Transom Glass Replacementin DC, MD & VA

A transom is a glazed lite sitting above the head of a door or a window, separated from it by a horizontal member called the transom bar. It is a separate frame carrying separate glass, which is the single most useful fact about it: a broken or fogged transom is re-glazed in place without the door, the leaf hardware, the casing or the surrounding finish being touched at all. Historically the transom over an interior door was operable, hinged at the head or the sill and worked with a chain or a rod, because before mechanical ventilation it was how air moved through a house with the doors shut. Over an entrance the transom is almost always fixed and is there for daylight into a hall that has no other window, and to give the doorway the proportion the elevation was designed around.

Operable transoms survive in enormous numbers across DC rowhouses, older Alexandria and Georgetown properties, pre-war apartment buildings and school and institutional corridors, and the overwhelming majority of them have been painted shut rather than removed. Freeing one is worth doing: the sash is usually sound, the chain or rod operator is usually repairable, and a working transom over an interior door still moves air exactly as well as it did a century ago. Where a sash has rotted or the operator hardware is unobtainable, the opening can be glazed fixed in a way that keeps the appearance of the original from the room. What we do not do is default to fixing it shut because that is quicker.

This is where curved transom work is either right or wasted. A segmental or half-round head is an arc of a single circle, and the span across the springline together with the rise from the springline to the crown fixes that radius completely — two measurements taken on site and the glass can be cut to a dimension. A half-round is simply the case where the rise equals half the span. An ellipse is a different animal: its curvature changes continuously along its length, so it has no single radius, and no amount of measuring the span and rise will describe it. Elliptical and irregular heads are templated, physically, in card taken off the opening, and the template goes to the bench with the order. Trying to cut an ellipse as an arc produces a lite that will not enter the rebate and cannot be adjusted, which is the most wasteful mistake available on this kind of work.

Because a transom sits at the top of an opening, people assume that touching it means touching structure. It does not. The load over the opening is carried by the structural header or lintel above the whole rough opening, and everything below that — the transom frame, the transom bar, the door head jamb — is infill hanging inside it. That is why a transom can be re-glazed, and why a transom that a previous owner boarded or plastered over can usually be reopened, without any structural work at all. The exception is a masonry opening where a failed or missing lintel is the reason the head has moved and cracked the glass in the first place, and in that case the structural fault gets addressed before any new glass goes near it.

Two separate rules can catch a transom and it is worth knowing which. Section R308.4.2 of the International Residential Code, as adopted across DC, Maryland and Virginia, covers glazing adjacent to a door: it applies where the nearest vertical edge is within a 24-inch arc of either vertical edge of the door in its closed position AND the bottom exposed edge is less than 60 inches above the walking surface. A transom over a standard door head is well above that 60-inch line, so this clause usually does not catch it. Section R308.4.3 is the one that does: any individual lite with an exposed area greater than 9 square feet, a bottom edge less than 18 inches above the floor, a top edge more than 36 inches above the floor and a walking surface within 36 inches of it. Tall transom-and-window assemblies, and shop-front transom bars over full-height glazing, frequently satisfy every part of that. Commercial buildings are governed by the equivalent provisions in IBC 2406.

A transom set into a sloped wall, a barrel front, a canopy or the head of a conservatory is overhead glazing, and it is a different specification entirely. Under IRC R308.6, and IBC 2405 for commercial work, the inboard lite of a multiple-layer overhead unit must be laminated, so that if the glass fractures the interlayer holds the pieces rather than dropping them into the space below. The outboard lite is normally fully tempered to deal with hail and debris. This is not a judgement call to make on site or a place to take a view, and it is worth confirming what you actually have before ordering: a lite that looks like a transom from inside can be overhead glazing from the roof.

A large share of the transom work in the DMV is on properties where the glass is part of what the building is: leaded fanlights in Georgetown and Capitol Hill, house numbers sandblasted into the lite over a rowhouse door, chipped and bevelled panels over shopfronts in Old Town Alexandria and Annapolis. Historic district review may have a say in what may go back on a street-facing elevation, including pattern, reflectivity and caming, and safety glazing requirements still apply in full alongside it — the two sets of rules have to be satisfied together rather than traded off. Original leaded panels can very often be repaired rather than replaced, and where a decorative panel is sound but thermally hopeless it can be encapsulated inside a modern sealed unit, so the original glass is protected in the cavity while the assembly performs like a current one.