I am trying to decipher some as-built drawings for an old wood frame building. The purpose is to generate a preliminary load takeoff. I cannot figure out what the following is saying: enter image description here

It looks like "3 Ply Built up roof " but the rest makes no sense. Any seasoned structural engineers can tell me what this says?

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    $\begingroup$ It would help if you could post a higher resolution image. And it may help if you could post an image showing more of the as-built drawings. That could provide additional context to help decipher the annotations. Finally, please give a rough idea of the location of the building along with the building's age. Building codes vary by jurisdiction and knowing approximately where the building is at can help identify relevant code. $\endgroup$
    – user16
    Feb 19, 2016 at 14:34
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    $\begingroup$ If I am not mistaken, it seems that the draw was made on a vegetal paper ( and it could explain why is so messed). so many old projects there was been made like that before the appearence of the CAD. So its possible that rest of drawing could be so messed as that detail. $\endgroup$
    – h1k3rpath
    Feb 19, 2016 at 14:59
  • $\begingroup$ @GlenH7 This is the highest resolution image I have access to. Additional context includes that this is a residential building in Southern California. It is a wood frame building with block walls and a flat roof. $\endgroup$
    – user32882
    Feb 19, 2016 at 23:20

1 Answer 1


I am a practicing structural engineer with plenty of wood experience. That said, this is a bit of a tricky note because I believe it was probably written by an architect on a set of architectural drawings (as opposed to by a structural engineer on a set of structural drawings). Therefore, I think the note includes information about both the structural makeup of the roof diaphragm as well as the architectural components of the roofing system. That said, I can at least get you started. I am assuming that you are in the US, but if you're in another country, the abbreviations may be a bit different. I am less sure about the second line, but am confident about the meaning of the other 4 lines.

  1. The first line reads, 3 PLY BU ROOF

    I agree that it is communicating something related to the (mostly) architectural composition of a built up roof. It likely means that the roof is a 3 ply built up roof. Depending on your intent here, if it's structural for example, this doesn't have too much importance to you.

  2. The second line, o/ 3/4" O.F. PLYWOOD INO.

    This line is a description, of some sort, of the plywood. o/ is, I think, an abbreviation for the word "over." So the objects on this line are under the 3 ply built up roof. 3/4" is describing the plywood thickness, OF is an old way to describe "oriented fiber" material, this is similar to oriented strand board (OSB) that you see at the hardware store today. This is a little confusing, since plywood is not oriented fiber board. It could have another meaning. OF could also mean "outside face" and this may be an instruction about how to apply a coating related to the built up roof to the outside face of the plywood. Also, since the plywood is given a span rating (see next point), no plywood thickness note is required so the 3/4" may not be referring to the plywood at all.

  3. The third and fourth line, 32/16 (UNBLKD) W/ 8d @ 6" EN & RO

    These lines are decribing the strucutral requirements of the plywood and associated fasteners.

    • 32/16 is a "span rating" for the plywood. From the APA,

      Two numbers separated by a slash. The left-hand number is the maximum recommended center-to-center spacing for supports in inches when the panel is used for roof sheathing with long dimensions across supports. The right-hand number is the maximum center-to-center spacing of supports in inches when the panel is used for subflooring with the long dimension across supports.

    • (UNBLKD) is instructing that the edges of the plywood do not need to be blocked.

    • W/ is an abbreviation for the word "with"

    • 8d @ 6" is instructing the use of 8d nail at a spacing of 6" on center.

    • EN & RO is instructing that the previously mentioned nailing is applied as Edge Nailing and at Rough Openings. This means that the 6" oc nailing is applicable at the endges of the roof and around any openings.

  4. The fifth line, 12" FN is instructing a 12" on center "field nailing." This means that the plywood should be nailed to all the rafters/joists in areas that are not diaphragm edges at a 12 oc spacing.

Good luck and please comment when you figure it out. I've passed your image around the office and it's generated quite the lively discussion!

  • $\begingroup$ William. Thanks so much for the detailed response. I have a couple of follow up comments: The second line symbol "o/" apparently stands for "over". This being said, The first two lines would read "3 Ply Builtup roof over 3/4" oriented fiber plywood INO". Do we know what INO means? Also does this imply there are two wood sheet layers? Namely one layer of 3 Ply and over one layer of 3/4" plywood? $\endgroup$
    – user32882
    Feb 19, 2016 at 17:03
  • $\begingroup$ With respect to your question about the plywood, no, I do not think it is calling out for 2 layers of wood sheathing. 3 PLY is likely referring to the layers in the built up roof system. So the 3 ply built up roofing is over the structural system described in the lines below. I'm still not sure about INO. $\endgroup$ Feb 20, 2016 at 13:52
  • $\begingroup$ Yes. But it says 3 ply bur OVER 3/4" plywood. This to me implies 3 ply bur and 3/4" plywood are two different layers, superimposed. Otherwise there wouldn't be a need to say OVER, right? $\endgroup$
    – user32882
    Feb 21, 2016 at 16:42
  • $\begingroup$ Yes, I agree with you. I think we're miscommunicating. '3 PLY BU ROOF` is a description of the built of roofing system. Likely 3 layers of bitumen soaked/synthetic membranes which are placed on top of the structural plywood. The system probably looks something like this. $\endgroup$ Feb 21, 2016 at 19:21
  • $\begingroup$ I see ... interesting. Again thanks for following up. Nice to see we have structural engineers contributing to this site! :D $\endgroup$
    – user32882
    Feb 22, 2016 at 3:16

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