Building envelope · Junctions & Terminations
Roof-to-Wall Abutment and Flashing System
This entry explains why an abutment needs weathering contributed by both the roof and the vertical element it stops against, and why a flashing without the tray behind it relocates water rather than excluding it.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What abutment detail is
An abutment exists wherever a roof stops against something taller than itself: a higher wall above a lower extension, a chimney rising through the slope, or a neighbouring flank wall. The roof cannot simply run into the vertical face, because the joint between them is exactly where water collects and where the two constructions move to schedules of their own.
Weathering at such a junction is deliberately layered rather than sealed. The roof turns its own covering or membrane up the face. The vertical element then oversails that upstand with a flashing retained in a chase or a bed joint. If the vertical element is a cavity construction, a tray inside it collects water descending within the cavity and directs it out above the flashing. Each stage assumes the one below it exists.
The test against a parapet and roof upstand system is ownership of the vertical element. At an abutment the wall belongs to something else and continues upward past the roof, so the concealed tray is buried in another trade's masonry where the roof detailer cannot see it. A parapet is part of the roof's own perimeter, capped by a coping and exposed on its outer face as well as its inner one. A valley, by contrast, is a geometric condition inside a covering assembly rather than a junction with a separate element.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- roof to wall junction
- flashing and soaker detail
- chimney and stack weathering
- wall abutment detail
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Carry water arriving on the roof surface away from the line where the roof meets a taller element, rather than sealing that line shut.
- Intercept water descending inside the abutting construction and return it to the roof surface above the point at which the covering can take it.
- Allow the roof and the vertical element to move independently of one another without opening the weathering between them.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
This system is itself the junction between two others, so there is no build-up to read. The roles below are the conditions that have to be resolved where those systems meet, and no order is implied between them.
- Edge and terminationRoof-side upstand
The roof's own covering, membrane or soakers turned up the vertical face. It raises the level at which water would have to stand before crossing the junction, but it is open at its top edge and is not a termination in its own right.
- Drainage planeCover or stepped flashing
The sheet fixed to the vertical element and dressed down over the upstand, shedding water arriving on that face back onto the roof. On a sloping roof it steps with the coursing, which is why the wall's own joint pattern governs how the flashing can be formed.
- Control layerCavity tray or through-wall flashing
A concealed layer inside the abutting construction that catches water running down within it and discharges it outward above the cover flashing. It is invisible once built, cannot be added later without opening the wall, and its outlets are the only evidence it is working.
- Drainage planeBack gutter and front apron at a stack
Where the vertical element interrupts the flow of water down the slope, the upslope face needs a channel that carries water around it and the downslope face needs an apron returning water onto the covering below. Both belong to the same weathering as the side flashings.
- Jointing and sealingChase, wedges and pointing
The mechanical retention of the flashing in the vertical element and the closing of the slot above it. The wedges or clips are what resist wind lift, and the pointing closes the slot above them as the sacrificial part of the detail, renewed on a cycle while the flashing itself stays in place.
- Thermal layerInsulation carried across the junction
The continuation of the roof insulation to meet the insulation of the abutting construction. An abutment is a change of plane as well as a change of trade, so the insulation line and the weathering line often have to be resolved by different people at the same spot.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
Each stage of an abutment is useful only because of the stage above it. An upstand with no cover flashing over it is an open channel. A cover flashing with no tray behind it in a cavity construction weathers the wall face while leaving the cavity free to deliver water below the flashing line. Because the layers overlap rather than seal, removing any one of them does not create a leak at that point; it relocates the water to somewhere further down.
That relocation is why abutment failures are so often misdiagnosed. Water getting past a stepped flashing may travel within the cavity, along a mortar bed or down a rafter before it appears, so the damp patch commonly shows on a different wall, on a lower storey or well away from the junction. Repairing the ceiling where the stain appears treats a symptom whose cause is somewhere nobody has looked.
The flashing and the wall are mechanically interdependent as well. The chase or joint the flashing is dressed into has to be formed in the vertical element, so whoever builds the wall determines what whoever weathers the roof can achieve. Where the wall is finished first with no provision made, the usual outcomes are a shallow chase, a surface-sealed edge or reliance on a bead of sealant, all of which shift the junction from mechanical retention to adhesion.
