In the US, Marine is a grade in PS 1, the standard for structural plywood. A Marine panel meets all the Exterior rules, uses only Group 1 Douglas-fir and western larch veneers, has inner plies of B grade or better running full length and width, and has no filled gaps.
PS 1's Marine rules
- Grades: A-A, A-B, B-B, High Density Overlay (HDO) or Medium Density Overlay (MDO), each meeting the Exterior requirements.
- Species: Group 1 Douglas-fir and western larch veneers only.
- Faces: an A face can have no more than nine wood repairs in a 48" x 96" sheet, and repairs are bonded with an Exterior adhesive using heat and pressure.
- Inner plies: B grade or better, full length and width. Where a ply is made of more than one piece, the edges are straight and square and don't overlap.
- Gaps: no crossband gap or edge split wider than 1/8" at either panel edge, no more than four per 96" of crossband, and none of them filled.
Exterior and Special Exterior
An Exterior panel in an ordinary grade such as A-C or B-C can have C grade inner plies, where Marine calls for B grade or better. PS 1 also has a Special Exterior grade, which meets every Marine requirement except the species rule and can be made from any species the standard covers.
The face and back letters work the same way as on other plywood. See plywood grades explained for what A, B and C veneer allow.
A marine panel's stamp
Roseburg's AB Marine panel has the end stamp AB-EXT-PS 1-Marine-PTL-#, and Roseburg's brochure lists an A grade Douglas-fir face, a B grade Douglas-fir back and a core of B grade or better western softwood, and calls it a PS 1 panel with an exterior rating.
BS 1088
BS 1088 is a British Standard for marine plywood, a separate document from PS 1. Its current edition, BS 1088:2018, covers two classes, standard and lightweight, intended mainly for building marine craft and for other marine and waterway uses. BSI's summary says the standard bases its choice of veneer species only on density and resistance to bio-deterioration, where PS 1's Marine grade names its species. The full text is sold by BSI, so for a panel sold as BS 1088, check the maker's data sheet for its species and construction.
Treated plywood is a different thing
Pressure treatment with waterborne preservatives or fire retardants is a separate process from the grade, and specialist treating companies typically do it rather than the plywood mill. Marine in PS 1 sets the veneers, species and construction, and the PS 1 rules for it don't include a treatment. For a treated panel, check its own label or tag.
Boats
For a boat, use the plywood the boat's plans call for.