Reusing a Rotten Wall Plate on a Hip Roof
A hip roof is to cover a 10.0 m by 8.0 m house near Negombo. The pitch is 30° and the covering is clay tiles. The existing wall plate is 100 mm by 75 mm jak; the rear section is soft and stained. The carpenter has set rafters at 600 mm centres with 50 mm by 100 mm kumbuk, purlins at 450 mm, and a 25 mm ridge board. There is no bracing and no strap over the rafters. The site is exposed to monsoon wind. The owner wants to reuse the old wall plate to save money. The tile supplier says the tile weight is about 45 kg/m². You are asked to inspect the roof before the tiles are loaded.
1. Evaluate the roof structure and decide whether the old wall plate can be reused.
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Do not reuse the soft, stained rear wall plate. Softness and staining suggest decay or termite damage, and a wall plate carries the rafter loads and ties the roof to the walls. A locally soft section can crush under the rafters, and a splice in a wall plate is a structural alteration that needs design. The owner's saving is small compared with the cost of roof failure.
What to do: probe the plate; if the damage is isolated and the timber is sound elsewhere, replace the affected length with matching treated timber and join it over a support with a proper splice detail approved by the engineer. If the plate is generally sound but the bearing is uneven, level it with a mortar bed or packing that will not compress.
Separate checks: the plate must be strapped to the wall, the rafters must be fixed to the plate against wind uplift, and the plate must be treated for termites. Reusing a rotten plate undermines all of those. The decision should be recorded and shown to the owner, not hidden.
2. Check the rafter and purlin arrangement and specify the missing structural and wind-uplift items.
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The spans must be checked against a span table for the actual species and grade, not assumed from the member sizes. Kumbuk is strong, but 50 mm by 100 mm rafters at 600 mm centres may be near their limit depending on the purlin spacing and tile weight; the 450 mm purlin spacing may be too wide for the tile gauge and wind load, or it may be acceptable, but it needs verification. The 25 mm ridge board is a spacer, not a structural ridge beam; it does not support the rafters.
Missing items: rafter ties or ceiling joists to resist outward thrust, diagonal bracing in the roof plane, wind bracing at the hip, galvanised straps or hurricane ties at every rafter-to-plate connection, and a continuous tie-down from plate to wall. Also check purlin laps and fixing, and the fascia and barge details. On an exposed Negombo site, uplift can exceed the weight of the tiles, so straps and bracing are not optional. Refer the sizing to the engineer or a recognised span table before loading tiles.
3. Plan the sequence for setting out the hip rafters and the checks you would make.
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Set out from the plan, not by eye. Mark the ridge line at the centre of the 8.0 m span, with the ridge length equal to the difference between the length and width (10.0 m - 8.0 m = 2.0 m) for a regular hip. Mark the hip positions at the four corners. Calculate each hip rafter from its plan run (half the span in each direction) and the 30° pitch using Pythagoras, then add the correct allowance for the ridge and the birdsmouth; do not step it out on the roof and hope. Cut one hip as a pattern, trial-fit it, then cut the rest.
Checks: diagonals of the wall plate equal; ridge level and straight; hip rafters in the same plane as the common rafters; crown of each rafter up; birdsmouth not cut deeper than one-third of the rafter depth; purlin lines straight and at the correct spacing; jack rafters cut to the correct lengths on both sides. Check the first course of tiles for gauge and alignment before loading the whole roof.