Design Roof Assembly

A wall in a temperate climate usually gets a second chance: it can dry one way or the other. A compact roof often cannot dry in either direction, so anything that gets into it stays. That single difference is why a roof stack has to be decided rather than adapted from a wall.

Workflow

Step 1. Take the demands, not the layers

Ask, in one message:

1. What is the covering, or is it open? Membrane, and adhered or ballasted or mechanically attached? Or a steep-slope covering with laps?
2. What is the deck, and what spans it?
3. What climate zone, and from what source?
4. What runs below: conditioned or not, and how humid? A natatorium, a commercial kitchen and an office are three different roofs.
5. Where does the slope come from: the structure, tapered insulation, or a fill? And is it enough to drain, given what you have to work with?
6. What sits on the roof, what passes through it, and does anyone walk on it?
7. What is already fixed: a floor-to-floor, a parapet height, an equipment layout, a warranty already selected?

If the user offers a stack instead, take it as a proposal to be tested against the demands, not as the answer.

Step 2. Decide whether this roof can dry at all, then place the control layers

Answer the drying question before placing anything, because it constrains every position that follows. A compact roof with a membrane above and a vapor retarder below has no direction to dry in; that is a legitimate design and it means the assembly has to be built to stay dry rather than to recover.

State the answer, then present the control layers as a table with the reason column in full sentences:

Control layer	Where it sits	Why there	Continuous at

• Water. The covering, and what happens to water that gets past it. Name the secondary drainage path or say plainly there is none.
• Air. Which plane stops air, and note in the last column every place it has to carry: down the parapet to meet the wall's air barrier, around every penetration, across every joint in the deck.
• Vapor. Whether there is a vapor retarder and on which side of the insulation. On a roof this is more consequential than on a wall, because a membrane above is already close to vapor-tight and a second retarder below closes the assembly from both sides.
• Thermal. Above the deck, below it, or split. If any is below, say plainly that the deck itself is then cold and the vapor question changes with it.

Where one material serves two functions, say so, and say that one failure then takes out both.

Step 3. Build the stack, deck upward, and read the gaps between rows

#	Layer	Its job	Control layer?	Value needed from you

The last column stays as questions. A layer with no job comes out.

Then read between the rows:

• Is anything trapped between two vapor-tight layers with no way out?
• Does a fastener pass through a control layer, and what closes it?
• Does the slope get built inside the stack, and does that change any layer's thickness across the roof?
• Do two materials meet that should not touch, chemically or by temperature?

Step 4. Work the conditions, where the roof stops or gets interrupted

The field of a roof rarely fails. Name every one of these and say what each of the four control layers does there:

• The perimeter: parapet, or edge and fascia. Where the roof's air barrier meets the wall's.
• Every drain and every overflow or scupper.
• Every penetration: pipes, conduit, vents, structure passing through.
• Every curb: equipment, skylights, hatches, ductwork.
• Expansion joints, and any change in deck direction or level.
• Where this roof area meets another at a different height.
• Any tie-back, davit, or fall arrest anchor already planned.

An empty list here means one was forgotten, not that the roof is simple.

Step 5. Name the drying direction the assembly actually has

Read the stack back and state which way it can dry, or that it cannot. Compare that to what Step 2 assumed. If the assembly turns out to be closed both ways, say so plainly and name the consequence: every joint, curb and penetration is now the whole waterproofing strategy, because nothing gets a second chance.

State a direction, never a rate.

Step 6. Report, and hand off

The control layer table, the stack table with its between-row notes, the conditions list, then prose on what is unresolved. Close with the hand-offs, every time:

• The arithmetic has not been run. condensation-check takes it.
• Air barrier continuity has not been audited. air-barrier-trace takes it.
• Drainage sizing, structural capacity and the warranty are all open, and named as open.

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

Rules

• Never supply a number. Not a thickness, not an R-value, not a permeance, not a slope, not a drain count, not as a placeholder that looks like a value.
• Never recall a code requirement. Minimum slope, required insulation, vapor retarder rules by zone, overflow provisions: all quoted by you.
• Answer the drying question before placing a layer. On a roof it constrains everything after it.
• Every layer carries a job in a sentence, or it comes out.
• Name every condition in Step 4 even when the answer is short.
• Say on every run that the arithmetic has not been done here, and that the manufacturer's warranty has not been checked against this stack.

Anti-patterns

• Adapting a wall stack by turning it on its side. A wall can usually dry; a compact roof usually cannot, and the vapor decision is not the same one.
• Adding a vapor retarder below the deck by habit when the membrane above is already vapor-tight. That closes the assembly from both directions.
• Recalling a minimum slope, a per-inch R-value, or a "typical" tapered build-up because the figure feels standard.
• Quoting a code clause found by web search. Search identifies which document governs; it does not verify what it says.
• Calling the roof resolved with an empty conditions list.
• Treating drainage as sized because the drains are located.
• Running condensation-check or air-barrier-trace from inside this skill. Both are hand-offs, named and not executed.

Resources

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