Third-Layer Overlay on a Three-Unit Building With a Low-Slope Porch Roof
You are the estimator for a licensed roofing company in Quincy. The owner of a three-story, three-unit wood-frame building built in 1928 wants the cheapest possible reroof before spring. The main roof is 6:12 with two existing layers of three-tab asphalt shingles. The rear porch roof is 3:12 with failed roll roofing. The masonry chimney is 36 inches wide and carries painted aluminum counterflashing. The owner wants a third layer of architectural shingles over the main roof, reuse of all existing flashing, and the same shingles on the porch. She will not pull a permit and says the last roofer 'did it without one.' On the walk you find the 1x6 plank roof deck is sound but gapped up to 1/2 inch, aluminum drip edge is missing at three rake edges, and the attic has only two gable vents, partly blocked by stored furniture. You must decide what you can legally and practically sell her.
1. Can you install a third layer of asphalt shingles over the two existing layers? Explain the governing limits in Massachusetts and state the minimum slope for asphalt shingles and what changes at 3:12.
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No. Three layers is not a defensible installation here.
Code basis. The roofing-replacement provisions adopted through 780 CMR (the IRC R907-R908 series) limit how much existing covering may remain. Recovery is generally permitted over no more than one existing layer, and only where the existing covering is sound, flat, and compatible, and where the new covering can be fastened and flashed correctly. Two layers already exist, so a third exceeds the recovery allowance and also stacks dead load: roughly 2 to 2.5 psf per three-tab layer plus 3 to 4 psf for architectural shingles, on 1928 rafters that were never analyzed for it.
Practical failures on this job. You cannot install the required ice-dam membrane at the eaves over existing shingles, you cannot inspect or re-nail a gapped plank deck, and you cannot rebuild chimney and rake flashing from the deck up. The manufacturer will also void the shingle warranty over two layers. The correct scope is full tear-off to the plank deck.
Slope. Asphalt shingles are permitted down to 2:12. Between 2:12 and 4:12 the underlayment must be doubled (two layers, lapped per the code table) rather than a single layer. So the 3:12 porch is not below the code minimum, but it sits in the doubled-underlayment band and most shingle warranties require 4:12 or steeper for standard installation. A self-adhered modified-bitumen membrane is the better judgment call on that porch.
2. Specify the eave and edge detailing and address the attic ventilation deficiency, explaining why each item matters.
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Ice-dam protection. Massachusetts amendments require a self-adhered ice-dam protection membrane at the eaves, applied directly to the deck and extending from the eave edge to at least 24 inches inside the interior wall line. Install it also in valleys and along the chimney sides. This can only be done after tear-off, which is another reason overlay is not an option.
Drip edge. Drip edge is required at eaves and rake edges. At the eave, the underlayment laps over the drip edge flange; at the rake, the drip edge laps over the underlayment. The three missing rake sections must be added, since water tracking behind a shingle edge at a rake is a common rot path on this vintage of building.
Ventilation. Two partly blocked gable vents provide neither balanced intake nor adequate net free area. The code baseline is 1 square foot of net free vent area per 150 square feet of vented ceiling area, relaxed to 1/300 where a balanced intake/exhaust system is provided or a vapor retarder is detailed. Gable vents alone rarely achieve balance. The practical answer is continuous ridge vent plus continuous soffit intake with baffles to keep insulation out of the air path, sized to the 1/300 or 1/150 ratio. Since you are already stripping to the deck, doing this now avoids trapping the moisture problem you would otherwise inherit.
3. The owner refuses a permit. How do you handle permitting, licensing, and inspection sequencing on this building?
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Reroofing this building requires a building permit, and the sequence is part of the job, not paperwork.
Who pulls it. This is a three-unit building, so it is not the owner-occupied single-family situation in which a homeowner may take out the permit for their own dwelling. The permit should be applied for by the licensed roofing contractor, and the contractor must hold the appropriate Massachusetts roofing license for the scope. Working without a permit exposes both parties: the owner to enforcement and disclosure problems at sale, the contractor to cease-and-desist orders, fines, and license discipline.
Inspection points. Call for the deck inspection after tear-off, re-nailing, and sheathing repair but before underlayment, so the building official can see the plank deck condition and any replacement. Then a flashing/ice-dam inspection before shingles go on, specifically at the chimney and the porch transition, and a final inspection after completion.
Client conversation. Do not frame the permit as optional. Explain that the permit is what documents the ice-dam membrane, sheathing repairs, and flashing that a future buyer's inspector and lender will look for, and that no legitimate manufacturer warranty registration or insurance coverage attaches to unpermitted work. If she still refuses, decline the contract in writing. A verbal 'the last roofer did it' is not an indemnity.