Surface water absorption and total water absorption are separate gypsum-panel results. A favorable result for one does not establish the other, and neither should be assigned to a loose fiberglass facer. When a facer or board construction changes, keep the tested panel, evaluated side, method, unit and requirement attached to each result.
For a board-development team, the practical decision is whether the evidence addresses the revised panel’s surface-water question, its total-absorption question, or both. Start with those two questions rather than reducing the review to a single description such as “moisture resistant.”
This guide concerns evidence for manufactured glass-mat-faced gypsum panels. The named product example is a gypsum roof board; its specifications are not presented as requirements for every plasterboard, sheathing panel or backing board.
1. Separate the panel evaluations before comparing numbers
ASTM C473-24’s public scope separately lists a water-resistance evaluation for core-treated water-repellent gypsum panels and a surface-water-resistance evaluation for panels with water-repellent surfaces. The distinction is between two panel evaluations, not simply two names for one result. [1]
Two object boundaries matter. A surface result concerns the evaluated surface of a panel specimen; it is not automatically a measurement of an unbonded facer. Equally, the reference to a core-treated panel does not turn the other evaluation into a test of bare gypsum core alone.
In a comparison record, give the results separate entries. For the surface entry, identify the evaluated side. For the total-absorption entry, retain the panel-specimen identity and reported mass basis. Read the actual method and report rather than assuming either object from a short spreadsheet heading.
Only the public method scope is used here. It does not supply an exposure procedure or acceptance limit. The article does not establish equivalence to another method or determine which specification applies to a European project.
2. Read a manufacturer’s two rows without merging their meaning
Georgia-Pacific’s DensDeck Roof Boards Technical Guide provides a concrete example. Its DensDeck Prime physical-properties table lists water absorption as a maximum percentage and surface water absorption as a nominal quantity in grams. The product overview also identifies face and back surface performance separately from total absorption by weight. [2]
The useful lesson is the separation of the fields, not the transfer of that manufacturer’s values. The rows differ in the property named, the unit and the stated nature of the value. A nominal entry should not silently become a maximum limit, and a manufacturer’s specification is not a measured result for a GRECHO sample.
Keep the guide’s row-specific footnotes when extracting information. This article does not reconcile those references into a test procedure. An unclear method reference should go back to the manufacturer or testing team for clarification, not be replaced with a guessed section number.
For the wider distinction between nominal figures, limits and sample results, use the existing guide to reading facer technical-data values.
3. Grams and mass percent are clues, not a conversion route
A quantity in grams and a mass percentage have different reporting bases. However, the unit alone does not reveal the specimen preparation, exposed surface or complete method. Those details must come from the accompanying record.
Even where enough information exists to express a water quantity relative to a specimen mass, the arithmetic does not change how the specimen was exposed or which property was evaluated. Converting units is not the same as converting a surface-water evaluation into a total-absorption evaluation.
The same caution applies to comparisons within one column. Two entries marked “surface water absorption” may still refer to different sides or panel revisions. Match the evaluated side, construction and method before interpreting their difference.
A useful question to send with an incomplete result is: “Which panel revision and side does this result describe, and where are its method, reporting basis and applicable requirement recorded?” That is more precise than asking only for a better water-resistance number.
4. Review the facer change against both evidence needs
The following table is an editorial review aid, not a standardized test protocol. Use it to identify the next information needed for the particular panel under development.
| Information received | What to match | Useful next decision | Conclusion not supported by this alone |
|---|---|---|---|
| A surface-water result reported in grams | Panel revision, evaluated face or back, method and exposure record | Assess that identified surface against its relevant requirement | Total absorption, the other side’s result or a waterproof assembly |
| Total water absorption reported as mass percent | Panel-specimen identity, reporting basis and method | Review the total-absorption item separately | Automatic satisfaction of the surface-water item |
| A nominal surface entry beside a maximum percentage entry | Original row labels, footnotes and whether each entry is a specification or test result | Clarify the requirement and evidence needed for each field | That both entries are guaranteed limits or observed trial results |
| A revised facer with incomplete core or reverse-side records | The actual old and new panel constructions | Resolve the construction difference before attributing a changed result | That the facer, coating or bond caused the difference |
| Data for loose fiberglass mat only | Whether the tested object was material or a prepared panel | Keep material screening separate and identify missing panel evidence | Either result for the manufactured gypsum panel |
| A face-surface record but no back-surface evidence | Which sides the applicable panel requirement covers | Ask whether an additional side-specific result is needed | That both surfaces have the same behavior |
| A favorable surface record with the total-absorption item absent | The requirement and evidence status of the missing item | Ask the board/testing team how the gap should be addressed | Unrestricted panel, storage, installation or mold-performance approval |
Illustrative document example—not a GRECHO trial: a revised board has a face-surface record, while its total-absorption sheet still identifies the previous panel revision. The review has two documents, but they do not yet describe the same construction. The next step is to establish whether the earlier evidence applies, not to mark both items complete because both files are present.
5. Separate an evidence gap from a material diagnosis
A missing result is a documentation or evaluation gap. A difference between two results is an observation. Neither, by itself, identifies why the board behaved differently.
For a facer-only comparison, confirm what actually remained unchanged in the panel. Where other construction or preparation details also changed, keep that information visible instead of assigning the outcome to the facer. Do not infer that a clean outer surface proves the required core-facing connection or drying outcome.
Manufacturing-process questions belong to the separate paper-to-glass-mat gypsum-line qualification guide. Broader exposure questions are covered in the moisture-related gypsum facer review. The present task remains narrower: identifying which of the two panel water-absorption questions each record answers.
6. Request the missing panel evidence precisely
For a focused discussion, share the panel or development reference, current and proposed facer references, the side being changed, and the construction information that can be disclosed. Include both result descriptions in their original units, with the method/version, specimen conditions, document date and nominal/limit/test-result labels.
State the intended use and the specific requirement still under review. Confidential formulations are not necessary merely to explain that a face-specific result is available while another panel item remains unresolved.
GRECHO supports application-matched facer discussions, sample review and technical-document coordination. The responsible board manufacturer and testing team determine the applicable evaluation and acceptance basis. Explore gypsum board facer solutions or request technical-data coordination.
Frequently asked questions
Can a favorable surface-water result replace total water absorption?
No. Treat them as separate panel items. Establish what the available result measures and whether the requirement also calls for the other evaluation.
Why can one gypsum-board datasheet use grams and mass percent?
The entries may describe different properties and reporting bases, as in the named roof-board example. Read their labels and footnotes together; unit conversion does not make different evaluations equivalent.
Is surface water absorption a test of the loose facer?
Not in the panel evaluations discussed here. Identify the prepared panel and evaluated side. A loose-mat result should retain its material-level identity.
Does a changed absorption result identify a defective facer?
Not by itself. First establish the actual construction and method differences. The observation does not isolate a cause or supply an acceptance criterion.
Must the face and back be identified separately?
Identify the side each surface record addresses. Whether both sides require evaluation depends on the applicable panel requirement; do not assume one side’s result covers the other.
Sources and scope
[1] ASTM International: C473-24, physical testing of gypsum panel products. Public abstract and scope only; the full procedure was not used.
[2] Georgia-Pacific: DensDeck Roof Boards Technical Guide, product overview and notes on pages 2–3; DensDeck Prime table and footnotes on page 18. The example remains specific to that manufacturer’s roof boards.
The comparison table and illustrative document example are editorial review aids, not reported GRECHO test results. No absorption limit, exposure procedure, waterproofing claim or equivalence between board-product specifications is established here.
