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Mold on exterior cladding and decking

qthisup | Posted in General Questions on

I’ve recently visited a holiday cabin we finished last summer. Unfortunately I’ve discovered a lot of what appears to be surface mold on various surfaces on the exterior of the building. There are lots of articles here on GBA regarding interior mold/sheathing mold etc but I couldn’t find anything on this topic. 

There is Cedar T&G cladding treated with Osmo UV and locally milled 4×10 Douglas Fir beams treated with the same product, both showing significant mold. This product was chosen to maintain the natural wood colour.

In addition the Cedar decking, posts and stringers also have significant mold growth. These surfaces have been treated with Sansin Wood Sealer to provide some protection while allowing the wood to ‘silver’ naturally. 

Climate 4C – specifically Sunshine Coast of BC, Canada

Photos attached.

Is there anything I can/should do to prevent this? 

Any product suggestions for cleaning the surfacing without damaging them? 

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Is there anything I can do to prevent this?"

    A. Either cut down all the trees around your house to encourage sunlight and wind to access the house, or lift the house off its foundation and move it from British Columbia to Manitoba.

  2. Jon_R | | #2

    Wet&Forget has worked for me.

  3. Peter Yost | | #3

    Hi Quinn -

    You might find some insight from this article:

    It looks to me as though your mold is growing on the surface and not penetrating to your cladding; the mold could be "feeding" off of available oil in the finish, or just surface "dirt" that is on the finish. In any event, the simplest thing is just to scrub the mold off where you can.

    I see here that the manufacturer of OSMO UV has a product that they claim prevents biological growth, but I am certainly not vouching for this product but probably worth contacting them about your situation.

    Best - Peter

  4. Expert Member


    I live on Vancouver Island. I have mold on my north side cladding, mold on the underside of my roof sheathing, mold on the soffits of my shed, and the road outside my house has moss over the whole surface where it is sheltered by trees. I've never found any satisfactory solution, but I've also never found it to be anything more than an aesthetic concern.

  5. qthisup | | #5

    Thanks for the feedback.
    Sounds like it is something we'll need live with, perhaps only coming out in winter when there is so much moisture around and no sun.
    Peter - I have indeed followed up with both stain manufactures as they are suppose to be mold resistance. Thanks again.

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