Roof underlay is a critical layer of protection sitting between your roofing material (iron, tiles, or membrane) and your roof structure. If it's damaged or missing, water, wind-driven rain, and condensation can penetrate your home — often without obvious signs until serious damage has already occurred. Here's how to check its condition safely and what to look for.
What Is Roof Underlay and Why Does It Matter?
Roof underlay (sometimes called sarking or building wrap) is a water-resistant or waterproof sheet installed over the roof framing before the outer cladding goes on. In New Zealand, roof underlay requirements are governed by NZ Building Code Clause E2 (External Moisture), which requires buildings to prevent moisture from entering and causing damage. MBIE Building Performance provides guidance on these requirements.
Underlay serves several purposes:
- Acts as a secondary barrier against wind-driven rain
- Allows moisture vapour to escape (breathable underlays)
- Protects the structure if the outer roofing is breached
- Reduces condensation dripping onto insulation and ceilings
Older homes (pre-1990s especially) may have bitumen-impregnated felt underlay, which becomes brittle and cracks over time. Modern homes typically use a synthetic breathable membrane.
Safety First Before You Inspect
Before doing anything, understand that working at height is dangerous. Falls from roofs are a leading cause of serious injury in New Zealand. WorkSafe NZ requires that anyone working at height takes appropriate precautions — including using a safety harness, roof ladder, and having a spotter on the ground.
If you're not confident working at height, or your roof pitch is steep, do not climb up. Many of the checks below can be done from inside the roof space or from the ground.
Step 1: Check From Inside the Roof Space (Safest Option)
The easiest and safest way to inspect your underlay is from inside your ceiling cavity or roof space:
- Access your manhole — most NZ homes have a ceiling access hatch.
- Take a torch and look up at the underside of the roof cladding.
- Look for daylight showing through — this indicates missing or torn underlay.
- Check for water staining, mould, or damp timber on rafters and battens.
- Look for sagging or torn sections of underlay hanging down between rafters.
- After rain, check for active drips or wet patches on the underlay or timber.
This internal inspection is the most reliable method and requires no climbing on the roof.
Step 2: Look for Warning Signs Inside Your Home
Damaged or missing underlay often shows up as interior symptoms before you ever get on the roof:
- Water stains on ceilings or walls — brown rings or discolouration
- Mould or mildew smell in the ceiling space or upper rooms
- Damp or wet insulation — insulation that's wet loses its effectiveness and can cause structural rot
- Peeling paint or bubbling plasterboard on ceilings
- Increased heating costs — wet or compressed insulation performs poorly
If you notice any of these, underlay damage is a strong possibility and warrants further investigation.
Step 3: Ground-Level External Inspection
Using binoculars from the ground, scan your roof for:
- Lifted, cracked, or missing roofing iron or tiles — these expose the underlay to UV and physical damage
- Visible gaps or holes in the roof surface
- Rust or corrosion on metal roofing that may have allowed water through
- Sagging roof sections which can indicate structural or underlay failure
Step 4: Careful Roof-Level Inspection (If Safe to Do So)
If you're confident working at height and have the right safety equipment:
- Use a roof ladder hooked over the ridge to distribute your weight
- Look for underlay visible at eaves or verges — you may be able to see its condition at the edges
- Check flashings around chimneys, skylights, and penetrations — these are common failure points where underlay can be cut or torn
- Look for patches of underlay that appear darker, brittle, or crinkled compared to surrounding areas
Common Problem Areas to Focus On
| Location | What to Look For |
|---|---|
| Ridge and hips | Torn or missing underlay under ridge caps |
| Valleys | Pooling water, split or blocked valley liners |
| Around penetrations | Gaps where underlay was cut for pipes or skylights |
| Eaves and verges | Underlay not lapped correctly or missing entirely |
| Older sections of roof | Brittle, cracked felt underlay |
How Old Is Your Underlay?
If your home was built before the 1990s, there's a good chance the original bitumen felt underlay is reaching or past the end of its serviceable life. BRANZ is a good resource for understanding building material lifespans and performance in NZ conditions. Synthetic underlays used from the 1990s onwards generally last longer but can still be damaged by UV exposure at edges or physical puncture.
When to Call a Professional
You should contact a qualified roofer if:
- You find or suspect any missing or damaged underlay
- There is active water ingress or significant staining
- Your roof is steep, high, or difficult to access safely
- Your roof is older than 20–25 years and hasn't been inspected recently
- You're unsure what type of underlay you have or whether it meets current standards
Replacing or repairing roof underlay typically requires removing and re-laying roofing material — this is skilled work that should be done by an experienced roofer. Depending on the scope of work, a building consent may be required; always check with your local council, as Schedule 1 exemptions under the Building Act may apply to like-for-like repairs but not full re-roofing in all cases. Visit MBIE Building Performance for consent guidance.
You can find qualified, local roofers on tradedirectory.co.nz to carry out a professional roof inspection or underlay replacement.