Home & Construction⏱ 5 min read
How to Calculate Roof Truss Spacing and Load
Roof trusses span wall to wall and support the roof covering, insulation, and any snow load. Here is how spacing is determined, what loads are calculated, and when an engineer is needed.
For most domestic extensions and new builds, roof trusses are supplied by a truss manufacturer who does the structural calculations. But understanding the logic helps you specify correctly, avoid installation errors, and communicate with contractors.
Standard Truss Spacing
UK domestic roof trusses are most commonly spaced at:
600mm centres (most common for slate/tile roofs)
400mm centres (heavier loads, or where specified by engineer)
900mm centres (used in some modern construction with structural insulated panels)
Why 600mm is the default:
Matches standard roof boarding/sarking spacing
Compatible with standard insulation roll widths
Tested performance under UK snow and wind loads
The spacing is set by the truss designer based on:
Roof span | Pitch | Roof covering weight | Wind zone | Snow zone
Loads That Trusses Must Support
Dead load (permanent, constant):
Roof tiles or slate: 40-80 kg/m2 depending on type
Roofing felt/membrane: 2 kg/m2
Insulation (between rafters): 2-5 kg/m2
Any services or boarding in loft: add as required
Imposed load (variable):
Snow load (UK most areas): 0.6 kN/m2 = 60 kg/m2
Wind load (pressure component): 0.4-0.8 kN/m2 depending on exposure
Access maintenance load: 0.25 kN/m2
Total design load for typical UK domestic (45 degree pitch, moderate exposure):
Dead + imposed: approximately 1.2-1.5 kN/m2 on plan area
Truss Count for a Roof
Number of trusses = (Roof length / Spacing) + 1
(+1 because you need a truss at each end)
Roof length: 9m, trusses at 600mm centres:
Number = (9 / 0.6) + 1 = 15 + 1 = 16 trusses
For hip ends (roofs that slope on all four sides):
Hip trusses are stacked progressively shorter
Additional jack trusses required at each hip
Manufacturer calculates these -- specify the hip layout
Gable ends:
Typically a standard full truss at gable
Plus a rafter at each end against the gable wall (with strap anchors)
When You Need an Engineer
Standard truss manufacturer: handles typical domestic spans up to 12m
An engineer is required for:
Non-standard spans (over 12-14m): requires engineer certification
Attic trusses (habitable loft): heavier floor loading, complex design
Modifications to existing truss: NEVER cut or alter a truss without engineer sign-off
Partial load-bearing walls removed: changes truss bearing points
Flat or very low pitch (under 17.5 degrees): special design required
High wind zones (Orkney, Shetland, exposed coastal): enhanced design
Altering a truss without engineering advice:
Even removing one timber can cause progressive collapse.
This is one of the most dangerous DIY mistakes in construction.