Wednesday, July 15, 2009

Be Nice To Your Historic Brick!

One of the members of our historical society missed our June fireplace/chimney workshop and contacted me via email with a follow-up question about the best way to clean mildew/algae/moss from historic bricks (a common problem around New Castle). It affects a wall of her house that borders a shady alleyway. She also asked if using a sealer on brick might be a good way to keep water from driving rains from penetrating her home’s brick walls.

She suggested that I add my response to the blog - a great idea! So…here’s a summary of my response to her:

In general, for historic brick it is always best to use the gentlest means possible for cleaning. That usually means just water and a natural bristle brush. You can try that if you haven't yet, but sometimes staining doesn't respond to that so here are a few solutions that our mason recommends (listed from gentlest to harshest – though all should be safe for historic masonry):

Vinegar and water – though he didn't give me a ratio for this - I'd start with one part vinegar to one part water and increase the vinegar as necessary.

Bleach and water - start with one cup of bleach to one gallon of water. Increase bleach slowly if necessary.

Muriatic acid and water - one part muriatic acid to 10 parts water (Remember when mixing these together to add the acid to the water. Don't add the water to the acid or else it might boil and spit acid on you. Ouch!) FYI...The masons used this solution to wash down our chimney at the Amstel House to remove excess lime from the bricks and mortar, and also to clean the salvaged bricks before using them.

Whichever of these solutions you try, its a good idea to test it first on a small section of the wall and let it weather for a while before going on with the whole project. Always better safe than sorry!

Whenever a discussion of cleaning brick comes up It’s worth noting that there are two cleaning methods that should always be avoided - powerwashing and sandblasting (which is about the worst thing). These methods are generally too harsh for historic brick, and will destroy the hard outer surface of the brick. Don’t let anyone talk you into using these methods on your historic brick (or any other historic materials for that matter!).

Regarding the second question…It's almost always a bad idea to apply any type of exterior sealer or coating (including paint) to historic masonry. Historic masonry - particularly if it was built with lime mortar, needs to breathe to allow water vapor to pass through the masonry to the exterior of the house. Water gets inside walls from a variety of sources including driving rain, condensation, rising damp, etc. I should mention that an exterior sealer will keep driving rain out - but it won't do anything about water in the walls from condensation or rising damp - it just makes those issues worse.

Any water in the wall needs to get out of the wall somehow - normally that's through the exterior mortar joints. If you seal up the exterior surface the water will escape through the next easiest path - usually that is through your interior wall surface which is typically plaster or drywall. So you'll eventually start to see plaster failure on walls that have an exterior sealer. The same thing can happen if you re-point your historic brick with a portland cement mortar instead of a lime mortar since portland cement is so hard (it also will cause your historic bricks, which are softer than the portland cement, to fail).

In looking around the web, you may find that there are some sealers that claim to be breathable (usually these are siloxane-based sealers). They look promising, but don't have a long enough track record to recommend them on historic buildings yet. One of these was actually applied to the garden house at the Amstel House and I'm pretty nervous about it - I'll let you know how it's going in about 10 years. One problem with all sealers is that once they are on the brick they are not reversible. Any sealer that is silicone-based is one that should be avoided at all costs.

If you have any painted woodwork on the wall that has been affected by mildew, a safe cleaning mixture is one cup bleach to one gallon of water. That's very diluted so if it doesn't work right away you can add more bleach a little at a time. Again use a natural bristle brush to do the cleaning. I've used this on the windows at the Amstel House with good results - though I need to do it again (ugh!).

If there is any way that you can trim branches to allow more sunlight into the area or increase the airflow to the alley that might help retard future growth. Also, it might be helpful if during the next heavy rain you take a look outside to see how the water is draining away from the area, if you are getting any splash up onto the walls from roof or gutter water, of if the gutters are working correctly.

Finally, here’s a link to an article from the National Park Service on cleaning historic masonry ( It should help with the basics though I don't think it covers mildew/algae specifically. The Park Service has a whole series of these articles about various maintenance topics for owners of historic houses. You can see and download them all for free at

Always remember...Be Nice To Your Historic Brick!


  1. what a totally inept blog. Please dont write rubbish about something you dont have any knowledge of. Telling someone to use bleach on historical buildings should be a hangable offence

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  3. just a quick reply on the sealer issue,,,, they have come a long way from the topical barrier filming that is safe to use and user friendly..."water based silane,siloxane blend penetrating repellent which is 100% vapor breathable, in return moisture can escape from interior masonry through substrate while still protecting exterior surface from damaging elements... this is something of concern if living on the east coast..due to having numerous rapid freeze weather cycles which damage brick and mortar by thermal expansion. this sealer type is a saturate applied sealer, if applied correctly over "sound masonry" meaning any repairs should be addressed before appication..tested to give sevice life of approx 10-15 years...i use few hundred gallons a year for masonry restoration and preservation..the main thing is to use as conditions and temps are big factor in its effectivness...most of my projects that i have done in repair etc...could of been avoided if they were treated with a repellent....on a second note topical filming sealers simply "dont work" they do seal alright.. the entrap moisture into the walls and it will create damage rott to the masonry and migrate into the interior of walls to sum it up know what ur doing and what ur using it for..."siloxane/silane sealer repellents " are a great way of protecting the masonry new or old !!hope this info helps !! Bryan Mabius " owner and custom mason of mabius masonry serving the east coast for 25 years,worked in the masonry trade for over 27 years with over 1,000 projects completed.. proudly carrying on a family tradition for over 150 years !! best regards and search us on the web !!

  4. Thanks for the comments, particularly Bryan for your additional insight into siloxane/silane sealers!