Flashing a deck post that penetrates a roof is one of the most critical weathertightness details in New Zealand residential construction. Done poorly, it is a leading cause of water ingress, rot, and costly structural damage. Done correctly — with the right materials, sequence, and attention to detail — it will keep your home dry for decades.
Why This Detail Matters So Much
New Zealand's NZ Building Code Clause E2 (External Moisture) requires that buildings are constructed to prevent the penetration of water that could cause undue dampness or damage. A post penetrating a roof is a high-risk junction because water can track down the post, pool around the penetration, and enter the building envelope. MBIE Building Performance provides detailed guidance on E2 compliance that is worth reading before you start.
In most cases, adding a deck with posts through a roof will require a building consent from your local council. While some minor work is exempt under Schedule 1 of the Building Act, a structural penetration through a roof is unlikely to qualify. Always confirm with your council before proceeding.
Materials You Will Need
- Lead flashing or a quality aluminium/copper flashing (lead is the traditional NZ preference for durability around posts)
- Butyl or polyurethane sealant (e.g. Sika Sikaflex or equivalent)
- Roofing screws with neoprene washers
- Flashing tape (self-adhesive, compatible with your roofing membrane or underlay)
- Galvanised or stainless steel saddle/soaker (if using corrugated iron or tray roofing)
- Waterproof membrane or self-adhesive flashing tape for the upstand
- Appropriate post wrap or sleeve if the post is timber
Always match your flashing metals to avoid galvanic corrosion — for example, do not use copper flashing against aluminium roofing.
Step-by-Step Flashing Method
1. Plan the Post Location Carefully
Before cutting any holes, position the post to avoid roof valleys, gutters, and low-drainage zones where possible. Ideally, the post should penetrate near a high point of the roof profile (e.g. the crown of a corrugated sheet) to minimise standing water risk.
2. Cut the Penetration
Cut the roof cladding opening slightly larger than the post — typically 10–15 mm clearance all around — to allow for the flashing collar to sit properly. Do not cut the roofing underlay yet.
3. Install the Post and Structural Connection
The post must be structurally connected to the building framing below, not just resting on the roof. This is restricted building work if it involves primary structure — see the Licensed Building Practitioners website for what work requires an LBP.
4. Form the Flashing Collar
This is the most important step. The flashing collar (sometimes called a boot flashing or pipe flashing) must:
- Wrap tightly around the post with no gaps
- Extend at least 150 mm up the post above the roof surface
- Extend at least 100 mm onto the roof surface in all directions
- Be sealed at the top of the upstand with a continuous bead of sealant and a stainless steel jubilee clip or clamping band
For corrugated iron or tray roofing, a lead soaker is typically dressed (shaped by hand) to follow the roof profile around the post, then sealed and lapped under the uphill sheets and over the downhill sheets.
5. Integrate with the Roofing Underlay
Slit the underlay carefully and tape it up and around the flashing collar using compatible self-adhesive flashing tape. The underlay must lap over the top of the collar on the uphill side so any water that gets under the cladding is directed away from the penetration.
6. Lap the Roof Cladding
Replace or cut the roof cladding sheets so that:
- Uphill sheets lap over the top of the flashing collar
- Side sheets are sealed to the collar
- Downhill sheets sit below the collar, allowing water to run clear
Seal all laps with appropriate sealant and fasten with roofing screws.
7. Protect the Post
Timber posts should be wrapped or sleeved above the flashing to prevent water tracking down into the collar. A stainless steel or aluminium cap over the top of the post is also good practice.
Common Mistakes to Avoid
| Mistake | Why It Causes Problems |
|---|---|
| Sealing the bottom of the collar | Traps water inside — always leave a drainage path |
| Using incompatible metals | Galvanic corrosion eats through flashing within years |
| Insufficient upstand height | Wind-driven rain overtops the collar |
| Not integrating with underlay | Water bypasses the flashing at the underlay level |
| Relying on sealant alone | Sealant degrades; mechanical lapping is the primary defence |
When to Call a Professional
This type of work sits at the intersection of structural building work and weathertightness detailing — both areas where mistakes are expensive and sometimes invisible until serious damage has occurred.
You should engage a Licensed Building Practitioner (LBP) if:
- The work involves structural connections (almost always the case with deck posts)
- A building consent is required (check with your council)
- You are not confident forming and dressing lead or metal flashings
- Your roof is under warranty and you don't want to void it
BRANZ research consistently identifies penetration flashings as a top cause of weathertightness failures in NZ homes. Getting this right is worth the investment in professional help.
You can find qualified local builders and roofers on tradedirectory.co.nz.