Four dimensions describe every window and they are never the same number. Here is what each one means, who owns it, and why a landmark opening changes the arithmetic.
More replacement windows go wrong on paper than on site. The window arrives, it is a fine window, and it is three-quarters of an inch too wide for the hole — or it fits, but the glass is so much smaller than the old glass that the room is darker than it was, and on a landmarked block the Commission wants to know why the front now looks different from the neighbours.
Nearly every one of those failures comes from two people using the word “size” to mean two different things. A window has at least four sizes. The architect draws one, the manufacturer builds another, the installer measures a third, and the person living behind it experiences a fourth. This article sets out what each is, in the order they exist on a job.
Not to scale. Each dimension sits inside the one before it.
Each dimension sits inside the one before. MO is the biggest; it is a fact about the building. If bucks are used, the RO is the MO less the buck thickness. The unit is the RO less a clearance on each side — enough to shim the frame plumb, foam the gap and get a sealant joint that can move, which on most systems means a fraction of an inch per side and is set by the installer from the system’s installation instructions, not by guesswork. The DLO is the unit less the frame, sash and stop widths of whatever system was chosen.
A worked example, illustrative rather than a specification. A rowhouse window opening measures 36 inches wide by 60 tall in the brick. Bucks are not used; the window is set directly to the masonry. The installer orders a unit at 35 by 59 to leave half an inch a side. In a slim aluminum tilt-turn the DLO might come out around 31 by 55; in a wood double-hung with a meeting rail and heavier sash it might be closer to 30 by 24 per sash. Same hole, same unit size, very different glass — which is the point.
On new construction the sequence runs from the drawing outward: the architect sets the RO on the plans, the framer builds it, the window is ordered to fit it, and the site is checked before release to fabrication. On replacement it runs the other way: nobody trusts the drawing, the old window comes out or is measured around, the MO is taken in three places and checked for square with diagonals, and the unit is ordered to the smallest reading.
Old New York masonry is rarely square. A hundred-year-old opening can be half an inch wider at the sill than at the head, with a sill that slopes and a lintel that sags. That is why replacement units are ordered to the tightest measurement, and why an installer who measures once, in the middle, is the one who calls the office to say the window does not fit. On Surfview Towers in Long Beach, a whole-building replacement, every opening was measured individually before anything was ordered, and the units were sized to each opening rather than to the drawing.
On an ordinary replacement the owner cares about the unit size (does it fit?) and the DLO (is the room bright?). The Landmarks Preservation Commission cares about neither directly. It cares about what the street sees: how wide the frame reads against the brick, where the meeting rail falls, whether the glass proportions match the original and the neighbours, and whether the window sits in the masonry the way the old one did.
Every one of those is a unit-size or DLO question in disguise, and on a landmark they are answered by the original window rather than the manufacturer’s catalogue. Three things follow.
Brick-to-brick, not pocket. The fastest replacement is a pocket or insert unit set inside the old frame, which leaves the old frame in the wall and shrinks the DLO by two frame widths. It is often not acceptable on a landmark street facade because it changes the visible frame width and the glass proportion. The Commission’s guidance generally expects full replacement to the masonry, so the MO is the number that matters and the old frame comes out.
The DLO is a target, not a result. On a normal job the DLO is whatever the chosen system leaves. On a landmark the DLO is set by the historic window, and the system has to be chosen — or custom-profiled — to hit it. That is why a landmark double-hung can cost more than a larger ordinary window: the profile is being built to a proportion, not picked from a list.
The arch is part of the size. Rowhouse openings are often segmental arches. The unit is square-headed; the arch is closed with a shaped panel or the frame is made to the curve. Either way the MO includes the arch and the unit does not, and the Commission wants the opening to keep its shape from the street. At 343 Hoyt Street in the Carroll Gardens Historic District the Pella double-hungs were set on the original bluestone sills in arched openings, with the head detailed so the arch reads intact from the sidewalk.
None of this applies to a rear facade or a non-visible elevation, which is why the same building can have brick-to-brick landmark units at the front and standard replacements at the back, as at 165 East 64th Street. If you are unsure whether a facade is visible, the answer is usually yes.
A window shop drawing shows all four numbers, and knowing which is which saves the phone call. The opening dimension, marked MO or RO, is at the outside of the drawing. The frame dimension, often marked OD or “unit,” is the outside of the drawn frame. The DLO is dimensioned across the glass, usually per lite. If the drawing shows only one width, ask which one it is before you sign it; and if it shows an opening dimension the site has not been measured for, do not sign it at all.
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Send it to us with a photo of the opening. We will tell you which dimension is which, whether the clearance is right, and whether a landmark reviewer will accept it.
