Junctions deserve careful attention because control layers must connect across changes in geometry and material. Leaks can also occur through the field of a wall; the model focuses on junctions to make these connections visible.
The rule that governs every junction
Each control layer has to get from one assembly to the next. The Atlas uses three simplified labels:
- Carry straight through — the layer is continuous across the junction
- Hand over by lapping — one layer overlaps the next in the direction water travels
- Be interrupted — the layer stops, and something else must take the job
Interruption is not a defect. A window interrupts every layer in a wall, and that is fine — the frame takes over. What is not fine is an interruption where nothing takes over.
Lap direction decides everything
At a window head, water running down the wall has to be thrown out over the window. That depends on one thing: the barrier above laps over the flashing, not under it.
Look at the headGet it backwards and the flashing collects water and delivers it into the wall. That can defeat the drainage strategy, and the lap is concealed once the cladding is on.
The same rule, one storey up, governs where the roof meets the wall.
Roof to wallNot every junction is about water
Switch the layer you are tracing and the same junction tells a different story. At the floor edge, water passes by on the outside without incident — but the air barrier stops dead.
Follow air through the floor edgeWhere the ground meets the building
At the bottom of the wall, two separate water systems meet and must not connect. The wall drains down its cavity and out at the base. The foundation sheds groundwater sideways into the perimeter drain. Bridge them — with cladding carried down to the soil, or a cavity closed off at the bottom — and the wall wicks from the ground.
Wall to foundationCheck yourself
A head flashing is installed, but the weather-resistive barrier above it was tucked underneath instead of lapped over. What is the result?
The one layer that must never be interrupted
Every control layer in this lesson is allowed to stop, as long as something takes over. Load is the exception. It has to reach the ground, continuously, through every junction it passes.
Watch load find the groundThat is why the hardest details are the ones where structure passes through the enclosure — a balcony slab, a deck ledger, a beam landing on a foundation wall. The layer that cannot be interrupted is running straight through the layers that would rather it did not.
See it at a balconyWhat this means when you are looking at a house
A sidewalk view cannot establish the condition of concealed wall layers. Look for visible clues at junctions — under windows, at the base of the cladding, at the eaves, where a deck meets the wall. Visible staining or distortion can justify further investigation, but an absence of visible damage does not establish that the wall is dry.