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Low slope clay tile roof leaks (C) Daniel FriedmanWhite Efflorescence Stains on Slate, Clay or Concrete Roofs

Cause, Effects, Repairs

  • POST a QUESTION or COMMENT about white stains & deposits: efflorescence causes, significance, cures when found on building roofs

White efflorescence salts or stains on building clay, concrete, or slate roofs.

Salt Deposits, Brown & White Bubbly or Fluffy Crystalline Deposits Diagnostic & Identification Photographs.

This article series illustrates and explains white or sometimes reddish brown bubbly surfaces on walls and white powdery or crystalline deposits left on walls, especially masonry walls, by moisture - efflorescence.

Our photo at page top shows white efflorescence staining on the under-side of a clay tile roof in San Miguel de Allende, Guanajuato, Mexico.

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White Efflorescence & Other Stains on Clay, Concrete Tile or Slate Roofs

Slate effloresence (C) Daniel Friedman

White efflorescence, a light white or gray "powdery stain" can often appear on clay tile roofs, concrete roofs, and slate roofs as you can see in our slate roof photo here and on the clay tile roof at page-top.

Types of White Effloresence on Slate, Concrete, Clay Tile Roofing

  1. Age indication on slate roofs:

    Photo just above: on a slate roof round white deposits starting at the center of the slate and expanding towards its perimeter can be an indication of roof age and condition.

See those details

at SLATE ROOF LIFE EXPECTANCY

 

  1. Moisture-leaks on concrete or clay roofs:

    On concrete or clay tile roofs efflorescence may be a sign of moisture absorption, deterioration, or it may be due to use of concrete mortar used to set the roof tiles along a clay or concrete tile hip or ridge.

Unlike on slate, white effloresence stains on clay or concrete roof tiles doesn't usually threaten the remaining roof life.

Excerpting from "Hip and Ridge MOrtar Generated Surface Effloresence", from the Tile Roofing Industry Alliance TRIA -

Efflorescence is a temporary surface discoloration common to all concrete based building materials including mortar and concrete roof tiles.

It is superficial and in no way detrimental to the overall quality, structural integrity, or functional properties of the tile. [1]

  1. Other White, Red, Brown or Tan Stains on Slate or other Masonry Material Roofs

    White stains on clay or concrete tile roofs may be caused by natural weathering of mortar used to attach the tiles to the roof or mortar used as weather blocking at hips, ridges, valleys, and on some roofs, at the open ends of clay tiles

    Dark brown, black, and occasionally reddish or even green stains on slate, clay tile, or concrete tile roofs may be from algae.

    see ALGAE STAINS on ROOFS,

    Brown or red stains on slate may be due to oxidation of phyrrhotite or pyrite inclusions or iron sulfides in the roofing slates themselves. These inclusion occur depending on the quarry where the slate was mined and of course on individual slate variations. See the articles by Cárdenes Van den Eynde below on this page.

    If you see red streaks that appear to originate at a specific inclusion on a slate and that run down-roof from that point, I think you're seeing a combination of oxidation of an inclusion in the slate and the wash-down of iron orxides or sulfides from that point onto the surface of lower slates.

    Red or reddish brown or occasionally yellow stains on roofs,     found below a chimney, may be also due to rusting metal flashing or to run-down of chimney soot or smoke deposits.

    see at RUST STAINS on ROOFS.

    and STAINS on/near CHIMNEYS

    Other roof stain colors may be made worse or changed but not removed by use of the wrong cleaner or chemical on the roof surface.

    Watch out: At the reader Q&A below on this page we discuss with a reader stains left on her slate roof after they applied a rust stain remover.

    We don't have enough details but it's possible that the reader was trying to remove iron stains that occur as a result of weathering natural iron or other ore inclusions found in some slates.

    Doing so is not going to remove those "red spot" (for example) and risks damaging the roof and shortening its life.

Slate Standards Performance & Deterioration:

Effect of Using Acid Cleaners on Slate or Tile Roofs

White stain diagnosis on a clay roof tile in Mexico (C) Daniel Friedman

 

Cures for White Stains on Roofs

Now found at

at EFFLORESCENCE REMOVAL & PREVENTION - What steps should I do to remove efflorescence from my building?.

and

at WHITE STAINS on ROOFS - includes more References for Cause & Cure of White Calcium or Mineral Stains on Roofs

...


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: using an iron / rust stain remover on our natural slate roof left white "efflorescence" that won't come off

While cleaning a natural slate roof we applied a rust stain remover. Once the rust remover effectively removed the rust we now have various areas of efflorescence where the remover was applied.

We tried vinegar and water with no results. We tried an efflorescence remover from powerwash.com with no results. Any suggestions? - Amanda Dooley, 2022/09/24

This question was posted originally

at EFFLORESCENCE REMOVAL & PREVENTION

Moderator reply:

@Amanda Dooley,

I've been thinking about your question for a time before trying a reply as I don't have a great answer. But in researching the effect of an acid cleaner on slate roofs we did find some information that I'll share below.]

