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Thread: Building height datum

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    Cyburbian Otis's avatar
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    Building height datum

    My city council is looking at its options for measuring building height. Specifically, I need to give them options on where to measure height from. Presently we measure from the average finished grade. I know some places measure from the average original grade, and others from the lowest corner of the lot, and others the lowest point of the grade adjacent to the structure. I need input on other options.

  2. #2
    Cyburbian
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    Structural Height

    The City Council here has recently directed staff to look into design review for single-family dwellings, which in my opinion appears to be due in large part to unexpected on-the-ground-results of the way in which building height is measured here (Council-members don't understand why homes look so much taller and bulkier than they think they should). My suggestion, no matter what methodologies you give them to choose from, is that you draw it out on a series of elevations, illustrating different grade differentials, so that they are able to understand visually what it is really going to look like on the ground.
    -good luck-

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    Cyburbian Jeff's avatar
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    Cant give you a definitive answer, just an opinion. Measure it from where the Fire Department is going to throw ladders. If that section is higher than 35' think again. Reason being a 35' high roof.....a 28' ground ladder, propped at an angle can hit the 2nd story window.

    Again, probably not the answer your looking for, but something to consider.

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    Cyburbian permaplanjuneau's avatar
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    I think an important thing to consider in establishing a datum is the type of topography in your area. Is everything pretty flat? Steep hills and mountains? We have a little of both, so we have a two-part datum (see (b) below):

    49.25.420 Height of building.

    (a) The height of a building is the vertical distance above a reference datum measured to the highest point of the coping of a flat roof or to the deck line of a mansard roof or to the average height of the highest gable of a pitched or hipped roof. Roofs with slopes greater than 75 percent shall be regarded as walls. The height of a stepped or terraced building is the height of the highest segment thereof.

    (b) The reference datum shall be whichever of the following yields the greater height of building:

    (1) The highest point within a horizontal distance of five feet from the exterior wall of the building, when such point is not more than ten feet above the lowest point within said five-foot radius.

    (2) An elevation ten feet higher than the lowest grade, when the highest point described in subsection (b)(1) of this section is more than ten feet above the lowest point.

    (c) Exceptions.

    (1) Height limitations stipulated in this section shall not apply to tanks, church spires, belfries, cupolas, monuments, fire and hose towers, chimneys, flagpoles, masts, aerials, antennas, telecommunication and electrical transmission towers and other similar structures or facilities.

    (2) Height calculations shall disregard any fill or construction which the director finds to have no significant purpose other than elevating the reference datum. In reaching such finding, the director shall consider only those architectural, structural, safety, aesthetic, access or other purposes claimed by the developer and supported by reasonable evidence.

    I'd suggest that you follow Oregon Planner's suggestion: "My suggestion, no matter what methodologies you give them to choose from, is that you draw it out on a series of elevations, illustrating different grade differentials, so that they are able to understand visually what it is really going to look like on the ground."

    I once spent over an hour with two of our senior planners and one disgruntled neighbor who insisted that we were interpreting the code wrong in terms of establishing the datum under (b)(2) (he eventually realized we were right). Having some images in your code would help clarify the intent and reduce the chances that you'd have such a meeting.

  5. #5
    Cyburbian
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    Also, state that the height is measured to the peak or highest point of the roof line. It elimates a lot of confusion regarding mid points and is much easier to measure on a building under construction.

  6. #6
    Cyburbian Otis's avatar
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    Thanks for the suggestions. Permaplanjuneau, how do you use the exception in (c)(2) in practice? Do you require the applicant to submit a grading plan?

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    Cyburbian michaelskis's avatar
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    Average grade to midway between the eve and peak of a house.
    Invest in the things today, that provide the returns tomorrow.

  8. #8
    Cyburbian abrowne's avatar
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    Quote Originally posted by michaelskis
    Average grade to midway between the eve and peak of a house.
    Same in my municipality. I'll clarify average finished grade.

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