Attic conversion with dormer, notched beam and no structural calculations
A 1935 stone masonry house in Diekirch has a roof structure of 160 x 240 mm pine rafters at 800 mm centres spanning 4.8 m, with a collar tie at mid-height. The owner wants a habitable attic: 60 mm screed with acoustic mat, 18 mm OSB, a 2.0 kN/m2 imposed load, and a new 3.5 m wide dormer removing two rafters and the collar tie. A contractor proposes to notch the existing 160 x 240 mm ceiling beam at the support by 90 mm to pass a 100 mm soil pipe, and to support the dormer with a single 200 x 200 mm ridge beam bolted to the gable. The roof covering is clay tiles; snow load in the area is 0.8 kN/m2. The owner has no structural calculations; the contractor says the stone walls have held for 90 years.
1. Identify the load-path changes caused by the attic conversion and dormer.
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The existing roof is a coupled timber roof: rafters carry tile and snow load to the eaves and ridge, while the collar tie resists the outward thrust of the rafters at mid-height. Removing two rafters and the collar tie to form the dormer opening interrupts that load path. The dormer roof and its snow load must be carried by trimming rafters, doubled rafters and a header; those members must transfer load to the ridge and to the eaves or gable. A single 200 x 200 mm ridge beam bolted to the stone gable is suspect: the gable may not be designed for the horizontal thrust or the concentrated bolt loads, and the ridge beam itself must span between supports. The new floor load - 60 mm screed plus OSB plus 2.0 kN/m2 imposed - also increases the load on the ceiling joists, which may not have been sized for habitable use. The age of the walls is not a calculation.
2. Evaluate the proposed 90 mm notch in the 160 x 240 mm beam and propose an acceptable detail.
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The notch leaves only 70 mm of the 160 mm beam depth, i.e. about 44% removed at the support. That is a severe shear concentration at exactly the point of maximum shear, and it is well beyond the normal detailing limit of about one-quarter of the depth. It also creates a crack initiator and reduces fire performance. The proposal should be rejected.
Acceptable options, in order of preference: reroute the 100 mm soil pipe through the wall or a boxed-out soffit rather than the beam; use a smaller pipe if hydraulically possible; if a penetration is unavoidable, drill a concentric hole in the middle third of the depth, sized within the engineer's limit and away from the support, and reinforce if required. A notch, if truly unavoidable, should be shallow, located away from the support, and checked by the structural engineer, with side plates or a steel shoe. The detail must be drawn and approved before cutting.
3. What structural verification and temporary works should precede demolition?
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Before any rafter or collar tie is cut, a structural engineer should survey the roof, check timber condition and connections, and model the existing and new load paths for dead, snow and imposed loads. The verification should cover the trimmed rafters, ridge beam, gable anchorage, ceiling joists and the new floor. Temporary props should support the rafters and collar-tie line before removal, and the dormer opening should be sequenced so the roof is never left as an unbraced mechanism. The gable should be checked for overturning and the ridge beam for horizontal thrust; if the gable cannot take it, a steel frame or posts down to the floor structure are needed. The works also require the building permit and a statics dossier, and the contractor's method statement should be reviewed. The owner should not accept 'it has stood for 90 years' as a structural argument.