Structural timber design
Timber is light, quick to build with and, when graded and detailed properly, a strong and predictable structural material for floors, roofs and walls. We design solid timber structures to the Eurocodes for clients across the UK.
What is the difference between C16 and C24 timber?
They are strength classes for softwood. C24 is stronger and stiffer than C16, so it can span further or carry more at the same depth. We specify the class that suits the load and span, and for standard joists C24 is common because it often avoids going up a depth.
Timber as a structural material
Timber has a very good strength for its weight, which is why a floor of small joists can carry a room without heavy plant or cranage. Because it is a natural material it varies from piece to piece, so structural timber is strength graded, either visually or by machine, and stamped with a strength class such as C16 or C24 for softwood or D30 and above for hardwoods. The class fixes the design values for bending, shear, compression and stiffness that we use in the calculations, so a C24 joist is a known quantity even though the tree it came from was not.
Timber is an anisotropic material, meaning it is far stronger along the grain than across it, so the direction of load matters. It is strong in tension and compression parallel to the grain and much weaker perpendicular to it, which is why bearing at supports and notching near supports need care. It also moves with moisture, shrinking as it dries and swelling as it takes up damp, so the service class, whether the timber is in a dry interior or an exposed or humid setting, feeds directly into the design values and the detailing.
Designing to BS EN 1995
Timber structures are designed to BS EN 1995, Eurocode 5, with the UK National Annex. Unlike steel and concrete, timber design applies a modification factor, kmod, that adjusts the strength for how long the load acts and how damp the timber is, because timber carries a short gust of wind better than a permanent dead load. We check bending, shear, bearing and, for slender members, buckling, then deflection at the serviceability limit state. Deflection and vibration usually govern floor joists, so a floor that is strong enough can still need deepening to stop it feeling bouncy underfoot.
Connections carry the loads between members and often decide the layout. We design nailed, screwed, bolted and dowelled joints, joist hangers, framing anchors and, for heavier work, toothed plate and steel flitch arrangements. Where a timber beam is asked to do more than solid softwood can manage we introduce a steel flitch plate bolted between timbers, or move to an engineered product. The output is a set of calculations and details a carpenter can build from and that satisfy Approved Document A.
Moisture, durability and where timber suits
Timber's main enemy is moisture, because sustained damp above about twenty per cent moisture content lets decay fungi take hold, and it also drives movement that opens up joints. Keeping structural timber dry, or specifying it for its service class and treating it where it cannot stay dry, is the key to durability. For timber in ground contact or exposed to weather we specify preservative treatment to the right use class, and for anything permanently wet we would usually move to a more durable species or a different material altogether.
Solid timber suits domestic floors, pitched and flat roofs, stud walls and short to medium spans where its light weight and speed of build are an advantage. Its limits are span and predictability, since knots and grain mean a single joist can only be pushed so far before an engineered product earns its place. We advise where solid softwood is the right call and where glulam, laminated veneer lumber or an I-joist gives a better result, so the frame is neither over-specified nor asked to do more than the material allows.
What we check.
The points our calculations resolve for a project like this.
- Strength class selection, typically C16 or C24 softwood, D-class hardwood
- Service class and kmod for load duration and moisture
- Bending, shear and bearing checks to BS EN 1995
- Deflection and floor vibration limits for comfort
- Connection design, hangers, flitch plates and framing anchors
- Preservative treatment and durability for the exposure
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
What is the difference between C16 and C24 timber?
They are strength classes for softwood. C24 is stronger and stiffer than C16, so it can span further or carry more at the same depth. We specify the class that suits the load and span, and for standard joists C24 is common because it often avoids going up a depth.
Why do my floor joists feel bouncy even though they are strong?
Because floors are usually governed by deflection and vibration, not raw strength. A joist can pass the strength check yet still feel lively. Eurocode 5 sets deflection and vibration limits, and we design to them so the floor feels solid, which sometimes means a deeper joist or closer spacing.
Can a timber beam replace a steel one?
Sometimes. For modest spans and loads a solid timber or flitch beam works and is easier to handle than steel. For longer spans or heavy loads, steel, glulam or laminated veneer lumber is usually the better answer. We compare the options and specify what suits the opening.
Does structural timber need treatment?
It depends where it sits. Timber kept dry in a heated interior generally needs no treatment. Timber exposed to weather, damp or ground contact needs preservative treatment to the right use class, and permanently wet locations usually call for a different material. We set the requirement to the service conditions.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
Start a project