Subscribe or Renew Membership Subscribe Renew

mold/mildew on red cedar shingle siding exterior of house

745sarachal's picture

We built our own house several years ago and found an article in Fine Homebuilding magazine about cedar shingle siding.  We contacted the author of the article and hired him to teach us how to shingle our own house.  It was great.  We used Cabot - Clear Solutions - Weathered Gray as our stain.  Mind you, this was about 15 years ago.  We haven't washed our house down in about 6 years, and slowly, over time, mold/mildew has attached itself to our once beautiful red cedar shingles.  Our house is UGLY.  We are stripping it with various products to get rid of the mold and the stain as we must get down to bare wood in order to put a new stain on the house.  Should we use an oil based product or a water based product?  We love the look of the shingles but the possibility of new mold growth is not acceptable.  About 80% of our house is surrounded by a covered porch so UV exposure isn't the biggest concern.  Mold/mildew is the biggest concern.  We live in Sonoma County, CA which is about one hour Northeast of San Francisco.  Any thoughts?

Cabot studiously avoids (post #215964, reply #1 of 15)

Cabot studiously avoids saying whether that stain is water or oil.  Which is is?

You don't necesssarily need to get down to bare wood.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

mold/mildew on red cedar shingle siding exterior (post #215964, reply #3 of 15)

Thank you DanH.  The Cabot product used was oil based.  Clean up was with mineral spirits.  When you say that it's not necessary to get down to bare wood - most products I've looked at require that all prior product be removed for the new product to adhere properly.  Any suggestions?  Do you have much experience with cedar shingle siding?  If yes, what products do you use?  We live in CA so VOC can be an issue.  Thank you.

Completely removing the old (post #215964, reply #5 of 15)

Completely removing the old finish is only necessary if either the old or new coat is a paint rather than a stain.  Of course some products claim to be one and are really the other, so it's a touchy topic.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

 Because cedar shingles are (post #215964, reply #2 of 15)

 Because cedar shingles are rough and porus it makes a fertile breeding ground for  mold and mildew which will grow anywhere moisture is present. Try Oxyclean to get the mold off then figure on washing it every year to keep it looking n

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

mold/mildew on cedar shingles siding (post #215964, reply #4 of 15)

Hi Florida.  Thank you for the info.  We have been told that we have to get back to bare wood in order to put a new product on our shingles.  We live in CA so VOC is an issue.  The Cabot product we used was oil based.  Would a water based product be a better idea to keep the mold/mildew at bay?  I tried Oxyclean but it didn't remove the old Cabot Stain.  Thank you.

Oxyclean won't remove your (post #215964, reply #9 of 15)

Oxyclean won't remove your stain but will remove and kill the mildew on your house. Trust me, you do not have lichen, you have mildew. I've never heard of having to remove to bare wood and I wouldn't do son on my own house. Since you can't use oil your only choice is acrylic. A pressure cleaner, used very carefully with the grain will remove most of the stain but start slow if you go that route.  

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

Acrylic - any brand you recommend? (post #215964, reply #11 of 15)

Thank you.  I presume you looked at the photos of our house.

Are you saying that we "can't use oil" because we will have the same problem down the road?  When you say "acrylic", is there a product that you recommend?  Is there a water based, semi-transparent "stain" that will allow some of the wood characteristics to show through?  Is Benjamin Moore Arborcoat any good for this job?

My husband says that we have to remove the stain to remove all the mold because in areas that we cleaned with a "stripper", there are spotty remains of grayish discoloration.  We presume that any residual mold in the wood must be killed before we put any new product on the shingles. 

Any additional help you can provide will be greatly appreciated!!  Is there some way I could talk with you?

Thank you.

 

One thing I'm not sure you (post #215964, reply #6 of 15)

One thing I'm not sure you appreciate is the difference between mold/mildew (fungus) and moss/lichen.  Encountering mold on exterior surfaces is actually fairly rare (at least in these parts).


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

photos of our cedar shingles (post #215964, reply #7 of 15)

Well, you're right.  We have no idea if this is mold/mildew or imbedded dirt or moss/lichen.  We can't tell if it's growing on the stain, or on the wood.  Generally, once we get all the stain stripped off, they look pretty good.  I've put undiluted Clorox on a few areas and it gets rid of most of the bladk stuff  - but not all of it.  It still looks like there are dark areas on the cedar even after I clean it.  I hopefully correctly attached some photos.  We live in California about one hour Northeast of San Francisco.  Before I forget - I really like your Herman Melville quote. 

The photos show the ugliness of our house.

Thank you.

PreviewAttachmentSize
_MG_4997_copy.jpg729.26 KB
_MG_4998_copy.jpg894.59 KB
_MG_5003_copy.jpg938.38 KB
_MG_5006_copy.jpg1.03 MB

Are those pictures AFTER (post #215964, reply #8 of 15)

Are those pictures AFTER you've used some sort of cleaner/stripper?  Did you scrub?

Mold/mildew/moss/lichen would exhibit one common characteristic: being more concentrated on certain spots of the wall, based on exposure to weather and sunlight.

Your finish looks rather evenly discolored.  Unless this is a result of your cleaning process, I would say the discoloration is due to the oxidation of iron and other minerals that are naturally found in red cedar and redwood.

If oxidation is the cause of the discoloration, then a cleaner that uses oxalic acid (many "wood bleach" and deck cleaner products) will lighten things up significantly, while requiring very little scrubbing.  (Regular chlorine bleach, while effective on mold and, to a lesser degree, moss will have very little effect on oxidation-caused discoloration.)

If you do use oxalic acid, be careful to keep it away from any anodized aluminum, and go a little easy on the cleaning, as wood in the redwood/ceder category gains much of it's durability from the minerals in it that ox acid will wash away.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

BEFORE pictures - not after (post #215964, reply #10 of 15)

Thank you for your response.

The photos are BEFORE any cleaning.  This is what happened to our house after too many years of not washing it down.  The Cabot stain is still on the house - along with all the mold/mildew. 

I need to know if another oil based product is going to result in the same problem.  From what I've read, a water based product will be less likely to support the growth of mold/mildew, although the water based products will not last as long as the oil based stuff. 

At this time, I just know that we don't want to have this happen again.  If we need to go with water based products, and they require more frequent application, we can live with that.

Thank you.

Once again, the staining seen (post #215964, reply #12 of 15)

Once again, the staining seen in those pictures does not seem to fit the pattern for mold/mildew.  (Though it's impossible to say with certainty without a closer examination.)


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Jeeze, once again you step (post #215964, reply #13 of 15)

Jeeze, once again you step into the void. That looks exactl;y like mildew. My experience is washing well over 10,000 houses,

miles of sidewalks and driveways and acres of roofs. What's yours?

Florida Licensed Building Contractor, 50 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

My experience is watching (post #215964, reply #14 of 15)

My experience is watching people claim it was mildew when in fact it was efflorescence or oxidation or something else.  I've seen this several times, and there have been at least a couple of cases pictured here over the years.

A quick test for mildew is to sponge on full strength chlorine bleach.  If there is a major color change in 30-60 seconds, without scrubbing, then it's probably mildew.  A slight change, with some scrubbing, after a couple of minutes then it's probably moss/lichen.  If the bleach has little effect (without vigorous scrubbing) then it's probably some sort of chemical discoloration.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

The best approach is try (post #215964, reply #15 of 15)

The best approach is try various washes and options on a concealed or less conspicuous area. Start ith the least abrasive  and work your way up.