What Is Concealed SOSS Hinge Geometry? (Mortise Specs)
A concealed SOSS hinge uses linked arms that fold into matching mortises in the door and frame. Its geometry sets the hinge’s pivot path, opening angle, and the position and size of both mortises. There is no universal mortise specification: use the current template and load chart for the exact model, door, and frame.
What concealed SOSS hinge geometry means
Concealed SOSS hinge geometry is the relationship between the hinge’s moving parts, its pivot points, and the mortises that hold it. The geometry lets the hinge arms fold inside the door and frame as the door closes, keeping the hardware out of view. The model-specific template controls where the mortises go.
Pivot path and mortise alignment
The pivot path is the route the hinge follows as the door swings. Both mortises must hold the hinge in the position intended by its template, so the arms can move freely without binding. A small layout error can affect the swing or door alignment; the correct response is to inspect the layout, not enlarge the mortises at random.
The door mortise and frame mortise are a matched pair. Their location, depth, and orientation work together to preserve the hinge’s intended pivot geometry. Mark both from the same model-specific template and datum. A datum is the fixed reference edge or line used to locate measurements consistently.
Which SOSS mortise specs should you use?
Mortise specifications are the dimensions and locations needed to house a particular hinge model. They include mortise length, width, depth, and edge offsets. These dimensions vary by model, so I do not treat a measurement from one SOSS hinge as a safe substitute for another.
Read the template and load chart together
Use the current manufacturer template for the exact hinge model to lay out both mortises. Use the matching load chart to check the permitted door weight and width, hinge quantity, and any stated spacing or substrate conditions. The template gives cutting details; the chart explains the conditions behind the rating.
| Item to verify | Where to verify it | Why it matters |
|---|---|---|
| Mortise length, width, and depth | Current template for the exact model | Sets the hinge pocket and preserves arm movement |
| Mortise edge offsets and location | Same model-specific template | Positions the hinge correctly in the door and frame |
| Hinge count and spacing | Model’s load chart and installation instructions | Ratings depend on the stated setup |
| Door weight and width | Measure the actual door; compare with the load chart | A rating is not a blanket approval for every door |
| Frame and door substrate | Manufacturer’s installation requirements | Weak or unsuitable material may not hold fasteners |
| Opening angle | Model information and project requirements | Determines whether the hinge suits the intended swing |
Do not infer a mortise dimension from a hinge’s appearance, a generic online diagram, or a different model’s template. If the template is missing or unclear, obtain the correct current instructions before cutting.
What a published load rating does and does not mean
A load rating applies under the manufacturer’s stated conditions. It does not prove that a split door edge, hollow section, weak frame, or poorly reinforced attachment area can support the hinge. The material around the screws must be sound and suitable for the chosen hardware.
If a door is heavy, wide, or made from a construction that may not provide secure screw holding, check the model instructions and seek guidance from the manufacturer or a qualified door professional. Adding hinges or changing their spacing without checking the model requirements does not automatically make the installation safe.
Plan the door, frame, and clearances before cutting
Planning means confirming that the hinge suits the complete opening, not just the door edge. I record the exact model, door size and weight, frame material, handing, floor condition, and required opening angle before layout. This helps catch conflicts while changes are still easy.
Record the opening conditions
Check that the frame is plumb and square and that the door dimensions match the opening. Note the finish floor, threshold, and any floor covering that could affect clearance. Hinge geometry does not set one universal door-to-jamb, head, or floor clearance; confirm those details against the door and frame specifications.
Confirm the handing and swing direction against the selected hinge instructions. If the opening must meet an accessibility or egress requirement, check the full opening and the adopted code requirements for the project. Hinge selection alone does not establish compliance.
For U.S. accessibility requirements, the ADA Standards specify at least 32 inches of clear width with the door open 90 degrees, subject to the standard’s provisions. Measure the actual clear opening; nominal door width is not the same measurement. Verify applicable exceptions and project requirements rather than relying on this figure alone.
Machine and install the mortises in sequence
A controlled installation preserves the template geometry and protects the door and frame. Work in order: verify the opening, mark both parts from one reference, cut to the specified depth, install as instructed, then test the swing before finishing. This sequence helps separate layout problems from hardware or clearance problems.
Step-by-step layout and routing
- Survey and mock up. Check frame alignment, door size and weight, swing, floor clearance, hinge model, template, and load-chart requirements. Confirm the intended hinge count and spacing.
- Set one datum. Use the template instructions to establish a consistent reference on the door and frame. Transfer the locations carefully; do not measure each mortise independently from a different edge.
- Mark both mortises. Confirm orientation and location against the model template before cutting. Double-check that the door and frame marks correspond to the same hinge position.
- Route in controlled passes. Set the tool for the specified depth and remove material gradually. Keep the mortise square and within its marked limits. Protect visible edges from tear-out and verify the pocket before fastening.
- Fasten as specified. Use the manufacturer’s required screws and a compatible, sound substrate. Do not omit fasteners or reinforcement called for by the instructions.
