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The summed depth of horizontally laid timber the load path passes through at one level.
In a platform-framed floor that is the sole plate, plus the rim board or the joist depth, plus both top plates — commonly around 300 mm in total, though an engineered rim or a deep joist takes it well past that. For a single member being checked on its own, such as a beam bolted top and bottom to a steel plate, enter that member's depth instead and leave the level count at one. Studs are not included: they stand with the grain vertical and shorten by almost nothing.
Tools needed: Framing section or shop drawing, Tape
The meter reading taken on the timber at the moment it was fixed in place.
This is the single cheapest piece of evidence on a disputed split, and it takes a minute with a resistance meter pushed into a fresh face away from the surface. Stock delivered from an open, unheated store commonly reads in the high teens or low twenties; material that has stood in a conditioned space reads far lower. The upper end of this field sits at the fibre saturation point, above which timber holds free water and does not shrink at all until it dries past it.
Tools needed: Resistance moisture meter with insulated pins
Where the timber will settle once the building is finished and running.
Eurocode 5 service class 1 — a heated, occupied interior — corresponds to an average that will not exceed 12 per cent in most softwoods, and a continuously heated space with dry air settles lower still, near 8. Service class 2, a covered but unheated or humid space, corresponds to an average not exceeding 20 per cent. Entering the delivery figure here instead of the in-service figure is the commonest way to make this calculation report no movement at all.
The fraction of its depth the timber moves for each single point of moisture change.
Tangential coefficients — the face of a flat-sawn board — for common construction softwoods sit around 0.0026, meaning roughly a quarter of one per cent of the depth per point of moisture change. Radial coefficients, for quarter-sawn material, run near two-thirds of that. Dense hardwoods move considerably more. The Wood Handbook tabulates both directions species by species, and where a member is neither cleanly flat-sawn nor cleanly quarter-sawn the tangential figure is the conservative choice.
How many platform-framed levels the movement accumulates through.
Each level contributes its own cross-grain depth, and the movement adds all the way up: a facade, a stair, a service riser or a run of masonry cladding fixed against the frame has to absorb the total, not the amount at one level. Set this to one where you are checking a single member rather than a stacked building.
Cross-grain movement at one floor level
0.2749 in
Straight arithmetic on the coefficient and the moisture change entered. The movement is across the grain; along the grain the same change moves the timber by a small fraction of this and is normally ignored.
- Accumulated movement over all floor levels
- 0.82 in
- Depth remaining once the movement has happened
- 11.48 in
- Movement for a single point of moisture change
- 0.03 in
- Change in moisture content
- 9 %
- Proportion of the depth lost
- 2.34 %
They open the calculator with your figures already in it
Timber Cross-Grain Shrinkage Calculator: 0.2749 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- USDA Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material (General Technical Report FPL-GTR-190), Chapter 4 — the dimensional change coefficient method, in which the change in a dimension equals the dimension multiplied by the coefficient and by the change in moisture content, valid below the fibre saturation point
- EN 1995-1-1 (Eurocode 5) service classes — service class 1 describes an environment in which the average moisture content of most softwoods will not exceed 12 per cent, and service class 2 one in which it will not exceed 20 per cent; those are the end points this calculation runs to
- EN 335, Durability of wood and wood-based products — Use classes, which describe the exposure a detail will actually see and therefore the moisture the timber in it will reach
- RULE OF THUMB, stated as one: platform-framed shortening is estimated by summing the depth of horizontally grained timber at each floor — sole plate, rim board or joist depth, and top plates — because those pieces shrink across their depth while the studs between them barely move at all
Inputs used
- Depth of Cross-Grain Timber at One Floor Level
- 11.75 in
- Moisture Content at Fixing, per cent
- 19
- Equilibrium Moisture Content in Service, per cent
- 10
- Dimensional Change Coefficient
- 0
- Floor Levels Stacked Above One Another (count)
- 3
Intermediate steps
- Accumulated movement over all floor levels
- 0.82 in
- Depth remaining once the movement has happened
- 11.48 in
- Movement for a single point of moisture change
- 0.03 in
- Change in moisture content
- 9 %
- Proportion of the depth lost
- 2.34 %
Confidence note: Straight arithmetic on the coefficient and the moisture change entered. The movement is across the grain; along the grain the same change moves the timber by a small fraction of this and is normally ignored.
What this calculation does not cover
- Movement across the grain only. Timber moves along the grain by a small fraction of this, and a long member's length is effectively stable.
- Says nothing about whether the movement will be RESTRAINED. A member free to shrink simply shrinks; the split the guide describes happens when two rigid fixing lines hold a deep section apart while it tries to move, and that is a detailing question this arithmetic cannot answer.
- Uses one coefficient for the whole stack. Where the plates, the rim and the joists are different species or different sawing patterns, run them separately and add the results.
- Ignores creep, mechano-sorptive movement under sustained load, and any moisture cycling. A detail that dries, wets and dries again does not simply return to where it started.
Add the equipment this sizes
This result is a specification — 0.2749 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-08-30 · in the site-wide review of 2026-09-06 · v1.0.0
Regulatory standards & verification citations4
- USDA Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material (General Technical Report FPL-GTR-190), Chapter 4 — the dimensional change coefficient method, in which the change in a dimension equals the dimension multiplied by the coefficient and by the change in moisture content, valid below the fibre saturation point
- EN 1995-1-1 (Eurocode 5) service classes — service class 1 describes an environment in which the average moisture content of most softwoods will not exceed 12 per cent, and service class 2 one in which it will not exceed 20 per cent; those are the end points this calculation runs to
- EN 335, Durability of wood and wood-based products — Use classes, which describe the exposure a detail will actually see and therefore the moisture the timber in it will reach
- RULE OF THUMB, stated as one: platform-framed shortening is estimated by summing the depth of horizontally grained timber at each floor — sole plate, rim board or joist depth, and top plates — because those pieces shrink across their depth while the studs between them barely move at all
Which documents these citations point at
- Eurocode 5 — Design of timber structures (BS EN 1995) (European (EN))Timber design, including the service classes and the modification factors applied for moisture and load duration.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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