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T1-11 — overlay or replacement?

Charles Thompson | Posted in Green Products and Materials on

My 1979 T1-11 plywood sheathing has suffered what Martin has called similar to not drying faster than rate of being wetted. My intention is to overlay rather than replace simply because, if we can reduce the inrush of moisture and increase the drying potential, the labor of removing (organic) mass is not warranted. This is a (2) two story DOE #3 moist home. So–Here is my question– Is there a concrete type (Hardie like) panel that lines up with the T1-11 reverse board and batten pattern, because I don’t want to overlay /replace the upper panels that are appearing good?

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  1. GBA Editor
    Martin Holladay | | #1

    As far as I know, so-called "texture 111" or "T-111" is a type of plywood, not a brand name. Different manufacturers make different configurations of T-111. If you want to try to match your T-111 to a brand of fiber-cement siding, you'll have to measure the width of the pattern and do some research.

    The photo below shows a fiber-cement panel siding from Allura USA.


  2. GBA Editor
    Martin Holladay | | #2

    I'm not sure what condition your T-111 is in; you wrote that the siding is "not drying faster than [its] rate of being wetted." That makes it sound as if your T-111 is beginning to rot. If it is, you don't want to leave it in place.

  3. D Dorsett | | #3

    T1-11 siding is usually the structural sheathing, not merely siding. (T1-11 was a bad idea in damp climates for a number of reasons. )

    Hardie panels are not structural- you can't just replace one with the other. Nor can you overlay compromised T1-11 with a non-structural panel and assume that it still meets the structural requirements.

    If the upper panels are in good shape you can replace the lower panels with OSB or CDX, apply a housewrap or #15 felt over the whole thing (taking care to re-work the flashing details properly) and put Hardie or other panels over that, preferably with at least 1/4" of rainscreen gap between the new siding and the housewrap. (Cutting 2" strips of 1/4-1/2" OSB/plywood nailed through to the studs,, and adding the additional depth to the siding fasteners works.) With a rainscreen between the siding and structural sheathing you have a capillary break to keep the moisture from easily wicking into the structural sheathing, and a rapid drying path for the siding and any moisture that gets by the siding during wind-driven rain.

    It would be cheaper to skip the furring & rainscreen, and use vinyl siding (which is inherently back-ventilated, and a good capillary break), after you've replaced the compromised T1-11 with structural OSB or CDX

    A short-sheet on T1-11 repair requirements per CA code lives here:

    This older thread on JLC covers some of the other issues related to siding over T1-11:

  4. Charles Thompson | | #4

    Agree on the two (2) panels that are coming apart need replacement. With that exception, The rest of the house is sound. Referencing your graphic, the repeat of Hardie is every 8 inches composed of appx. 7 1/4 in. planks and 3/4 in. groove or reverse batten. The two patterns stacked one above the other would look weird - even with a 1 foot wide horizontal plank separating them! We measured my T-111 and it has repeat of 12 inches composed of appx. 10 1/2 in. planks and 1 1/2 in. groove or reverse batten. The Allura USA shown appears to have 8 repeats in the 96 inch dimension. Hallellalula and thank you! I will try to find some Allura USA fiber-cement siding and measure.

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