The only question a detail has to answer
Wood decays when it stays above roughly 20 % moisture content, at a temperature fungi like, for long enough. Everything below follows from one question: after this detail gets wet, how quickly does it dry?
A good timber frame detail does four things in order — deflect water before it lands, drain what does land, dry what drains behind the face, and tolerate the movement all of that causes. Details that skip the third step are the ones that fail, because they look perfectly watertight on a drawing.
1. Base of wall and ground contact
Ground clearance to cladding
≥ 300 mm to soil · ≥ 150 mm to hard drained surface
Splash-back keeps the base above fibre saturation long enough for decay fungi to establish.
Sole plate isolation
DPC + capillary break under every bearing
End grain in contact with concrete wicks moisture upward faster than the wall can dry it.
Base of cavity
Open, drained, insect-meshed, no mortar bridging
A blocked cavity base converts the drainage plane into a reservoir.
2. Wall build-up and the drying path
Ventilated cavity depth
25 mm min · 38–50 mm for cross-battens
Enough free area for buoyancy-driven airflow to dry the back face between wetting events.
Vapour gradient
Vapour-tight inside, vapour-open outside (≈5:1)
Guarantees an outward drying path so interstitial condensation does not accumulate.
Breather membrane
UV-stable, taped laps, shingled downwards
Secondary drainage plane; reverse laps drive water into the frame instead of out of it.
Board moisture content at fixing
16 ± 2 % exterior · ≤ 12 % heated interior
Matching equilibrium moisture content limits cupping, shrinkage gaps and fixing loosening.
3. Openings, sills and reveals
Sill pan and end dams
Continuous, upstand at jambs, sloped ≥ 5°
Openings are the highest-risk leak path in any timber frame; water must be returned outwards.
Head flashing
Drip edge past cladding face, sealed to membrane
Stops runoff tracking back along the soffit of the reveal into the frame.
Reveal linings
Ventilated or fully sealed — never half-sealed
Partially sealed reveals trap water without a drying path.
Horizontal surfaces
Slope ≥ 10° or protect with metal
Flat timber surfaces retain water for hours and are where coatings fail first.
4. Eaves, verges and cavity termination
Eaves overhang
300–600 mm typical; more for exposed sites
Overhang reduces wall wetting; a 600 mm eaves can halve driving-rain load on the top storey.
Cavity head ventilation
Continuous outlet at eaves, insect-meshed
Without a head outlet the cavity does not ventilate — a base gap alone is not enough.
Verge and gable ends
Drip terminations, no end grain exposed upwards
Upward end grain absorbs water axially 10–100× faster than the face.
5. Fixings, movement and end grain
Fastener metallurgy
A2 stainless inland · A4 stainless coastal or with tannin-rich species
Oak, western red cedar and chestnut corrode plain steel and cause black staining.
Fixings per board width
One fixing per bearing up to 125 mm wide boards
Two fixings across a wide board restrain movement and split it as it cups.
End distances
≥ 10× fastener diameter from board end
Prevents splitting at the most moisture-cycled part of the board.
End grain sealing
Seal or prime all cuts on site
Every site cut re-opens the fastest moisture pathway in the board.
6. Use class before anything else
Detailing changes the use class, and the use class changes which species and treatments are viable. Move a detail from Use Class 3.2 to 3.1 with an overhang and a ventilated cavity and a durability class 3 species becomes a defensible choice; leave the same board unventilated and trapping water and even a class 1 species will disappoint.
UC 1
Interior, dry
No decay risk; movement and finish govern.
UC 2
Interior, occasional condensation
Ventilate voids; avoid cold-side vapour barriers.
UC 3.1
Exterior, above ground, protected, quick drying
Overhang, cavity, drained joints.
UC 3.2
Exterior, above ground, frequently wet
Higher durability or treatment; no traps.
UC 4
Ground or fresh-water contact
Detail it out if at all possible.
7. The detailing checklist
- Every horizontal surface is sloped, flashed, or both.
- Every cavity has an inlet at the bottom and an outlet at the top.
- Every membrane lap sheds downward and outward.
- No end grain points upward, and every site cut is sealed.
- No timber bears directly on concrete, masonry or soil.
- Fasteners are stainless where the species or exposure demands it.
- Boards are fixed at their in-service moisture content.
- Every element that will need maintenance can be reached and replaced.
Frequently asked
How much ground clearance does timber cladding need?
At least 300 mm above soil, or 150 mm above a hard free-draining surface. Splash-back is the most common cause of decay at the base of a timber frame.
How wide should the ventilated cavity be?
25 mm minimum, 38–50 mm where cross-battens or profiled boards are used, with continuous openings top and bottom. Mesh should not cut free area below roughly 10 mm equivalent.
Where does the vapour control layer go?
Warm side of the insulation, with layers becoming progressively more vapour-open outwards — roughly 5:1 between inner and outer vapour resistance in temperate climates.
What moisture content should timber be installed at?
16 ± 2 % for exposed exterior cladding, 12 % or lower for heated interiors.
Apply it to your project
Give the engine your exposure, orientation and species and it will return the detailing consequences — not generic advice.