  1. What was the source of "iron stains"

    on your roof that you found a concern. Iron staining from rusting metal flashing or from a chimney byproduct is completely different from iron inclusions in the slate itself .

    Watch out: some slate contains iron inclusions, iron sulphides, including pyrite (FeS2) which can be oxidized by acids (hence using an iron remover) BUT if you dissolved inclusions in the slate you could have left it seriously-damaged, even with holes or thinning that could reduce the slate's remaining life.

    Particularly if there are iron (Pyrrhotite) inclusions in the slate that are quite thick, dissolving them could leave a thin area or in the most-extreme cases a hole in the slate. (Cardenes 2013)
    So we need to know more about your roof.

    It would be most helpful to see some sharp photos (one per comment) both from a distance and up close of one or two slates so that we can see what you're seeing.
  1. What are the slate roof's environment, location, condition:

    Second I need to know the location of the building (country & city) and age of the slate.

    And if your roof is true slate, not a manufactured product, we'll want to see if your slates had iron inclusions.

    Watch out: other readers: Amanda's roof is "natural slate" - but if your "slate" roof isn't true slate ( a mined stone product) but is a manufactured slate such as made by Eternit your acid could have actually removed a coloring agent from the roof material. That's not going to be cured by further cleaning
  1. Was your Iron Remover Approves as Safe for Slate Roofs & applied as directed?

    I see that the Rust Remover Plus product sold by power wash (cited below) whom you cite, is intended to be used with a power washer.

    I would not recommend power-washing a slate roof - but, then, perhaps you didn't apply a high-pressure spray to the roof: do tell me how you applied this chemical.

    The manufacturer's instructions are simple:

    1. Rust Remover Plus™ is a concentrated rust stain remover. It can be applied directly, or diluted with water up to a 1:5 mixture.

    The exact dilution ratio will depend on the severity of the rust stain.

    2. Apply product with a pump up garden or chemical sprayer using a cone or fan tip.

    Apply evenly with overlapping passes to insure uniform coverage. Thoroughly wet area to be cleaned with product.

    3. Allow a 5 to 10 minute dwell time for product to soak into surface.

    When using product for rust stain removal, do not proceed to step 4 until stain disappears. Reapply if necessary.

    4. Pressure wash using high pressure with a 40 degree tip. Use overlapping passes approximately 6 inches from the surface.

    5. Allow surface to dry before evaluating results. Product brightens concrete as it dries.

    Do not allow product or overspray to come into contact with glass, metal or tile surfaces. Protect these areas from contact.

    Always perform a test in an inconspicuous area to determine results achieved before using on a wide scale basis.

    Allow surface to dry prior to evaluating results.

    A one-gallon jug of Rust Remover Plus™ will cover approximately 150 to 300 square feet of concrete when used straight.

    - IS THAT how you applied the chemical? Including testing it on a small area first?

  1. What's in the chemical you used and what might be its effect on slate?

    Ammonium Bifluoride
    Phosphoric Acid
    Oxalic Acid

    Basically acids

    Here is the
  1. Did you use high pressure power washer?

    If you used a pressure sprayer you may have soaked the slates more than would normal rain or snow, thus speeding the leaching of minerals to the slate surface.
  2. About using Vinegar to try to Get Rid of Efflorescence on a slate roof

    Vinegar will dissolve many surface efflorescent salts but if the white is actually in the body of the slates - as you'll see in the 2 articles I cite below, it is not so easily-removed.

    But we don't know yet if the white deposits you see are mineral efflorescence!
  3. What else might the white stains be?

    It is possible that the acid in the chemical you used (perhaps not intended for use on slate roofs) not only will release iron oxide but also carbonates that may be what you're seeing as white discoloration that led you to our Efflorescence articles.

    Watch out: If so, you may have damaged the slate - though the extent and effect on remaining roof life no one with any sense would predict based merely on some text. The process, among historic preservationists who work on slate, is gypsification.
  4. Efflorescence on slate can be an indication of slate roof age and wear - as we discuss at

    SLATE ROOF DEFECTS https://inspectapedia.com/roof/Slate_Roof_Defects.php

    and at

    SLATE ROOF LIFE EXPECTANCY https://inspectapedia.com/roof/Slate_Roof_Life_Expectancy.php
  5. Cleaners that may work to remove remaining stains on your roof:

    I've emphasized that once we've established that the stains of concern on your specific roof are NOT normal effloresence, we need to have an idea of the chemistry of the stain before choosing a stain remover. For example iron stains may respond to iron stain removers but not to other cleaners.

    Watch out: using acids on slate and other sensitive stone materials may damage the roof. Don't use any cleaner product before reviewing its manufacturer's instructions as they apply to your roof.

    At EFFLORESCENCE REMOVAL & PREVENTION we discuss some cleaning products that might be used safely on acid-sensitive stone materials such as slate.

    So if you are up to answering my blizzard of technical questions we can continue to figure out what happened to your slate roof and what we can do about it.



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