Movement runs across this junction rather than along it. The roof deflects, warms and cools on its own schedule and the abutting element on another, so the weathering has to accommodate differential movement while remaining lapped. That is why the roof-side and wall-side components are separate pieces rather than a single continuous sheet, and why a rigid fill between them tends to fracture at precisely the line it was meant to protect.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Edge and termination
Malleable and corrosion-resistant sheet metals are commonly encountered in cover flashings, soakers and aprons. Which metal can be used alongside a given covering and a given masonry is governed by compatibility, exposure and the manufacturers' documentation.
Control layer
Preformed damp-proof course materials and self-adhesive membranes are commonly encountered in cavity trays and through-wall flashings. Whether a product can be used in a concealed position of this kind rests with its documentation and with the designer.
Jointing and sealing
Pointing mortars and gun-applied sealants are commonly encountered where a flashing is retained and its slot closed. Sealants are conventionally treated as secondary to mechanical retention, and their role in any given detail is a matter for the designer.
Substrate
The vertical element at an abutment is commonly encountered in brick, block, stone or rendered construction. What each allows by way of chasing, coursing and concealed trays differs considerably, and the consequences are a matter for the designer.
Components
Building components that fill these roles
The discrete elements commonly occupying each role in this assembly. This is a different statement from the material families above: those name what a role is commonly made of, these name the positioned elements that occupy it. Commonly encountered, never recommended.
Control layer
Drainage plane
Edge and termination
- Back Gutter →The drainage half of an abutment, as distinct from the covering half formed by flashings and soakers.
- Stack →The obstruction the system's flashings, soakers and back gutter are formed around.
- Dormer Cheek →Its base is an abutment between a vertical surface and a sloping covering, running the depth of the dormer.
- Flashing →Covers the junction where a roof meets a wall and closes the upstand beneath it.
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
Concealed tray within the abutting wall
The tray at an abutment is the same device used elsewhere in a cavity wall, but here its line is set by the roof rather than by an opening. Its weeps have to discharge above the cover flashing, which means two trades have to agree a level that neither of them controls alone.
Read about Cavity Trays →The wall the roof abuts
A cavity wall delivers water down the back of its outer leaf, so an abutment against one is not weathered by a face flashing alone. Whether the wall was built with a tray at the right line is usually undiscoverable without opening the construction up.
Read about Twin-Leaf Cavity Wall →Covering and soakers at the junction
Discontinuous coverings weather an abutment with individual soakers interleaved with the courses and a flashing dressed over them, so the coursing of the roof and the coursing of the wall have to be reconciled at the junction itself.
Read about Tile and Slate Covering →Membrane upstand and mechanical termination
On a membrane roof the upstand is a continuation of the field sheet and is mechanically held near its top edge, with the cover flashing above. The termination line, not the membrane, is what fixes how far water can rise at the junction before it is behind everything.
Read about Single Ply Roof →Handover of air and vapour control
An abutment is also where the roof's control layers have to meet the wall's, and those layers sit inboard of everything the weathering is concerned with. Their continuity is decided long before any flashing becomes visible.
Read about Control Layer Transition →Roof Carcass
Where a slope meets a taller wall the framing has to bear or tie there, and the weathering detail follows that arrangement.
Read about Roof Carcass →Water Control Layer
Where a roof runs into a wall, the wall plane and the roof underlay have to be lapped in the right order with the flashing between them. This is the junction where two trades, two materials and two directions of flow meet in one detail.
Read about Water Control Layer →Eaves System
Arrives at the same corner wherever a roof also stops against a taller wall, so the eaves discharge and the abutment weathering have to be resolved together there.
Read about Eaves System →Cold Pitched Roof
An abutment interrupts the run of the covering and the underlay and often blocks the high-level air path along that edge. What replaces the interrupted route, and how the underlay is turned up against the wall, are decided together.
Read about Cold Pitched Roof →Warm Pitched Roof
Where the slope runs into a wall, the covering, the underlay and the insulated line all stop at different heights. A raised covering plane moves the flashing line, so the abutment is redrawn whenever insulation is added above the rafters.
