Different properties
Water absorption, wet strength, dimensional change and frost resistance describe different properties. A board may tolerate moisture without forming a waterproof roof layer. Joints, edges and fixings also influence practical performance.
MgO is not a full specification
The term MgO board covers different binders and compositions. Danish research documents moisture and corrosion damage in investigated magnesium-salt-containing boards. This does not condemn every MgO board, but it is a reason to assess a specific product rather than a generic material name.
Even below the membrane
Construction moisture, condensation and later leaks remain relevant even when a board sits below waterproofing. Ask for product-specific tests, storage and installation conditions, and instructions for wet boards. A dry office sample does not replace that information.
Assess the specific board, expected exposure and construction details together.
Technical background: orbit.dtu.dk ↗
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