Measuring a Slide Room Opening Before You Order a Seal

Before any dimension, establish how the seal is held on. Three families cover nearly everything. Adhesive backed seals have a flat foot with tape on it and stick directly to a painted or extruded surface. Kerf mounted seals have a barbed leg that pushes into a narrow slot and grips by friction. T slot seals have a leg that ends in a wider shape which slides into a matching channel from one end and cannot be pushed in from the side.

Those three do not interchange in either direction. A kerf seal will not stay in a T slot, and a T slot profile cannot be forced into a kerf without destroying the barb. Look at an end of the existing run where it terminates, or lift a short section with a plastic pick, and identify which family you have before you measure anything else.

• Adhesive backed: flat foot with tape, bonds to a surface
• Kerf mounted: barbed leg friction fitted into a narrow slot
• T slot: shaped leg slid into a channel from the end
• None of the three substitute for another

An installed seal is deformed. It is compressed, it may be stretched along its length, and its visible face tells you very little about its free dimensions. So take a clean cut through the run at a point where an extra half inch will never be missed, usually within a couple of inches of a terminated end behind a corner trim piece, and work on that sample.

With the sample in hand, measure four figures with a digital caliper: base width across the foot, free height from the foot to the top of the bulb or the tip of the lip, leg length from the foot to the shoulder, and wall thickness of the hollow section if there is one. Write all four down in thousandths rather than in fractions, because supplier tables are published in decimal inches.

Stand the cut sample on a flat surface with a steel rule laid against it so the graduations and the cross section are in the same plane, then photograph it square on from about eight inches away with good light. That single image communicates the profile shape better than any set of numbers, because shape is what a supplier matches on.

Send that photograph with your four measurements. Any competent seal supplier will identify a profile from it in one exchange, and the ones who cannot are telling you something useful about themselves. We keep these photographs on file by unit, so an owner who replaced a front room seal in 2024 does not have to measure again when the rear room needs one.

Measure the actual path the seal follows rather than the room dimensions. Run a flexible tape along the retention channel, around every corner, and add the vertical legs. On a room with mitered corners, measure each straight segment separately and note them individually, because you will be cutting to those lengths rather than bending a continuous piece.

Then add about ten percent. Extrusion shrinks slightly off the reel, corners consume more material than the geometry suggests, and a botched first cut is far cheaper than a second freight charge. On a twenty foot run, two extra feet costs very little and removes the possibility of coming up eleven inches short with the room already stripped.

A wiper seal is directional. The lip faces one way, either toward the outside of the coach or toward the room, depending on which position it is sealing and where the designer intended water to be shed. Once the old seal is out of the channel, that information is gone, and reinstalling a wiper backwards produces a seal that scoops water inward instead of shedding it.

So photograph it in place before removal, from an angle that shows the lip clearly, and take one shot at each corner. Two minutes of photography prevents an afternoon of confusion and a callback in the next rain. This is the step experienced people skip on a room they have done before, and it is the step that costs them.

If the room has been leaking and you suspect the original profile was marginal rather than worn out, measure the gap the seal is being asked to close. Press a strip of modelling clay onto the sealing surface, cycle the room to its seated position, then bring it back and measure the flattened clay with a caliper. That is your actual compressed gap.

Compare that against the free height of the profile you are about to order. A bulb seal should be compressed meaningfully, roughly to half its free height at the seated position, not merely touching. If the clay says the gap is larger than the profile can fill, the problem is geometry rather than rubber, and ordering more seal will not solve it.

1. Identify retention family from a terminated end
2. Cut a short sample and caliper base, height, leg and wall
3. Photograph the cut end square on against a steel rule
4. Tape the true run path and add ten percent
5. Photograph wiper lip direction at every corner before removal
6. Take a clay impression to confirm the compressed gap