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Community and Q&A

Protecting Redwood siding

user-917907 | Posted in General Questions on

I have moved into a house with unpainted vertical channel-rustic redwood siding. Unfortunately, some of the siding extends down to within a few inches of the ground. In spite of there being a 2-foot gravel perimeter around the house, and in spite of there being a 2-1/2 foot eave overhang, rain splashes against the lower portion of the siding, and after 35 years it is showing signs of decay.

I am unprepared to replace the siding and so was wondering what my options might be? Recently the shingle roof was replaced with a standing seam roof. As a result the one to three feet of snow that used to stay on the roof now slides off, and over the course of the winter builds up enough to lay against the house siding. Because of the sliding snow I am reluctant to install a gutter, fearing that it would be torn off.

I’ve thought of attaching a 1-1/2′ horizontal strip of 20 mil black PVC pool liner or 50 mil EPDM rubber roofing sheet to the lower portion of the siding, to protect if from getting wet further, and hopefully stopping any further decay.

My question is whether loosely covering the redwood siding with a vapor-proof barrier would somehow further promote rot, in spite of this covering protecting the siding from getting wet directly? I could either attach the plastic strip directly to the siding, or I could attach the plastic strip to a 3/4″ thick batten, which in turn would hold the plastic away from the siding.

Any advice, warnings, suggestions?

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  1. GBA Editor
    Martin Holladay | | #1

    You don't want to install a vapor barrier on the exterior of your siding.

    There are lots of questions here. For the long-term benefit of your house, you need to lower the grade. Ideally, your grade will be at least 8 inches lower than the lowest wooden components of your building. Ten or 12 inches is even better.

    How you will repair your siding depends on the type of siding you have on your house. Is it horizontal siding (like clapboard) or vertical siding (like board-and-batten siding)?

  2. user-917907 | | #2

    Martin, the siding is unpainted, vertical, channel-rustic redwood. I agree, lowering the grade would be ideal, but not very practical in this situation.

    I'm hoping I won't need to "repair" the siding, I just want to keep deterioration from getting worse. I don't see the hanging of a water barrier in front of the siding as a vapor barrier because there will be a small gap below the sheet, and the channel-rustic siding provides 1"x3/8" channel for ventilation every seven inches. Plus, if I space the sheet out away from the siding by the thickness of a batten there will be even more ventilation.

    The area I am most concerned about is on the north (dark and damp) side of the house. On the south (sunny) side of the house I've already hung a section of water barrier for one year, with no obvious bad effects. But I am less confident that this method will work on the north side, so thought I would bounce the idea off the forum's members.

  3. GBA Editor
    Martin Holladay | | #3

    If you already know what you want to do, you don't need my advice.

    The right way to repair the siding is to snap a horizontal line and cut off the bottom of the siding (all of the siding that shows signs of rot). If you replace the siding with new "water table" trim, you may be able to disguise the repair so that it looks like a planned trim detail. Don't forget the metal Z-flashing at the top of the new water table trim.

  4. user-917907 | | #4

    I had to look up what "water table trim" is. It appears to be a waterproof synthetic PVC board, installed at the bottom of siding, meant to replace that portion of the siding which was getting wet and therefore rotting.

    Since this PVC board is waterproof I assume it is also vapor proof? If putting up a sheet of plastic in front of the siding is a bad idea because it (may) act as a vapor barrier, why would replacing the siding with a vapor-proof board not also cause the same type of problem?

    (I'm actually not clear what problems would occur if a band of vapor-proof material was installed at the floor joist level, provided bulk water was not getting in behind the siding?)

  5. GBA Editor
    Martin Holladay | | #5

    Architects have been installing water tables for at least 200 years. Traditionally, this type of horizontal trim was made of rot-resistant lumber, not plastic -- so redwood or cedar would be fine.

    Photos below.


  6. GBA Editor
    Martin Holladay | | #6

    Once again, the long-term solution involves lowering your grade.

  7. GBA Editor
    Martin Holladay | | #7

    This is equivalent to a high foundation. Traditionally, builders brought the masonry foundation up about 24 inches above grade, and then installed the horizontal sill and started framing. There was usually a graceful entry porch with several steps up to the porch.

    It's possible to mimic this look retroactively by using stone cladding over the wood framing of a house with a foundation that is too close to grade. Be careful, though, or the stone cladding will trap moisture against the OSB sheathing.

    You end up with a house that, from a distance, looks like a graceful old house -- but it may still be rotting.

  8. iLikeDirt | | #8

    Is the sheathing OSB? On a house with old redwood siding, I'm guessing it may be plywood or even board sheathing. Regardless, it goes without saying that if my approach is to be taken, once the siding is cut off, the area under it should be scrupulously detailed with a new double-layer WRB (perhaps even a drainage mat) and appropriate flashings to make sure that the water in the stone doesn't get into the wood.

  9. Expert Member
    Dana Dorsett | | #9

    In an early 1980s single family house ("...after 35 years it is showing signs of decay...") CDX sheathing is more likely than OSB, though the latter is still possible. Plank or ship lap sheathing would have been unusual in that era.

    But it's all susceptible to high exterior moisture drives and moisture traps.

  10. iLikeDirt | | #10

    Traditionally (i.e. for thousands of years), people protected the bottom of the walls of important buildings by using mortared stone on the bottom few feet, which would get wet but not deteriorate. This will probably be a smaller job than redoing the grade, and will represent a substantial aesthetic upgrade to boot. I think it looks a lot better than water table trim since the bottom part of the wall appears thicker than the top--not thinner, which is a jarring and disconcerting look, if you ask me. Makes the building look top-heavy and unstable.

