Frame Roof and Create Tie-In
The roof is where your addition meets the sky—and where water wants to enter. How you frame the roof and connect it to your existing house determines both the look of your addition and its long-term weather resistance.
Quick Summary
Time needed
3-7 days
Cost
$5,000-$15,000
Professional help
Framing crew
Why This Step Matters
Roof design affects everything: how your addition looks, how water drains, how snow loads distribute, and how prone the junction is to leaks. A poorly planned roof tie-in creates a maintenance nightmare—ice dams, trapped debris, and chronic leaking at the junction.
Leak Risk Zone
The junction between new and existing roofs is the #1 leak location on home additions. Proper flashing here is critical—a $500 flashing job done right prevents $10,000+ in water damage repairs later.
Roof Tie-In Options
How your new roof connects to the existing house depends on the addition's location and your aesthetic preferences. Here are the three main approaches:
Extended Roof (Same Pitch)
The addition's roof continues the existing roof plane outward, as if you're simply making the house bigger. This works when the addition is behind or beside the house.
Pros:
- • Seamless appearance
- • Best water flow
- • Simplest flashing
Cons:
- • Requires matching pitch exactly
- • May need full re-roof
- • Limited placement options
Perpendicular Gable
The addition has its own gable roof that meets the existing roof at a 90-degree angle, creating a valley where they intersect. This is common for side additions.
Pros:
- • Flexible placement
- • Architecturally interesting
- • Can have different ridge height
Cons:
- • Valley collects debris/snow
- • More complex flashing
- • Higher leak risk
Shed Roof (Single Slope)
A single-slope roof that starts high at the existing wall and slopes away. Often used for bump-outs, sunrooms, and smaller additions.
Pros:
- • Simplest to frame
- • Easy flashing at wall
- • Good for narrow additions
Cons:
- • May look "tacked on"
- • Lower ceiling at far end
- • Limited attic space
Design advice: When possible, match your existing roof pitch and style. A roof that looks "original" adds value; a mismatched roof screams "addition" and can hurt resale value.
How to Frame the Roof
Determine the Roof Pitch
Measure your existing roof pitch—expressed as rise over run (e.g., 6/12 means 6 inches of rise for every 12 inches of horizontal run). Your new roof should match unless there's a design reason not to.
How to measure pitch: Place a level horizontally against the roof, mark 12 inches, then measure vertically from that point to the roof surface. That measurement is your "rise" in a rise/12 pitch.
Choose Trusses vs. Stick-Framed Rafters
For most additions, you'll choose between factory-made trusses and site-built rafters:
Trusses
- • Factory-engineered, stronger
- • Faster to install
- • Requires crane access
- • Less flexible for complex shapes
Stick-Framed Rafters
- • Built on-site, more flexible
- • No crane needed
- • Allows vaulted ceilings
- • More labor-intensive
For additions: Stick-framed rafters are often preferred because they're easier to fit to existing conditions and don't require crane access that may not fit in your backyard.
Frame the Ridge and Rafters
For a gable roof, start with the ridge board or ridge beam at the peak, then install rafters from ridge to wall plates.
- Rafter spacing: 16" or 24" on center
- Rafter size: 2×8 to 2×12 depending on span and pitch
- Bird's mouth cut: Notch where rafter meets wall plate
- Collar ties: Horizontal members connecting opposing rafters
Create the Tie-In to Existing Roof
This is the critical junction. The method depends on your roof style:
- Extended roof: Remove existing shingles, connect new rafters to existing with splice plates
- Perpendicular gable: Frame valley rafters, install valley jacks
- Shed roof: Attach ledger board to existing wall, rafters bear on ledger
Critical: Strip back existing shingles at least 12 inches from the tie-in point. You need to flash under the existing shingles, not just up against them.
Install Flashing at the Junction
Proper flashing is what keeps water out. At roof-to-roof and roof-to-wall junctions, use multiple layers of protection:
- Ice and water shield: Self-adhering membrane at valleys and tie-ins
- Step flashing: L-shaped metal at wall-to-roof junctions
- Valley flashing: W-shaped metal or woven shingle valley
- Counter flashing: Cap flashing that covers step flashing
Best practice: Use minimum 36" wide ice and water shield in all valleys and at wall tie-ins. It's cheap insurance against ice dams and wind-driven rain.
Install Fascia and Soffits
Fascia boards cap the rafter tails; soffits enclose the underside of the roof overhang. Both should match your existing house.
- • Match existing fascia size and material
- • Install drip edge over fascia
- • Include soffit vents for attic ventilation
- • Connect soffit vents to ridge vent or gable vents
Common Roof Pitches
| Pitch | Angle | Common Use |
|---|---|---|
| 2/12 - 3/12 | 9° - 14° | Low slope, requires special roofing |
| 4/12 - 6/12 | 18° - 27° | Standard residential, most common |
| 8/12 - 10/12 | 34° - 40° | Steep, sheds snow well |
| 12/12+ | 45°+ | Very steep, A-frame style |
Match your existing pitch unless there's a specific design reason to differ.
Frequently Asked Questions
Should the new roof ridge be higher, lower, or the same as the existing?
For the best appearance, the addition ridge should be lower than or equal to the main house ridge. A higher addition ridge looks odd and can create drainage problems. If matching isn't possible, aim for 12-24 inches lower.
Do I need to re-roof the entire house to match?
Not necessarily. If your existing shingles are in good condition and the same shingles are still available, you can often tie in without a full re-roof. However, if shingles have been discontinued or are weathered differently, a full re-roof may be worth considering for aesthetics.
What's the difference between a ridge board and ridge beam?
A ridge board is a non-structural board that rafters meet at—the rafters push against each other for support. A ridge beam is a structural beam that actually carries the roof load, required when you want a vaulted ceiling with no collar ties. Ridge beams need proper sizing and support posts.
How do I prevent ice dams at the roof junction?
Ice dams form when heat escapes through the roof, melting snow that refreezes at the cold eave. Prevent them with: (1) proper attic insulation (R-49+), (2) air sealing at the ceiling plane, (3) adequate attic ventilation, and (4) ice and water shield membrane at eaves and valleys.
Ready for the Next Step?
With the roof framed and tied in, you're ready to make the structure weather-tight by installing sheathing and weather barriers on walls and roof.