Two types of oak
Oak comes to a joinery workshop in two states: green and seasoned. Understanding the difference is fundamental to understanding why a piece of oak joinery was built the way it was, and whether it will behave the way the client expects.
Green oak is freshly felled timber. Its moisture content is typically 60–100% — the wood is full of sap and has not yet begun to dry. Green oak is the correct material for traditional pegged mortise-and-tenon frame construction. As both the mortise and tenon dry after assembly, they shrink. The oak peg that locks the joint is typically made from seasoned oak, so it retains its dimensions while the surrounding green wood shrinks around it. The result is a joint that tightens with age rather than loosening. You cannot replicate this mechanical behaviour with seasoned timber.
Seasoned oak has been dried — either air-dried over a period of years or kiln-dried to a controlled moisture content. our kiln-dried stock is brought to 8–12% moisture content before use in internal joinery. At this moisture content, the timber is dimensionally stable: it will not move significantly in service under typical interior humidity conditions. This stability is essential for windows, doors, staircase strings, and furniture where gaps at joints would cause draught problems, squeaks, or visible failures at glue lines.
Structural properties
Oak is a dense, hard hardwood. The dry density of European oak runs around 720 kg/m³ — roughly 35% denser than Douglas Fir at equivalent moisture content. Its Janka hardness is significantly higher than any structural softwood, making it resistant to mechanical damage in high-traffic applications like staircases and thresholds. Its natural durability is classified as Class 2 to BS EN 350 — moderately durable without any preservative treatment. For external structural applications like porches, frames, and exposed beams, oak will perform without treatment for the design life of the structure.
For structural grading, We source oak to BS 5756 — the British standard for visual strength grading of hardwoods. GS (General Structural) and SS (Special Structural) are the two grades relevant to framing work. SS grade oak has tighter limits on knot size, slope of grain, and other strength-reducing characteristics; it is specified for principal structural members where engineering calculations drive the grade requirement. GS grade is appropriate for secondary structural elements and non-critical framing applications.
Visual character
Oak’s visual appeal is not uniform — it varies substantially depending on how the board was cut from the log. Quarter-sawn oak, cut radially from the centre of the log, reveals medullary rays: the distinctive silver fleck pattern that runs across the face of the board. Quarter-sawn boards are also more dimensionally stable because the growth rings run perpendicular to the face. They are more expensive to produce because more waste is generated in cutting.
Crown-sawn (through-and-through) oak is cut tangentially through the log. The figure is bolder and more varied — cathedral arch patterns from the growth rings, with knots and grain variation that reflect the particular life of the tree. Crown-sawn boards are cheaper to produce and are the standard for structural grading. For furniture and decorative work, the choice between quarter and crown sawn is an aesthetic decision made at specification stage.
Fresh-cut heartwood is pale gold to warm brown. Sapwood — the outer zone of the tree — is cream to white and is generally excluded from structural sections because it is less durable and grades out differently. As the heartwood is exposed to light and air, it darkens. Left outside, it silvers. Inside a building, it deepens toward a rich amber-brown over years. These colour changes are predictable and can be managed: UV-stabilising oils slow the initial darkening; Danish oil enriches and stabilises the colour; bare untreated oak moves freely through its natural colour range.
Applications in framing
Oak is the traditional framing species for exposed structural work in British building. King post trusses in oak — with the post rising from the tie beam to the ridge, principal rafters either side, and straining beams between — are the visual centrepiece of many extensions and barn conversions we work on. The joint work in a king post truss is complex: the feet of the principal rafters are housed into the tie beam, the post is tenoned into both the tie beam and the ridge, and every joint is pinned with a seasoned oak peg. Done in green oak and left to dry in situ, this is construction that has worked in British buildings for eight hundred years.
Feature beams — structural members that carry a real load but are also intended to be seen — are a common request in renovation projects where the client is opening up a ceiling or removing a wall. We size these to the structural requirement first, then considers the aesthetic result. A beam that is visually proportionate is usually one that is structurally appropriate. A beam that needs to be deeper than looks good is a beam where the structural span or load is pushing the limits of oak at that section — and that conversation belongs at design stage, not during installation.
Applications in joinery
Seasoned oak is used throughout our joinery work for windows, doors, staircases, and fitted furniture. Oak windows for a natural or oiled finish are a direct alternative to softwood windows for a painted finish: both are legitimate choices, and the right one depends on the building’s character and the client’s maintenance preferences. An oak window that is oiled annually will outlast a painted softwood window maintained on the same schedule.
Oak staircases are a particular strength of the workshop. A straight-flight or quarter-turn staircase in oak — with newel posts, balusters, a continuous handrail, and treads in solid oak rather than veneered softwood — is built to last the life of the building. The treads are typically 40–50mm thick solid oak, housed into the strings with a glued and wedged joint. The construction is traditional and has no proprietary components that need sourcing when something needs repair in thirty years’ time.
Sourcing
Some of our oak comes from our own woodland in North Wales. This is primarily Douglas Fir land, but the woodland includes oak stands, and Citadel fells and saws oak from their own managed land when the sections match what the workshop requires. They supplement with Welsh and UK-sourced oak from suppliers they have established relationships with. Where provenance matters — in heritage work, conservation area projects, or barn conversions where like-for-like species sourcing is required — Citadel can specify Welsh oak from known sources and document the provenance for the application.
Working characteristics: what joiners know about oak
Oak is demanding to work. It is hard enough to dull tooling faster than any common British softwood — carbide tooling is mandatory for CNC work, and hand tools need sharpening more frequently. The tannic acid in oak reacts with iron and steel: an uncoated steel screw driven into oak will stain the surrounding timber with a blue-black mark within weeks. we use stainless steel or brass fixings throughout all oak work — this is not an optional quality measure, it is basic practice that prevents visible failures in finished work.
Green oak is particularly demanding to joint because the high moisture content affects both the cutting and the fit: a mortise cut accurately in the morning may be slightly different in dimension by the afternoon as the timber moves. Experienced joiners work green oak quickly and assemble promptly. our workshop has the experience and the throughput to do this correctly. It is not work for a workshop that encounters green oak occasionally.