Factory-made trussed rafters are designed as a single integrated system. Cutting one without replacement engineering is one of the most common — and serious — DIY mistakes in South African homes.
The cardinal rule
Trussed rafters cannot be cut. Each member is sized assuming the whole truss is intact. Remove one and the load redistribution can collapse the truss — sometimes immediately, sometimes years later under a heavy snow load.
What you can usually do
- Install loft floor boards on cross-braces (light storage only)
- Add hatch access between trusses
- Mount water tanks on properly designed platform between multiple trusses
What you cannot do
- Cut webs to create storage space
- Cut chord members for any reason
- Use the loft as habitable space without re-engineering
- Install dormers without complete redesign
Loft conversion options with trussed roofs
Truss removal and replacement
Standard solution. Trusses removed one at a time, replaced with cut-roof construction (rafters, purlins, ridge beam, struts). Allows full loft conversion.
Cost: typically R300,000–R700,000 in addition to standard loft conversion cost.
Attic trusses (factory-modified)
If you catch the design early enough, attic trusses can be specified for new builds — they provide habitable loft space within the truss design.
Structural strengthening
Some trussed roofs can be reinforced with steel beams and posts to allow partial use without full removal. Specialist design.
Modification process
- Engineer surveys existing trusses and measures spans
- Design replacement structure
- Temporary support of roof during removal
- Truss-by-truss removal and replacement
- New ceiling and finishes
Warning signs of unauthorised modification
- Cut truss members (often disguised behind boarding)
- Sagging ridge or roof slope
- Ceiling cracking in rooms below loft
- Difficulty closing first-floor doors
MCFAR designs roof truss modifications and replacements for loft conversions.
Need expert engineering on your project?
MCFAR GROUP has been delivering structural engineering, building, and plumbing services since 1998. Talk to our team about your build, retrofit, or renovation.
Request a QuoteFrequently Asked Questions
Can I just add a steel beam to support a cut truss?
Only with full engineering design. Most DIY "fixes" aren't load-path-equivalent and put the roof at risk.
How do I know if my roof has trusses or cut rafters?
Trusses are factory-made W or A shapes with timber connector plates. Cut roofs have larger individual timbers with no connector plates.