- Test before finishing. Move the door through its intended swing. Check for binding, rubbing, alignment, and clearance. If something is wrong, inspect the mortise location, depth, hinge seating, and frame alignment before making any change.
- Finish and recheck. Keep finish out of hinge joints and adjustment features. After finishing, confirm smooth operation, fastener security, clearances, and any accessibility or egress needs.
Diagnose binding without altering the geometry
A door that binds may have a mislocated mortise, a hinge that is not seated as intended, a frame alignment issue, or a clearance conflict. First compare the installed hinge position with the template. Then inspect the door and frame for plumb, square, and movement.
Do not enlarge a mortise simply to make the door move. Removing extra material can weaken the attachment area or shift the hinge from its intended geometry. Correct the cause using the manufacturer’s instructions, and get qualified help if the door remains unsupported or the attachment material is damaged.
Job-site scenarios: applying the geometry
These representative scenarios show how the same checks apply in different conditions. They are practical examples, not reports of a specific documented project. In each case, the model template and load chart remain the controlling source for dimensions and capacity.
Scenario: a new interior door
A builder has a new door and frame, but has not selected the hinge model. The reliable sequence is to choose a model that fits the door’s dimensions, weight, substrate, and required swing, then use its template to lay out both mortises. Cutting first and choosing a hinge later risks a mismatch.
Before routing, the builder also checks the floor finish and the frame alignment. This prevents a correct hinge layout from being blamed for a clearance or frame problem.
Scenario: replacing hardware on an existing opening
A homeowner finds a concealed hinge in an existing door but cannot identify its model. The visible shape is not enough to confirm the mortise dimensions or load rating. The homeowner should identify the hinge and locate its current instructions before modifying either pocket.
If the exact model cannot be confirmed, avoid copying the existing mortise as a template for new hardware. Existing cuts may reflect a different model, prior alteration, or a poor installation.
Scenario: a heavy door with a weak edge
A door’s weight appears to fall within a hinge chart, but its edge is split or does not provide the attachment support required by the manufacturer. The load rating does not resolve this issue. The installer must assess the door and frame material and follow the required reinforcement details.
If the substrate cannot safely hold the specified fasteners, stop and resolve that condition before installation. A hinge rating cannot compensate for a weak connection.
Code and fire-door checks
Code-conscious installation means checking the complete opening and its intended use, not only the hinge. Egress, accessibility, and fire-door rules depend on the project and adopted requirements. For a rated opening, the door, frame, and hardware must be approved for that use and installed under the applicable listing and NFPA 80.
Do not field-mortise a fire-rated door or frame unless the modification is allowed by the assembly’s approval and instructions. An unapproved cut or hardware change can affect the fire-door approval. Confirm acceptable hardware and preparation with the listing information or the responsible qualified professional before work begins.
For accessibility, measure the clear width with the door at the required open position. For egress, check the adopted code and project conditions for clear width, swing, and hardware operation. A concealed hinge’s opening angle or load chart does not, by itself, show that an opening meets these requirements.
Conclusion: preserve the model’s geometry
The core rule is simple: select the exact hinge first, then use its template and load chart to plan and cut. The mortise dimensions, pivot position, fasteners, substrate, and door clearances all work as one system. If the door binds, diagnose the alignment and installation before removing more material.
FAQ: concealed SOSS hinge geometry and mortise specs
These answers cover common measurement and installation questions. For a specific hinge, the current manufacturer instructions take priority over general guidance. Check the exact model, door, frame, and applicable code conditions before cutting or changing an opening.
Are all SOSS hinge mortises the same size?
No. Mortise length, width, depth, and location vary by model. Use the current template for the exact hinge; do not copy dimensions from another SOSS model.
How deep should I cut a SOSS hinge mortise?
Use the depth specified on the selected model’s template. There is no universal SOSS mortise depth that applies to every hinge.
Can I use the load rating to choose the number of hinges?
Use the model’s load chart and installation instructions. The rating may depend on hinge count, spacing, door dimensions, and substrate conditions.
Can I enlarge the mortise if the door binds?
Not as a first fix. Check hinge position, seating, frame alignment, and clearances against the template. Arbitrary enlargement can weaken the attachment area or alter the intended geometry.
Does a concealed hinge rating guarantee a weak door edge is safe?
No. The door edge and frame must provide the required fastener support. A published rating does not make split, hollow, weak, or inadequately reinforced material suitable.
How do I check accessibility clearance?
Measure the actual clear opening with the door at 90 degrees. The U.S. ADA Standards require at least 32 inches in the applicable condition, subject to the standard’s provisions.
Can I cut a concealed hinge mortise into a fire-rated door?
Only when the modification is allowed by the door assembly’s approval and instructions. Confirm the listing and applicable NFPA 80 requirements before altering the door or frame.
What should I do if I cannot identify an existing hinge?
Do not assume its mortise matches a replacement model. Identify the hinge and obtain its instructions before reusing or changing the mortise.
(This article was written by one of our staff writers, Garrett Vance. Visit our Meet the Team page.)
