Calculate preliminary truss member sizes

Choose the workflow

Manual keeps the section sizes you enter. Automatic searches supported timber sections using Code_Aster results and Autostrek’s preliminary member checks. It retains the chosen truss type, geometry, CC spacing and declared supports.

Automatic currently supports W-frame, the supported A-frame/attic layout, lattice attic and gable-ended Mansard trusses. Full Mansard roofs with hipped ends use Manual framing and a separate read-only assembly analysis. Unsupported layouts cannot be accepted as calculated designs.

Calculations require an available Cloud service or the paired Windows local calculation engine. Choose the target under Calculation engine. If it is unavailable, no sizes are applied and the app does not switch targets silently. Manual modelling remains available.

In Build Generator

1. Set the building’s width, roof angle and overhang in the editable preview, and choose its CC spacing.
2. Select Gable > Roof Structure > Truss, then choose a type card or a saved truss.
3. Select Automatic. Enter characteristic ground snow load in kN/m² and review Advanced settings.
4. Choose Calculate and apply, or Calculate and review for an attic layout. Review calculated sections and any support/layout guidance. Large W-frame results also remain open for review.
5. Choose Apply to building when prompted, then generate the building. This updates a project copy; the reusable library original is preserved.

Use Edit beside the selected truss to revise it, download its report or Recalculate member sizes. Changing span, pitch, loads, CC or the selected bearing wall invalidates the previous assessment. The build review’s Fix link returns to the selection dialog.

In standalone Truss Builder, choose the template and Automatic under Member sizing, review Loads and design assumptions, then select Calculate member sizes. Save or add the accepted result. Switching to Manual retains the selected sections.

Enter loads, not a snow-zone number

The ground-snow input is a load value for the site, including its altitude adjustment. A Norwegian snøsone identifier is not automatically converted into kN/m².

Roof dead load is entered per square metre of sloping roof; ceiling dead load is separate and uses horizontal area. Timber self-weight is added automatically. Review the visible assumptions for the intended materials. Selecting a roof covering does not calculate its weight, and ceiling weight should not be included twice.

Attic types additionally check the occupied floor zone between the knee posts. The starting assumptions include 0.5 kN/m² additional floor dead load, 2 kN/m² imposed floor load and separate 2 kN point-load cases. These still apply when the deck is hidden. Partitions, stairs, openings, storage outside the room and floor vibration require further assessment. W-frame sizing does not include an occupied attic floor.

Timber, room size and bracing

New designs use C30; C24 is also selectable. Existing designs retain their saved grade. Use the grade of the timber that will actually be supplied. A stronger grade does not improve every governing check or guarantee a larger span.

Automatic keeps a common 36 or 48 mm thickness. Member depths may differ: the catalogue reaches 223 mm for 36 mm stock and 298 mm for 48 mm stock. Attic rafters, bottom chord, posts and collar can have different depths. The search does not widen the truss or shrink the requested room to pass.

Required restraint assumed describes the lateral support credited in the checks. Advanced settings expose restraint distances for the relevant members. Zero means the full member is unrestrained. These distances are separate from truss CC. Rendered battens, roof boards, lining and connector plates do not prove that the required bracing exists or that its fixings work.

Internal bearings and long spans

Exterior walls are the default bearings. In supported Build layouts, you can explicitly select one eligible full-length, full-height longitudinal internal wall without doors. Other drawn walls are not counted automatically. Standalone Truss Builder can declare a bearing position, but Build requires a matching eligible wall.

This declares a support for the truss calculation; it does not size or approve the wall, openings below it, floor or foundation. Moving or removing that wall invalidates the assessment. Autostrek does not insert a load-bearing wall automatically.

When sizing fails, the closest tested sections and governing checks help explain why. Failed sizes are not applied. Passing sections deeper than 223 mm or the use of an internal bearing also prompt a practicality review. There is no universal maximum span or automatic switch to an I-beam. Compare the lattice attic option, layout and designed supports; glulam, LVL and proprietary I-beam alternatives need their own models.

What “Checking candidate” means

One candidate is a complete proposed combination of member sections. The app analyses it and evaluates all required load combinations and checks. It may then try a different combination or refine a passing result. The counter is not a percentage: an easy case may need only a few candidates, while a constrained attic can need many more.

The solver reuses the structure’s stiffness calculation across loads within a candidate, and repeated section combinations are skipped. These optimisations retain the load cases and member checks. Calculation can still take minutes. Cancelling, losing the service or changing an input applies no unfinished candidate.

Read the result within its scope

A passing result is a preliminary member assessment, not verified Norwegian Eurocode compliance or construction approval. Connection capacity and slip, bracing fixings, wind, fire, local snow accumulation, bearings and the complete load path need separate design. Compact connector plates are visual placeholders.

Save the report with the project information and arrange the remaining checks before construction. Previously generated geometry does not change when a new result is calculated: apply it and regenerate the building.