Design Parapet

A parapet is not a wall that keeps going. It is the place where two different assemblies have to hand four control layers to each other, around three changes of direction, at the most exposed point on the building. It is also usually the worst thermal bridge on the enclosure and the most reliable source of leaks. It earns its own pass.

Workflow

Step 1. Take both assemblies and what sets the height

Ask, in one message:

1. The wall build-up, inside to outside, with the four control layers located.
2. The roof build-up, deck upward, with the same.
3. What sets the parapet height: a guard, equipment screening, a datum, the roof drainage, or all of them?
4. Is the parapet backed by the wall structure continuing up, or by something added on top of it?
5. Does the roof drain toward this parapet? Where does overflow go, and is it through the parapet?
6. Is anything fixed to or through the parapet: davits, anchors, lightning protection, signage, railings, window washing?
7. What is already fixed: a coping profile from the facade module, a cladding already dimensioned, a height already agreed with planning?

If either assembly is missing, stop and say so. The parapet cannot be worked against an unknown.

Step 2. Carry each control layer around the corner and name the transfer

One row per control layer. The two middle columns are the point of this skill:

Control layer	Plane in the wall	Plane in the roof	How they transfer	Lap direction

• Water. The wall sheds down its drainage plane; the roof holds water on its membrane. Name the through-wall flashing that collects what runs down inside the parapet, and where it discharges. A parapet with no through-wall flashing drains into the wall below it.
• Air. This is the transfer most often drawn as a dotted line and never resolved. Name the physical material that connects the wall's air barrier to the roof's, over the top of the backup or around it, and say what holds it there.
• Vapor. Whether the two assemblies' vapor strategies are compatible at this junction, or whether the parapet creates a pocket closed from both sides.
• Thermal. Where the insulation goes around the parapet, or the fact that it does not. If the backup structure runs through uninsulated, state plainly that the parapet is a continuous linear thermal bridge along the whole roof perimeter, and that it has not been quantified here.

State explicitly for each row whether the parapet is insulated on the inside face, the outside face, over the top, or wrapped. The answer changes all four rows.

Step 3. Work the top, and read the gaps

The coping is the roof of the parapet and it fails the way roofs fail.

Element	Its job	Value needed from you

Then read between:

• Which way does the coping slope, and toward what? A coping draining outward stains the facade; draining inward loads the roof.
• What is under the coping, and is it a continuous waterproof layer in its own right, or is the coping the only barrier?
• How are the coping joints made, and what happens at those joints when the metal moves?
• Do the coping fixings penetrate a control layer, and what closes each one?
• Does the coping overhang enough to throw water clear on both faces?

Step 4. Work the corners and the interruptions

Name every one, with what each control layer does there:

• Outside corners and inside corners, where the coping and the flashing both have to turn.
• Every place the parapet changes height or steps.
• Every scupper or overflow through the parapet.
• Every anchor, davit base, railing post or lightning conductor that passes through the top or the face.
• Where the parapet ends: a return, an adjoining building, an expansion joint.
• Where the roof membrane turns up the inside face and terminates, and what holds that termination.

An empty list means one was forgotten.

Step 5. Name what fails first

For the parapet, report failure behaviour rather than a drying direction, since the parapet is where both assemblies are least able to dry:

• If the coping leaks, where does the water go, and does it appear inside or disappear into the wall?
• If the air barrier transfer is not made, where does the leak show as a stain, and in what season?
• If the through-wall flashing has no discharge, what does the wall below look like in two winters?

Step 6. Report, and hand off

The control layer transfer table, the coping table with its notes, the corners and interruptions, then the failure behaviour. Close with the hand-offs, every time:

• The thermal bridge here has not been quantified, and it runs the full length of the roof perimeter.
• Structural design of the parapet and its fixings is open.
• Air barrier continuity around the whole building belongs to air-barrier-trace, and this is one of the junctions it will check.
• The roof manufacturer's perimeter requirements have not been read.

Count the open values on the face of the output. Then ask whether to write the detail to a file and where, and write it directly, tables included.

Rules

• Never supply a number. Not a height, not a slope, not a flashing extension, not a fastener spacing, not as a placeholder.
• Never recall a code or a manufacturer requirement. Guard heights, flashing extensions above the roof and perimeter fastening patterns all come from documents you quote.
• Refuse to proceed without both assemblies. This skill is a junction, and a junction needs two sides.
• Name the physical material that makes each control layer transfer. A dotted line on a section is not a transfer.
• State where the insulation goes around the parapet, every run.
• Name every corner and interruption even when the answer is short.
• Say on every run that the thermal bridge has not been quantified.

Anti-patterns

• Drawing the parapet as the wall continuing up, with the roof arriving at it. Two assemblies meet here and both change.
• Showing the air barrier as continuous across the top without naming what makes it continuous.
• Omitting the through-wall flashing because the coping is assumed watertight. Copings are joints in a line; they leak by design.
• Recalling how far a membrane has to turn up above the roof surface. That is code and manufacturer text.
• Quoting a clause found by web search.
• Calling the parapet resolved with an empty corners list.
• Treating the coping slope direction as a detail rather than a decision about which face gets stained.
• Running air-barrier-trace from inside this skill. It is a hand-off.

Resources

None. This skill is one file. Output is written directly at the path you choose.