How to Flash a Roof Pitch Change or Slope Transition
Where a steep roof meets a low one, water slows down, debris collects and laps matter more. Here is how the transition is built.
- Advanced
- 2 to 6 hours
- Updated
The short answer
A pitch change is where one roof slope meets another at a different angle, such as a steep roof running into a low-slope porch roof or a mansard lower slope meeting a flatter upper one. Lap underlayment and ice and water shield across the break, add a transition flashing or a wide metal drip edge at the lower slope, and confirm the lower slope meets the roofing product's minimum pitch. Low slopes often need membrane roofing.
Many houses have more than one pitch: a steep main roof with a low-slope porch, a garage roof tied into a steeper house roof, or a mansard with a steep lower slope and a flat top. The line where the pitch changes is a weak point. Water coming off the steep slope is fast and concentrated, and as it reaches the low slope it slows, spreads and sits, so any shortcoming in the laps or a low slope shingle roof shows up there first.
The approach is to make the joint behave as if it were part of a continuous plane. Underlayment laps carry across the break, flashing covers the transition and the roofing on the low side is chosen for the low slope it has to handle. The governing principle is always the lapping rule.
For the shingle side of low slopes, see shingling a low-slope roof. For a wall where a roof meets a taller structure, see headwall flashing.
Tools
- Hook blade utility knife
- Tin snips
- Hand seamer or brake
- Roofing hammer or coil nailer
- Chalk line
- Tape measure
- Pitch gauge or level
Materials
- Ice and water shield
- Underlayment suited to the slope
- Transition flashing or wide drip edge metal, 26 gauge
- Roofing nails, 1-1/4 inch
- Sealant for laps and exposed nail heads
- Membrane roofing for the low slope where needed
Safety first
- Transitions are where pitch changes under your feet. Wear boots with good grip, stay tied off, and watch the break line. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- The steep side sheds loose material onto the low side, which is slippery with moss and granules. Clear it before walking.
- Cold metal flashing is brittle and sharp. Wear cut resistant gloves when bending.
Step by Step
Step 1: Measure both pitches and check the lower slope against the roofing product
Use a pitch gauge or a level and tape. Most asphalt shingle manufacturers set a minimum slope of 2/12 with special underlayment, and a standard installation is for 4/12 and steeper. Below those numbers, shingles are not suitable and a membrane or metal roofing is needed. Always check the shingle manufacturer's instructions, since the limits and conditions vary by product. See shingling a low-slope roof.
Watch out: Shingles on a slope below the manufacturer's minimum leak in rain and void the shingle warranty. Change the material, not just the sealant.
Step 2: Prepare the deck at the break
The break line often has a seam in the sheathing or a change in framing. Make sure the deck is flat and continuous on each side, with panels nailed tightly. Replace damaged sheathing. A deck with a sharp kink or a gap needs to be shimmed or patched first.
Step 3: Install ice and water shield across the break
Ice and water shield is self-sealing around fasteners and forgiving at the kink. Fold it gently at the break and press it in to avoid bridging a gap. On a low slope below 4/12, many manufacturers call for a full-coverage membrane. See how to install ice and water shield.
Step 4: Lay underlayment in the lapping order
Start at the eave of the low slope and work upslope, lapping each course over the last. At the break, the upper (steep) slope's underlayment goes over the lower (flat) slope's. Lap end joints by about 6 inches and stagger them. Follow the underlayment manufacturer's lap instructions. See synthetic underlayment.
Step 5: Install the transition flashing
A transition flashing is a bent piece of metal shaped to fit the change in angle. Use a width of at least 4 to 6 inches on each side of the break for a simple roof-to-roof joint, and wider where wind is a concern. Fasten only the upper leg where it will be covered, and lap pieces by 4 inches in the direction of water flow. In some cases a wide drip edge or a formed flashing with a hemmed lower edge does the job. See roof flashing types for the choices.
