Sophie Bennett
Why an exposed beam has to do two jobs
A beam hidden above a ceiling only has to carry its load. It can be twisted, wane-edged, still damp and generally ugly, and nobody will ever know. An exposed beam is different: it has to be structurally sound and look settled and deliberate in the room. That second requirement is where most of the specification happens, and it is why grade, moisture content and species matter so much more once timber is left on show.
Get this wrong and the problems appear slowly. A beam that was straight and tight in August can be bowed and gappy by February, with open splits down the face and a bearing sitting on nothing.
Choosing the species
For most domestic extensions and renovations, the choice comes down to oak or a graded softwood, with a few useful alternatives between them.
- Oak — strong, hard-wearing and visually unmistakable. European oak typically grades well above softwood in strength, and its heartwood is durable and resistant to insect attack. It is also the most demanding to get right: it moves more, is heavier, is harder to cut on site, and reacts with mild steel fixings.
- Softwood — spruce, pine and fir, supplied strength-graded and kiln-dried. Cheaper, more predictable in moisture terms, easy to work, and perfectly capable of carrying domestic spans. It looks softer and plainer, and takes paint or stain well.
- Douglas fir and larch — useful middle ground. Douglas fir has good stiffness for its weight and a warm, straight grain that suits a modern interior; larch is harder and more resinous, with a strong figure.
- Sweet chestnut — a traditional oak substitute with similar durability, usually at lower cost. It is more prone to shake and can be harder to source in structural sizes.
Think about the finish, not just the load. Oak wants to be seen as oak; if it is going to be painted, you are paying for appearance you will cover up.
Understanding grades
For softwood, look for timber marked with a strength class such as C16 or C24, graded to the relevant European standard. C24 is stiffer and stronger, and often the sensible choice for a longer visible span where you want to keep the section shallow. Machine grading is normal now, and the class stamp should be present on the piece or the documentation.
Hardwoods are graded differently. Oak for structural use is usually visually graded to a temperate hardwood standard, with grades often referenced as TH1 and TH2 — TH1 being the higher grade, with tighter limits on knots, slope of grain and other defects. Strength classes for hardwood such as D30 are also used.
Grade describes strength, not beauty. A beam can be perfectly strong and still full of knots, sapwood and wane that you would not want on show. If appearance matters, specify the visual quality separately from the structural grade.
Moisture content is the one that catches people out
Timber shrinks as it dries, and it shrinks unevenly — more around the growth rings than across them. That is what produces the classic split faces, twisted lengths and dropped bearings on green oak work.
Rough figures: freshly felled oak can sit at 60–80% moisture content or higher. Air-dried oak is typically around 18–22%. Kiln-dried softwood for structural use is generally in the region of 15–18%.
A heated interior settles at an equilibrium moisture content of roughly 10–12%. Any beam installed wetter than that will carry on drying in place, and a 200mm-wide oak beam can lose several millimetres across its width as it does so. On a long span, the cumulative shrinkage at a bearing can be enough to leave a visible gap.
This is not a reason to avoid green oak — a traditional green oak frame is designed to move. It is a reason to be deliberate: either accept the movement and detail for it, or specify well-seasoned timber and keep it dry on site.
What to check when the timber arrives
- Wane and sapwood — sapwood is not durable and invites beetle. Keep it off show faces and out of bearings.
- Splits and shakes — a shake running along the length, or a split near a fixing, is a structural concern, not just cosmetic.
- Bow, spring and twist — check with a string line and a square. Twist is the hardest defect to disguise.
- Knots — large knots on the tension edge reduce capacity. In softwood, they matter most where the beam is working hardest.
- Actual size — rough-sawn and planed-all-round sections differ. Confirm the finished dimensions against your calculations.
- Storage — keep timber off the ground, under cover, with air moving around it, and stickered if it is staying on site for weeks.
Detailing and fixings
Exposed beams inside a heated building sit in a benign environment, which works in your favour — no wet rot, minimal risk of decay. But oak is acidic and will corrode ordinary mild steel, staining the timber as it goes. Use stainless steel or silicon bronze fixings, plates and straps anywhere they touch oak, including hidden connections.
Design bearings so that shrinkage can be taken up without the beam dropping off its support, and allow for a small amount of movement at joints rather than forcing everything tight. If the beam passes through a wall, keep the ends ventilated and out of contact with damp masonry.
Finally, decide on finish early. A clear, breathable oil or wax lets oak settle and silver gently under light; a hard film finish will crack as the timber moves beneath it. Softwood is more forgiving and takes paint readily — though for exposed work, a good primer and two coats will always look better than a rushed job.
