Decide when a silane/siloxane water-repellent treatment is appropriate for masonry versus when it will trap moisture and cause damage

Steps

1. Consult NPS Preservation Brief 1's guidance distinguishing water-repellent (vapor-permeable) coatings from waterproof (film-forming, non-breathable) coatings before considering any product
2. Confirm the masonry has no active moisture problem (rising damp, chronic leaks, high interior humidity driving vapor outward) before applying any repellent, since sealing the exterior face of a wall that needs to dry outward can trap moisture and accelerate freeze-thaw spalling or subflorescence
3. Where a penetrating repellent is genuinely warranted (e.g., protecting sound, dry masonry from wind-driven rain in a harsh climate), select a vapor-permeable silane or siloxane product rather than a film-forming waterproof coating or sealer
4. Verify the specific product's compatibility with the substrate mineralogy (some formulations perform differently on cementitious versus non-cementitious/siliceous materials) against the manufacturer's technical data sheet rather than assuming all silane/siloxane products behave identically across brick, stone, and concrete
5. Apply on a test panel first and evaluate appearance (darkening, sheen change) and water behavior (beading vs. absorption) before full application, consistent with Brief 1's gentlest/most-reversible-first philosophy
6. Recognize that water repellents are not a substitute for fixing the actual water-entry problem (flashing, grading, roof leaks) — document that the underlying source has been addressed, not just coated over