative humidity (Table 4-2):

For a change in moisture content between about 6% and 14%, the Wood Handbook estimates movement as size x coefficient x change in moisture content (Equation 13-3). For American white oak the coefficients are 0.00376 per point tangentially and 0.00194 radially (Table 13-5). Applied to a solid panel 600 mm wide, in three cases:

Read the table for what it is. It is the free movement of one board of one species, by our arithmetic from US figures. The handbook says the method underestimates the true change by about 5%, and that because commercial lumber is rarely perfectly plain or quarter sawn it probably overestimates width movement. For a glued-up panel of mixed boards it advises the tangential figure where ring direction is unknown. European oak, the species most of our oak work uses, is not in the tables we read, so treat the figures as the order of magnitude, and ask the partner workshop for the species and cut being supplied.

The lesson is in the third row. A panel fitted damp into a room that later runs dry moves several times what a panel fitted at the right moisture content moves in a steady room. Most of that difference is under the specifier's control.

Moisture content at installation

The aim is to fit timber at the moisture content it will reach in service. The two documents we read give figures for the US, and neither is a UK table:

Board products are different. The Wood Handbook says hot-pressed products such as particleboard and hardboard usually settle at a lower moisture content than lumber, and recommends conditioning them at 30 to 40% relative humidity for interior use (p. 13-5). MDF is a hot-pressed fibreboard, so the same logic applies; the handbook text we read names particleboard and hardboard, and the MDF sheet gives the grades.

Why no UK figures. UK practice sets internal joinery moisture content by heating regime. Figures of this kind are quoted by others from the NHBC Standards and from an annex to BS EN 942, and they do not agree with each other. We could not read the NHBC page (it refused the request) and the standards are paywalled, so we print none. Ask the partner workshop for the moisture content it dries and dispatches to, the tolerance, and how it measures, and ask the project's warranty provider whether it has its own limit.

Fixing so that panelling can move

Solid timber that cannot move will split, push its frame apart or cup. The Wood Handbook is plain about it: design and install large solid pieces, such as wood panelling, so that the panels are free to move across the grain, and prefer narrow widths. It adds that wide door and window trim and base should be hollow-backed, and that large members such as cornices should be built up from comparatively small pieces (Interior Finish, p. 13-18).

For a framed panel that means a floating panel. The drawing shows the idea in section, and the worked example gives the allowance: a plain sawn white oak panel 300 mm wide moving through the seasonal case above changes by about 300 x 0.00376 x 5.1 = 5.8 mm, so about 2.9 mm into each groove, before any margin the workshop adds.

The points below are general joinery practice, not taken from the sources we read. We list them as questions to put to the partner workshop:

• Frames: no glue in the groove; a clear gap at the groove bottom for a panel that swells; finish on the panel edge before assembly so no bare strip shows when it shrinks.
• Boards and planks on a wall: leave room at the ends and at ceiling and floor, hidden or shown as a deliberate shadow gap, and say which in the drawing. See joinery tolerances and scribing for how scribes and gaps are drawn.
• Fixings: a fixing that grips the panel across its width stops it moving. Ask whether slotted, elastic or concealed clip fixings are used, and where a panel is held fixed (usually its centre, so movement runs out to both edges).
• Long runs: movement adds up along a wall. Ask where joints or relief breaks fall, and whether sequence-matched sets can still be split there. See wall panelling set-out.

Veneered and board-backed walls

A veneer is a thin slice of timber on a board, so the board largely decides how the panel behaves. It does not move like solid timber, but it is not still: the board has its own moisture response, and a long run of panels adds it up. For that the maker's figures govern, and the MDF sheet shows how to read them.

Balance. The Architectural Woodwork Standards define plywood as having the thickness and species either side of the core identical for a balanced effect (Section 4, 1.2.1), and say panel products need balanced coats of finish for stability and to stay free of warp (Section 5, 1.2.5). The Wood Handbook's glossary defines balanced construction as one in which uniformly distributed moisture changes will not cause warping, and, for decorative laminates bonded to a core, says a material with similar properties glued to the opposite face prevents bowing or cupping caused by moisture variation (p. 19-14). So a veneered panel wants a balancer or similar sheet on the reverse and finish on every face. The veneer sheet covers leaf and panel matching.

The wall behind. The same standards say that backing, blocking and furring that become part of the building are not furnished or installed by the woodwork manufacturer or installer (Section 8, 1.5.1), and that back-priming is not their responsibility unless the work is supplied prefinished (1.5.3). They recommend that concealed surfaces that might be exposed to moisture, such as those next to exterior concrete walls, be back-primed (Section 5, 1.3.1.2). So the specification should say who supplies the backing, whether the back is sealed, and how the wall will be shown to be dry.

Fire and finish. A wall lining has to meet the project's fire strategy, and the finish can change reaction to fire as well as movement. See the fire documentation matrix and finishes on timber joinery.

Site conditions, handover and who answers for what

Most movement faults are made on site before the panelling is fixed. The sources we read agree on the order of events.