    The stone bottom typically looks like this:

  11. user-917907 | | #11

    I appreciate the additional ideas suggested. The wall is framed in what, to my experience, is an unusual technique. The studs are 2x6, hung out over the foundation by 2" so that the foundation could have 2" of polystyrene insulation. Horizontal 2x4s, on 2' centers, were let into the 2x6 studs as nailers. Over the studs is a layer of Tyvek, then the vertical redwood siding. There is no plywood or OSB sheathing. Any remediation technique that resulted in cutting the siding back to above the floor joist level would require additional efforts to stabilize the now-flopping ends of the siding.

    Given my "recessed" foundation, plus lack of wall sheathing, I'm not sure how to attach a stone facade to the wall. I'm also concerned that Nate's suggestion to place a "double-layer WRB (perhaps even a drainage mat)" behind any new siding or stone facing will create an unwanted internal vapor barrier that Martin (and maybe Dana) has warned against.

    I have given some thought to dry laying a two-layer course of a decorative cinder block a foot or so away from the siding. They would set on the existing 2-foot wide gravel perimeter that surrounds much of the house. The blocks would block the splashing rain from hitting the siding. The problems that I foresee are: 1) the block may get knocked over by the snow falling off the roof, and 2) the blocks would have to be stacked much higher (three or four courses high?) to prevent the snow from laying against the siding.

    I'm still considering (and listening intently for arguments against) the idea of hanging a layer of plastic sheeting against the lower part of the siding. It will be as ugly as Hades, but will be on the back side of the house and not readily seen. Plus, if it doesn't work out, it can be easily removed, putting me back to square one. If the plastic is hung off a batten 3/4" away from the house I'm hoping there will be enough ventilation between it and the siding to allow any incidental moisture to escape.

  12. GBA Editor
    Martin Holladay | | #12

    You wrote, "I'm also concerned that Nate's suggestion to place a 'double-layer WRB (perhaps even a drainage mat)' behind any new siding or stone facing will create an unwanted internal vapor barrier that Martin (and maybe Dana) has warned against."

    Two points:

    1. Almost all WRBs are vapor-permeable, so a double-layer WRB will not create a vapor barrier.

    2. WRBs are placed on the exterior side of a wall, not the interior side of a wall, so I'm not sure why you mentioned "an unwanted internal vapor barrier."

  13. user-917907 | | #13

    Thank you for correcting my mistaken understanding of the vapor-permeability of WRB.

    When I said that the WRB would be internal to the wall I meant that it wouldn't be either the very most exterior layer (e.g. the siding), nor would it be the very most interior layer (e.g. the sheetrock), so it therefore would be a component in the interior of the wall. I guess I'm not using the correct nomenclature of the building trade. Sorry for the confusion.

    In thinking further about Martin's concern about my suggestion to put a vapor-proof barrier on the outside of the wall, I think about my standing seam roof. It, too, is a vapor-proof external layer covering a rot-able plywood sheathing. As far as moisture and vapor control is concerned, how does my standing seam roof differ from my proposed plastic barrier band? Come to think of it, how does my proposed plastic barrier band differ from the vinyl siding that is commonly retrofit over existing siding?

  14. GBA Editor
    Martin Holladay | | #14

    Q. "As far as moisture and vapor control is concerned, how does my standing-seam roof differ from my proposed plastic barrier band?"

    A. Moisture accumulation and rot are major problems with insulated sloped roof assemblies, and code requirements address these concerns by imposing details to prevent moisture accumulation. If your sloped roof assembly is insulated, codes require one of three approaches to address potential problems: either (a) you must include a ventilated air space between the roof sheathing and the upper layer of the insulation, or (b) you must install an adequately thick layer of air-impermeable insulation (usually closed-cell spray foam) on the underside of the roof sheathing, or (c) you must install an adequately thick layer of air-impermeable insulation (usually rigid foam) on the exterior side of your roof sheathing.

    Your wall doesn't have these features.

    For more information on designing a safe roof, see How to Build an Insulated Cathedral Ceiling.

    Q. "How does my proposed plastic barrier band differ from the vinyl siding that is commonly retrofit over existing siding?"

    A. Vinyl siding is safe because it includes a well-ventilated air space between the back of the siding and the WRB. If your "proposed plastic barrier" also includes a well-ventilated air space between the barrier and the siding, it will also be safe (though ugly).

  15. user-917907 | | #15

    As I tried to explain both in my opening post and in response #2, there will be, at minimum, ventilation channels created by the channel-rustic siding itself, plus, optionally, the gap created by the batten holding the plastic away from the siding. This seems to me to meet the requirements specified by roof assembly (a), and should provide as much ventilation as the air-gap behind vinyl siding.

  16. iLikeDirt | | #16

    It sounds like you really want to cover the bottom few feet of the siding in plastic. Knock yourself out. It will be cheap and likely effective. Keep in mind that if this moisture issue is a real problem, it demands a real solution--a permanent solution. Plastic sheeting will degrade, get blown off in the wind, etc. And it's ugly. If you can live with those drawbacks, then it seems like you've got your answer. And if it doesn't work, it will be easy enough to remove.

  17. Expert Member
    Dana Dorsett | | #17

    If the new cladding layer back ventilated by the same 2x4 girts onto which the board batten siding is attached, the vapor permeance of the cladding doesn't much matter. Trimming the board & batten at some point between the 24" o.c. girts is fine. Adding a new girt that spans the transition between the b & b and your stone or whatever, allows you to install a thin Z-flashing at the transition to direct bulk water streaming down the b & b to the exterior of the cladding below.

    Using a fiber-cement solution (perhaps faux-stucco finish look) for the new cladding below would be fairly resiliant to seasonal snow burial & roof edge splash back.

  18. user-917907 | | #18

    Thanks to all for your helpful ideas. GBA has become a real asset for those of us who are less experienced in the building trades.

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