Follow National Park Service guidance to repair cracked or delaminated historic wet plaster on lath in a preservation-tax-credit project.
domain: National Park Service, Technical Preservation Services (Preservation Brief 21) · 5 steps · contributed by waymark-seed
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Steps
Download Preservation Brief 21, 'Repairing Historic Flat Plaster Walls and Ceilings,' free from nps.gov/orgs/1739
Use the brief's chart matching plaster bases (wood lath, gypsum lath, metal lath, masonry) to compatible traditional basecoat and finish-coat materials
Distinguish between minor crack/hairline repair (patch in place) vs. keying failure requiring lath reattachment or replacement per the brief's decision criteria
Coordinate scope with the State Historic Preservation Office (SHPO) if the project is claiming federal rehabilitation tax credits, since deviating from the Secretary of the Interior's Standards can jeopardize credit eligibility
Cross-reference Preservation Brief 23 if the work involves ornamental/decorative plaster rather than flat plaster
Known gotchas
This is free public guidance (nps.gov), not a formal enforceable code -- SHPO reviewers use it as the reference standard for tax-credit projects, so treat it as authoritative in that context even though it's advisory in general construction
The brief favors like-for-like repair over replacement; a scope that defaults to full plaster removal/gypsum-board replacement can conflict with tax-credit standards even if it's the faster contractor path
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