• Gate the delivery. Deliver only when the area is enclosed, the wet work is dry, the overhead work is complete and the area is broom clean (Architectural Woodwork Standards, Section 2, 4.1.1).
• Dry the building, then heat it. Plastering a moderate six-room house puts about 450 kg of water into the building, all of which must leave before the interior finish goes on, so ventilate and heat (Wood Handbook, p. 13-20). Have the heating running before interior trim arrives, and keep outside doors and windows closed at night so the interior stays warm and dry (pp. 13-19 and 13-20).
• Do not flush with heat. Raising the temperature for a sustained period to eliminate off-gases is unacceptable; open joints and warped panelling caused that way are not a defect (Section 2, 1.5.1).
• Read, then fit. Take moisture readings of the panelling, and of the wall where it is in doubt, before fixing. The acclimatisation sheet sets out a logging and sign-off routine that works for panelling too.
• Hold the conditions after handover. The standards say the space needs humidity control to hold its optimum relative humidity, and treat extremes after occupancy as the owner's responsibility (1.2.3.1 and 1.4.1.3). So the operating notes should give the RH range.

These allocations are the US standard's. UK contracts allocate responsibility differently, so a contracts adviser should check how the project's documents say it.

Timber and wood-based panelling to be at a moisture content of [x]% (individual pieces [y to z]%) on delivery and at fixing, measured with a calibrated meter set for the species or board, on not fewer than [10]% of each item. Readings, meter and date to be recorded and issued to [designer] before fixing.

Panelling is not to be delivered until the building is enclosed, wet trades are complete and dry, overhead work is complete and the area is broom clean, and heating and ventilation have held [30 to 55]% relative humidity at [ ] degrees C for [ ] days, as shown by a logger record issued to [designer]. Conditions are to be held during fixing. The temperature is not to be raised for a sustained period to drive off off-gases.

Solid timber panels over [300] mm wide are to float in the frame. Clearance in each groove not less than [ ] mm, worked from [species and cut] and a [ ] to [ ]% RH range, and shown on the shop drawing. No glue or rigid fixing in the groove or across the width of the panel. Fixed point of each panel at [centre].

Veneered panels to be balanced: [veneer of the same species and thickness / a balancer of similar behaviour] on the reverse, all faces and edges finished with the same system, on [board] conditioned to the site relative humidity before fixing. Backing and blocking supplied by [ ]. Back of panels [sealed / back-primed]. Joints between panels [as a [ ] mm shadow gap].

Standards and sources

• USDA Forest Products Laboratory, Wood Handbook, FPL-GTR-282 (2021): Ch. 4 Tables 4-2, 4-3; Ch. 13; Ch. 19 p. 19-14. Read 5 Nov 2026.
• Architectural Woodwork Standards, 1st edition (2009): Sections 2, 4, 5 and 8. US standard. Read 5 Nov 2026.
• BS EN 942:2007, Timber in joinery: general requirements: Named, not read: confirm moisture content figures against the current edition.
• NHBC Standards, chapter 6.7.7, Timber doors and windows: Named, not read (the page refused the request): confirm.

Questions we are asked

Why is my wood panelling cracking or opening at the joints?

Almost always because the timber changed moisture content after it was fitted and was not free to move. Wood swells in damp air and shrinks in dry air, mostly across the grain. The Architectural Woodwork Standards say such problems are usually the result of faulty design or the wrong humidity during storage, fitting or use. The fixes are to fit at the right moisture content, hold the room conditions, and let solid panels float.

What moisture content should panelling be when it is installed?

It should be close to the moisture content it will reach in the finished room. For interior woodwork in most US areas the Wood Handbook gives an average of 8% with individual pieces between 6 and 10%, and 6% in the dry south-west. We could not read a UK table, so ask the partner workshop for the figure and tolerance it dries to, and check the project's warranty provider for any limit.

How much does solid oak panelling move?

By the Wood Handbook arithmetic for American white oak, a plain sawn panel 600 mm wide moves about 3.6 mm between 45% and 55% relative humidity and about 11.5 mm between 35% and 65%. A quarter sawn panel moves about half as much. That is free movement of one board; European oak was not in the tables we read, so treat the figures as an order of magnitude.

Do veneered MDF or plywood wall panels move?

Less than solid timber across the width, but not at all in the sense of being still: the board has its own response to humidity, and a long run of panels adds up. A veneered panel should be balanced, with a balancer or similar sheet on the reverse and finish on every face, or it can bow. Ask the board maker for its dimensional-change figures.

Should I leave an expansion gap?

Leave a designed one, not a hopeful one. Work out the movement for the widest piece, show where it goes (in the groove of a floating panel, or at a shadow gap at the ends and at ceiling and floor), and put the allowance on the shop drawing. Where a panel is fixed rigidly across its width it cannot use any gap you leave.

Can the heating be turned up to dry the building out before panelling goes in?

A steady regime, yes: the Wood Handbook advises having the heating in place before interior trim arrives and ventilating after plastering. A sustained rise in temperature to eliminate off-gases is the practice the Architectural Woodwork Standards call unacceptable, and open joints or warped panelling that follow are not treated as a defect.