Step 6: Shingle or roof up to and over the transition
On the steep slope, run shingles down over the upper leg of the flashing so water sheds onto the metal and continues. On the low slope, start the new course over the lower leg of the flashing, leaving a gap at the break so debris does not bridge it. Follow the manufacturer's low-slope nailing and sealant requirements. See how to lay shingle courses.
Step 7: Deal with where the pitch change meets a wall or valley
The corners are the vulnerable spots. Step flashing on the wall runs down to the transition metal, and the transition metal laps over the lower pieces. See step flashing and roof valley types.
Mansard and Gambrel Roofs
A mansard lower slope is very steep, often 60 degrees or more, and the upper slope is nearly flat. The break between them is a wide line of exposed metal, usually with a membrane roof above and shingles or metal below. Water runs fast down the steep slope and needs a generous drip and gutter or flashing at the break. A gambrel has two pitches on each side and the break is lapped like a steep-to-moderate transition, which is less demanding.
Low Slope Options
- Self-adhered membrane under shingles, for slopes down to the shingle manufacturer's minimum.
- Metal roofing with sealed seams or standing seam, for low slopes the manufacturer allows. See standing seam installation.
- A membrane roof such as EPDM, TPO or torch-on for very low or flat areas.
Fraser Valley Notes
Low-slope porch and addition roofs collect needles, moss and standing water under long wet spells, and the slower flow there puts heavy demand on laps. Keep the transition clear of debris, and do not let moss grow across the break.
Common Mistakes
- Shingling a low slope below the minimum pitch and expecting sealant to make up the difference.
- Reverse lapping underlayment at the break, so the steep side tucks under the low side.
- Flashing too narrow for the flow concentrating at the transition.
- Bridging the break with stiff sheathing or shingles that then crack as the roof moves.
- Leaving out membrane across the break, relying on felt alone.
- Ignoring the corners where the transition meets walls and valleys.
When to Call a Professional
If the lower slope is under 4/12, if the roof has a mansard or gambrel, or if an existing transition has leaked more than once, have a roofer assess the whole detail. The fix may involve changing material on the low slope, not only re-flashing the break.
Pitch changes with walls, valleys and skylights close by stack several flashings in one spot. A roofer can check that they work as a system.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Flash a Roof Pitch Change: Questions
How do you flash where a steep roof meets a low slope roof?
Lap ice and water shield across the break, lap the steep slope's underlayment over the low slope's, then install a formed transition flashing with its upper leg under the steep-slope shingles and its lower leg over the low slope. Use roofing on the low side that is approved for its pitch, often a membrane or metal, not standard shingles.
What is the minimum pitch for asphalt shingles?
Standard asphalt shingle installation is typically for 4/12 and steeper. Many manufacturers allow 2/12 to 4/12 with special underlayment and full-coverage membrane, and below 2/12 shingles are not recommended. The exact limits depend on the product and warranty, so check the shingle manufacturer's installation instructions before choosing the roofing.
Why does a roof leak where the pitch changes?
Water running fast off a steep slope slows and spreads on the low slope, so laps that were fine on the steep side get tested. Common causes are reverse laps in the underlayment, flashing that is too narrow, shingles on a slope that is too low, and debris damming the joint. Corners where the transition meets walls are another weak point.
Do you need flashing where two roof pitches meet?
Yes, in most cases. A formed transition flashing or wide metal piece, lapped with the underlayment and roofing on both slopes, carries water across the break. Without it, the joint relies on shingles alone to bridge a change in angle, and they tend to crack or lift. The right detail depends on the pitches and the roofing material.
What roofing goes on a porch roof that joins a steeper roof?
It depends on the porch roof's slope. At 4/12 or steeper, asphalt shingles work. Between about 2/12 and 4/12, shingles may be allowed with special underlayment per the manufacturer. Below 2/12, use a membrane such as EPDM or TPO, or a metal roof approved for low slopes. Flash the joint where they meet carefully.
Related Guides
All Flashing Details guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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