Short answer. Record the visible hinge-edge separation at several named positions only when that can be done safely without disturbing the window; a number by itself cannot select a replacement seal. The National Park Service (NPS) cautions that some historic steel casements may seal at three other edges without hinge-side weatherstripping, while a strip that is too thick can strain the hinges and make additional gaps. source
First check whether the opening can be observed safely
Do not force a sash, lever its hinge, loosen hardware, disturb old paint, or insert a probe, ruler, gauge or test strip into a closing joint. Work only from a safely accessible interior position. If the sash does not open and close normally, glass is loose, painted metal is flaking, corrosion is substantial, or the hinge location is not safely reachable, stop and hand the condition to a qualified historic-window professional. NPS recommends an annotated condition record that identifies the sash, frame, subframe, alignment, corrosion, paint and hardware. [Additional NPS source locator]
Three-position hinge-edge recording worksheet
This is an original, non-destructive documentation procedure, not a measurement tolerance or installation procedure published by NPS. The reason to distinguish positions is to avoid turning one observed opening into a universal hinge-side size.
- Identify the exact unit. Take an overall photograph and note the room/window identifier, viewing side, casement handing and whether it can close under its ordinary action. Photograph the hinge edge, latch edge, head and sill; use labels rather than guessing which part is the sash versus the fixed frame.
- Mark repeatable zones on your notes, not on historic finishes. Label hinge-side H-T (near the top), H-M (middle) and H-B (near the bottom). Describe their positions relative to the visible hinge hardware or a photographed stationary reference. If the three zones are not accessible, record which ones cannot be assessed.
- Observe the sash in its natural closed position only. At each zone, photograph the same visible sash-to-fixed-frame boundary with no added gasket and no extra closing pressure. Record apparent daylight, paint/rust obstruction, whether both opposing edges can be distinguished, and whether the sash looks aligned. Do not defeat a latch or lean out for a photograph.
- Measure only an already visible, directly accessible opening. If both edges of a naturally open visible gap can be seen clearly from the safe side, place a ruler beside (never inside) the opening, aligned in the same visible plane; photograph the reference, and record an approximate apparent width in millimetres and its location. If either edge or the ruler's plane is unclear, write NOT MEASURABLE instead of guessing. No touching or wedging the closing surfaces, no tracing a hidden compressed clearance, and no extrapolation from a perspective-distorted photograph.
- Record the pattern, not a pass/fail thickness. Keep the H-T/H-M/H-B observations, any approximate visible widths, unit labels and photos together. Differences along the edge are evidence to show a specialist; they are not an OEM clearance, gasket compression specification, alignment diagnosis or instruction to choose a strip of the same thickness.
| Hinge zone | Photo/visible reference | Natural closure and visible daylight | Apparent width if directly observable (mm) | Paint, rust, misalignment or obstruction | Confidence / stop reason |
|---|---|---|---|---|---|
| H-T (top) | ____ | ____ | ____ or NOT MEASURABLE | ____ | ____ |
| H-M (middle) | ____ | ____ | ____ or NOT MEASURABLE | ____ | ____ |
| H-B (bottom) | ____ | ____ | ____ or NOT MEASURABLE | ____ | ____ |
How to read the worksheet. A recorded number describes only the visible gap at that exact observation point and closure state. It does not reveal the concealed mating geometry, prove air leakage, establish a minimum hinge clearance, or justify a replacement profile. NPS Brief 13 supplies no universal numeric hinge-side gap or tolerance. Its separate example of a bowed tall sash and foam tape must not be generalized to this hinge-edge worksheet.
Decide what the observations justify
Possible three-side sealing design: For some casements, the NPS describes sealing on the three sides other than the hinge edge. Treat this as a conditional historic preservation consideration for professional assessment, not an instruction to strip off existing hinge seals. source
Possible interference from added material: A strip that makes the window harder to close is not a reason to use more force or choose thicker material. The NPS warns that overly thick weatherstripping can spring hinges and leave more gaps; it also notes that a folded V-shaped vinyl strip together with glue can be unsuitable because of its thickness. Do not trial-install or buy a seal based solely on the worksheet width. source
Uncertain sash condition or material: Do not select a generic spring-metal strip simply from an observed hinge gap. The NPS restricts its friction-clip spring-metal example to steel windows in good condition and describes painting steel before bronze/brass contact to reduce galvanic corrosion. Leave metal identification, paint condition and any treatment plan to a qualified assessor where uncertain. source
What to send for a fitting or conservation assessment
Send the overall unit ID and handing, safely obtained open/closed photographs, three labelled hinge-edge zone observations, exact points where an apparent width was directly visible, all NOT MEASURABLE fields, and any obstruction or unsafe-operation notes. Request an assessment of actual sash/frame section, hinges and conservation constraints before a product recommendation. No numeric fit, seal thickness, hinge adjustment, fire/egress approval or performance guarantee follows from this worksheet.
Manufacturer documents and safe interpretation
: NPS Sharon C. Park, The Repair and Thermal Upgrading of Historic Steel Windows, Preservation Brief 13 (1984), printed p. 10 (PDF page index 9), Weatherstripping, conditional hinge-side exception and three-side wedging; https://home.nps.gov/orgs/1739/upload/preservation-brief-13-steel-windows.pdf: same NPS original, printed p. 10, right column, warning that too-thick weatherstripping can spring hinges and introduce gaps.: same NPS original, printed p. 10, left column, V-shaped vinyl/adhesive thickness may make it unsuitable in some situations.: same NPS original, printed p. 10, left column and Fig. 8, good-condition steel spring-metal example and painted steel before bronze/brass contact.Additional NPS source locator: Additional source locator; same NPS original, printed p. 3 (PDF page index 2), Physical Evaluation and Rehabilitation Work Plan, annotated drawings/photos, corrosion/paint/bowing/hardware inventory and lead-paint hazards.
Method/source distinction: NPS supports preserving and recording physical conditions, plus the conditional weatherstripping statements above. The H-T/H-M/H-B worksheet and adjacent-ruler technique are this article's limited original observational aid—not an NPS specified measurement method, a universal numerical threshold or a safe instruction to repair old steel windows.