Nominal and finished dimensions
UK merchants describe timber in several ways. A sawn nominal section may finish smaller after planing or regularising, while CLS has rounded corners and a recognised finished size. For example, timber commonly sold from a 50 × 100 mm nominal origin may be listed as 47 × 100 mm regularised. The actual product data, not the casual name, controls geometry.
Make My Shed stores finished width and depth in millimetres. That keeps studs, plates and cladding lines consistent in the 3D model. When pricing, map each schedule description to the merchant’s exact finished section and check whether treatment or grading changes availability. Do not substitute a shallower member just because both products began with the same nominal label.
Wall studs, plates and noggins
Light sheds may use 38 × 63 mm CLS, while 47 × 75 mm and 47 × 100 mm regularised sections offer progressively more depth for stiffness, insulation and fixings. Stud centres of 400 mm or 600 mm are common design modules, but the chosen sheathing, cladding, wind exposure and openings influence what is appropriate.
Sole and top plates distribute loads and tie stud ends together. Noggins restrain studs, support sheet edges or linings and provide fixing zones. Their length is the clear distance between studs rather than the full bay module. Around doors and windows, king studs, trimmers, headers, sills and short studs redirect loads around the opening.
Floor joists and bearers
Floor members work harder than their modest span can suggest because garden sheds acquire dense point loads. A mower, workbench, machine or stack of paving does not behave like evenly spread lightweight storage. The standard preset uses 47 × 100 mm floor members, while workshop concepts step up to deeper sections and still require load-specific checking.
Bearers run beneath the floor cassette and transfer it to slabs, blocks, skids or a slab interface. Treatment is especially important near the ground, but moisture detailing remains primary. Blocking between joists helps restraint and load sharing where included. Sheet thickness and edge support must be coordinated with centres.
Rafters, ridge boards and ties
Rafter size depends on span, pitch, centres, covering weight, snow and wind, plus whether a deck or ceiling provides restraint. A ridge board helps locate opposing apex rafters but is not automatically a structural ridge beam. Rafter ties or an equivalent system resist outward spread where the roof arrangement needs them.
Pent roofs place the high and low walls at different levels and use single-span sloping members. Gambrel roofs divide each side into segments with additional joints. Flat roofs still have a designed fall and supporting members. The model generates each geometry separately instead of scaling one generic roof shape.
Grade, treatment and service conditions
Strength class such as C16 or C24 describes graded structural properties, not durability. Preservative use classes describe exposure conditions, not strength. A C24 untreated member and a C16 treated member answer different questions. Specify both where relevant and keep product certification with the project records.
UC2 is associated with protected internal situations where occasional wetting can occur, while UC3 products are intended for exterior above-ground exposure and UC4 for ground or fresh-water contact; confirm the treatment declaration and intended end use with the supplier. Re-treat cuts if the treatment system requires it and avoid details that trap water.
Centres and stock lengths
Closer centres increase member count and reduce the span of sheathing or decking between supports. They do not correct every weakness: connections, bearing and bracing still matter. Set out from a consistent reference face so standard sheets land on studs or joists, allowing for actual sheet size and expansion gaps.
Common stock lengths range from 2.4 m to 6.0 m, but local availability and straightness vary. The designer compares wall runs with the chosen plate stock length. When a run is too long, it places joints over studs and can add a staggered second top plate, reflecting standard framing logic rather than ordering imaginary extra-long timber.
Check substitutions as design changes
Merchant availability often prompts substitutions. Moving from 47 × 100 mm to another section affects bearing widths, overall wall or floor depth, opening components and every short infill cut. Changing grade may affect structural capacity without altering geometry; changing treatment may affect suitability without altering strength. Write down which property is being substituted and why, then update all dependent details.
Moisture content and straightness also influence build quality. Store timber off the ground, supported at close intervals and protected from direct rain while allowing air movement. Sort pieces before cutting: reserve the straightest stock for long plates, edges and opening members, and use sound shorter sections for noggins or blocking. Reject splits, damage or excessive distortion rather than burying problems behind cladding.
Read the whole specification on the schedule
A cut length without its role can be misleading. A short piece might be ordinary blocking, part of an opening or a critical roof connector zone. Keep section, grade, treatment, role and assembly attached to the quantity when exporting or ordering. Colour-coded labels or separate stacks help prevent visually similar treated and untreated material from being mixed.
Before final purchase, total each section independently and check that the merchant can supply the required grade and treatment in the chosen lengths. Ask how products are marked and whether planing changes the stated dimensions. If the answer differs from the model, revise the member build-up and regenerate the schedule so geometry, quantity and order remain one coordinated design.