Read about Warm Pitched Roof →Standing Seam Roof Assembly
Where the trays run into a wall or a chimney, the upstand and its cover flashing have to close the roof while still allowing the metal to travel. Fixing the upstand rigidly to both the tray and the wall is a common way of losing that freedom.
Read about Standing Seam Roof Assembly →
Internal junctions
Junction conditions inside this system
Junctions between the components within this assembly, rather than at its boundary. An entry exists only where the condition between two elements governs how the assembly behaves — two parts touching is not on its own a junction worth naming.
Back Gutter to Chimney Stack
Where water running down the slope arrives at the high face of a stack and has to be collected there and led out to each side.
Drainage · Moisture · Weathering · Buildability
Chimney Stack to Flashing
Where the roof covering is closed against an isolated mass passing through it, on faces that each meet the slope differently.
Weathering · Moisture · Movement · Buildability
Upstand to Flashing Termination
Where waterproofing turned up a vertical face stops, and where a flashing above closes that stop.
Moisture · Weathering · Movement · Buildability
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- Water reaches this junction from the roof surface, from the face of the vertical element and from inside a cavity construction. Whether the layered weathering handles each of them is a property of the completed junction, not of any single flashing.
- Movement
- The roof and the abutting element move independently, so the weathering is assembled as overlapping pieces rather than a continuous seal. Rigid infill across the junction concentrates that movement at a line which cannot accommodate it.
- Buildability
- Provision for the concealed tray has to be made while the wall is being built, by a trade that will have left before the roof is weathered. Where it is missed, the remedy involves taking part of the wall apart again.
- Maintenance and access
- Pointing above a flashing can be renewed from a ladder or scaffold; a cavity tray cannot be renewed at all without opening the construction. Treating the two as the same kind of maintenance item leads to repeated repairs at the visible layer.
- Documentation
- An abutment is drawn by the roof designer but built partly by the wall trade, so the detail that matters most often appears in neither party's package. Recording who provides the tray is more useful than specifying the flashing metal.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Is the abutting element a cavity construction, and if so has a tray been provided at the line where the roof meets it?
- Who forms the chase or bed joint that retains the flashing, and at what point in the programme does that happen?
- Does the weathering rely on mechanical retention, or on adhesion at the top edge of the flashing?
- Where does water intercepted inside the wall discharge, and is that point above the cover flashing?
- How does the roof insulation meet the insulation of the abutting element at this change of plane?
- If the abutting wall belongs to a neighbouring building, who is entitled to carry out work on it?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Sealant along the top of a flashing is often taken for the weathering itself, when it is at most a supplement to a mechanical fixing.
- A damp patch near an abutment is assumed to mark the point of entry, but layered weathering relocates water rather than releasing it where it got in.
- A cavity tray is frequently thought of as a roofing item, though it sits inside the wall and has to be built in by the masonry trade.
- Chimneys are treated as a single flashing job, when the upslope and downslope faces of a stack need entirely different components.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Has a cavity tray been provided at this abutment, and can its discharge points be seen from outside the building?
- Which package includes the tray, and has that been confirmed with the trade actually building the wall?
- How is the flashing retained in the vertical element, and what happens if the chase cannot be cut where it is drawn?
- Has anyone traced where water appearing internally could have entered, rather than assuming it entered nearby?
- What provision is made for movement between the roof and the element it abuts?
- If this is a party or boundary wall, what consents are needed before any work begins?
What this page does not do
- Nothing described here is watertight in isolation; weathering at an abutment depends on the whole layered detail being complete and correctly installed.
- Chimneys and flues carry combustion products, and any work affecting them belongs with a qualified professional in that field.
- This entry states no upstand height, lap or chase depth, and those are design determinations lying outside its scope.
- Working on a wall belonging to an adjoining owner may carry legal obligations that are outside the scope of this reference.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Commonly built alongside
Systems routinely present in the same building without a junction recorded between them and this one. Where two systems do meet, the junction is set out above instead.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- Pitched Roof · Envelope
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Envelope Junctions, Interfaces and Terminations
The places where continuous layers stop, turn, change plane or are pierced, and several duties collide inside a very small amount of construction.
Browse all junctions & terminations entries →