Install Dormer Siding & Trim
Completing the exterior with siding and trim that matches your home, creating a finished look that appears original to the house.
Why Proper Dormer Siding Matters
The siding and trim on your dormer serve two crucial functions: waterproofing and aesthetics. Properly installed siding with correct flashing keeps water out of your walls and protects the structure beneath. But siding is also the most visible part of your dormer - it's what you and your neighbors see every day. A well-finished dormer adds curb appeal and home value. A poorly finished one looks like an afterthought and can decrease property value. The key is matching your existing home's materials, proportions, and details so the dormer looks like it was always there.
Tools & Materials You'll Need
Step-by-Step Guide
Install House Wrap
The weather-resistant barrier is your first line of defense:
- Start at the bottom of dormer walls and work up
- Overlap horizontal seams by 6 inches (upper over lower)
- Overlap vertical seams by 12 inches
- Wrap corners, overlapping a minimum of 12 inches
- Lap house wrap over step flashing at roof intersection
- Tape all seams with approved seam tape
- Cut X-pattern at window openings, wrap edges into frame
Critical: House wrap must lap over the step flashing that sits on the roof shingles. Water hitting the siding runs down the house wrap and over the flashing onto the roof, not behind it.
Install Window Flashing
Proper window flashing prevents leaks at this vulnerable point:
Flashing Order (Bottom to Top):
- 1. Sill Pan: Install flexible flashing tape across sill, extending 6" up each side
- 2. Install Window: Set window in opening, shim and fasten
- 3. Side Jamb Flashing: Apply flashing tape from sill up both sides
- 4. Head Flashing: Install tape across top, extending over side tape
- 5. Drip Cap: Install metal drip cap above window, under house wrap
Product Note: Use flexible flashing tape designed for windows (like Vycor or similar). Don't substitute with roofing ice shield - it's too thick and doesn't conform well around window frames.
Install Corner Boards and Trim
Trim goes on before siding and establishes the finished look:
Trim Components:
- • Corner boards (typically 1x4 or 1x6)
- • Frieze board under soffit
- • Window casing (match existing)
- • Water table at roof line
Material Options:
- • PVC trim (rot-proof, paintable)
- • Cedar (traditional, needs paint)
- • Composite (middle ground)
- • Match existing house trim
- Install corner boards first, plumb and secure with stainless nails
- Add frieze board under soffit, level across the top
- Install window casing, keeping reveals consistent
- Pre-prime or pre-paint trim before installation if possible
Install Siding
Match your existing house siding style and installation method:
Common Siding Types:
Vinyl Siding:
Lock bottom, nail loosely at top. Allow 1/4" expansion gap at trim.
Wood Lap Siding:
Nail through top of each course. Maintain consistent exposure.
Fiber Cement:
Face nail with stainless nails. Pre-paint cut edges.
Cedar Shingles:
Double starter course. Offset joints. Nail above exposure line.
- Establish level starting line at bottom of dormer
- Install starter strip or first course
- Work up, maintaining consistent exposure to match house
- Cut carefully around windows and at corners
- Hold siding 1-2" above roof surface to prevent water wicking
Caulk and Paint
Final sealing and finishing completes the weatherproofing:
- Caulk all trim joints with quality paintable caulk
- Seal gaps between trim and siding
- Caulk around windows where casing meets frame
- Do not caulk bottom edges of siding (allows drainage)
- Prime all bare wood with exterior primer
- Apply two coats of exterior paint to trim
- Paint siding if required (wood or fiber cement)
Color Matching: If painting to match existing, bring a sample to the paint store for computer color matching. Test in a hidden spot first. Colors can look different when dry and in natural light.
Pro Tips
- ✓Install a kick-out flashing where the dormer meets the main roof - critical for directing water into gutter
- ✓Use stainless steel or hot-dipped galvanized fasteners - regular nails rust and stain siding
- ✓Pre-paint trim pieces on sawhorses before installation - easier and better coverage
- ✓Take photos of existing house details (corner treatment, window casing profile) for matching
- ✓Consider re-painting entire wall face for uniform color - spot painting rarely matches perfectly
Frequently Asked Questions
How do I match existing house siding on a new dormer?▼
For vinyl or aluminum siding, contact the original manufacturer with the product name or bring a sample piece to a supplier. For wood siding, measure the exposure and profile, then have a lumberyard mill matching pieces or find a close match. For older houses, consider re-siding the entire wall face for consistency. Color matching is easier if you paint both old and new siding together with fresh paint.
What is the correct order for installing dormer exterior components?▼
The correct order is: (1) House wrap/weather barrier, (2) Window pan flashing, (3) Window installation, (4) Window side and head flashing, (5) Corner boards and trim, (6) Siding from bottom up, (7) Final caulking, (8) Paint. Each layer overlaps the one below like shingles to shed water. Deviating from this order can create paths for water infiltration.
Should dormer trim match the house or can I use a contrasting color?▼
Dormer trim should coordinate with your home's overall exterior design. Most traditional homes use matching trim throughout. However, some architectural styles feature contrasting dormer trim as an intentional accent. Look at homes in your neighborhood with dormers for inspiration. When in doubt, match the existing house trim for a cohesive look that appears original to the home.
How do I prevent water damage at the dormer-roof intersection?▼
The dormer-roof intersection is critical. House wrap must lap over the roof step flashing by at least 4 inches. Install a kick-out flashing at the bottom corner where the dormer meets the main roof to direct water into the gutter. Caulk is not a substitute for proper flashing - use it only to seal trim joints, never as primary waterproofing. Ensure siding is held 1-2 inches above the roof surface.