A sagging or damaged garage roof can be a serious structural and safety issue, so it pays to catch problems early. The good news is that a basic visual inspection is something most homeowners can do safely from the ground or a stable ladder — no specialist tools required. Here's how to go about it in a methodical way.
Start With a Ground-Level Visual Check
Before you climb anywhere, stand back from your garage and look at the roofline from several angles. You're looking for:
- Visible dips or curves in what should be a straight roofline
- Uneven ridgeline — the peak of the roof should run in a straight, level line
- Bulging or bowing in the roof cladding (iron, tiles, or membrane)
- Gaps or lifted edges where the roof meets the walls or fascia
A healthy roof should have clean, straight lines. Any noticeable sag — even a gentle curve — warrants a closer look.
Inspect the Roof Cladding Up Close
If it's safe to do so, use a sturdy ladder to get a closer look at the roof surface. Always have someone with you to hold the ladder, and never walk on a roof you suspect is structurally compromised.
Common signs of damage to look for:
- Corrugated iron or steel roofing: Rust patches, holes, lifted or missing screws, and sheets that have pulled away from purlins
- Concrete or clay tiles: Cracked, slipped, or missing tiles; broken ridge capping
- Membrane or flat roofing: Bubbling, cracking, pooling water, or seam failures
- Fascia and barge boards: Rot, paint peeling, or boards pulling away from the structure
In New Zealand's variable climate — from the wet, humid conditions in Northland and the West Coast to the UV-intense summers in Central Otago — roofing materials can deteriorate faster than expected. Regular checks every one to two years are good practice.
Check Inside the Garage for Warning Signs
Some of the clearest signs of roof damage show up on the inside. On a dry day, go inside your garage and look up at the ceiling or exposed rafters:
- Water stains or damp patches on ceiling linings or timber
- Mould or mildew on rafters or wall framing
- Daylight visible through gaps in the roof structure
- Sagging ceiling lining (GIB board or ply) that has absorbed moisture
- Cracked or split rafters — look for fractures, especially near the ridge or wall plate connections
- Rafter or purlin deflection — stand at one end and sight along the length of a rafter; it should be straight
If you can see the structural framing, check that all joist hangers, straps, and fixings are still in place and free of rust.
Look for Water Ingress and Drainage Issues
Water is the number one enemy of any roof structure. Check your gutters and downpipes for blockages, sagging, or overflow marks on the fascia. Blocked gutters can cause water to back up under the roof cladding, leading to rot in the framing over time.
Also check:
- Flashings around any penetrations (skylights, vents, pipes) — lifted or cracked flashings are a common leak source
- Valley gutters where two roof planes meet — these collect a lot of water and can rust or block
- Ponding water on flat or low-pitch roofs, which accelerates membrane breakdown
Under the NZ Building Code, roofs must shed water effectively (Clause E2 — External Moisture). If your roof is consistently holding water, it may not meet this requirement.
Use a Simple Straightedge or String Line
For a more precise check of sagging, you can run a string line along the ridgeline or across the rafters (from inside the garage). Any noticeable deviation is worth having assessed by a professional. This is a quick, low-cost method that building inspectors often use as a starting point.
Document What You Find
Take photos of anything that looks out of place — both from outside and inside. Note the location, and if possible, measure the approximate size of any damage. This documentation is invaluable if you need to get quotes from tradespeople or make an insurance claim.
When to Call a Professional
Some findings go beyond a DIY check. You should contact a licensed building inspector or LBP-registered builder if you notice:
- Any visible structural sagging or deflection in rafters or purlins
- Cracked, split, or notched structural timber
- Evidence of ongoing water ingress or significant rot in framing
- The roof is more than 20–25 years old and hasn't been professionally assessed
- You're unsure whether the garage was built to comply with NZS 3604 (the standard for light timber-framed buildings)
- You're planning to add weight to the roof (e.g., solar panels) — this requires structural assessment
Repairs to structural framing may require a building consent depending on the scope of work. Always check with your local council before starting structural repairs — some work is exempt under Schedule 1 of the Building Act, but it pays to confirm. You can find guidance at MBIE Building Performance.
For a thorough, impartial assessment, consider hiring a qualified building inspector. You can find local inspectors and builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
BCITO Qualified
