French doors bowing outward in summer is a common problem in New Zealand homes, particularly with older timber doors exposed to harsh sun and humidity. The good news is that with the right diagnosis and a few targeted fixes, you can usually get your doors hanging true again — and prevent the problem from coming back.
Why Do French Doors Bow in Summer?
Timber is a natural, hygroscopic material, meaning it absorbs and releases moisture as conditions change. In summer, two things often happen at once:
- Heat and UV exposure dry out the exposed face of the door (usually the exterior), causing that face to shrink and pull the door into a bow.
- Moisture imbalance occurs when one face is sealed or painted and the other is not, causing uneven expansion and contraction.
This is sometimes called cupping or warping, and it's especially common in doors that face north or west and receive strong afternoon sun. New Zealand's UV index is among the highest in the world, which accelerates paint breakdown and timber drying.
Step 1 — Diagnose the Severity
Before reaching for tools, assess how bad the bow is:
- Place a long straightedge or spirit level across the face of the door.
- A bow of 3–5 mm is often manageable with adjustments and finishing work.
- A bow greater than 8–10 mm may mean the door needs to be replaced or professionally re-machined.
Also check whether the bow is consistent along the full height of the door, or whether it's worse at the top or bottom — this affects your repair approach.
Step 2 — Check and Adjust the Hinges
Sometimes what looks like a bow is actually a hinge alignment issue. Over time, hinge screws work loose in the frame, allowing the door to sag and twist.
- Tighten all hinge screws. If screw holes are stripped, remove the screws, pack the holes with wooden toothpicks and PVA glue, let it cure fully, then re-drive the screws.
- Check that the hinges themselves are not bent or worn — replace any that are.
- On French doors, also check the astragal (the centre meeting strip) is correctly aligned, as a misaligned astragal can make a door appear bowed when it isn't.
Step 3 — Rebalance Moisture with Finishing
If the bow is caused by moisture imbalance, the fix is to seal all six faces of each door panel — top, bottom, both sides, and both faces. Many installers only paint the visible faces, leaving the top and bottom edges bare. Bare timber absorbs moisture unevenly, leading to warping.
- Sand back any flaking or peeling paint on the exterior face.
- Apply a quality exterior primer and topcoat. Resene and Dulux both offer exterior timber systems suited to NZ conditions — check their technical datasheets for the right primer for your timber species.
- Pay particular attention to sealing the top and bottom rails of each door leaf.
- If the door is already bowed, allow it to acclimatise in a sheltered, dry space for several weeks before repainting — forcing paint onto a bowed door locks the warp in.
Step 4 — Apply Controlled Pressure to Reverse a Mild Bow
For a mild bow (under about 5 mm), you can sometimes reverse the warp using controlled moisture and weight:
- Lay the door flat on a clean, level surface (remove it from its hinges first).
- Place damp cloths on the concave face (the hollow side) and leave in a warm, sunny spot for several hours. This encourages the dry, contracted face to absorb moisture and relax.
- Once the bow has reduced, stack heavy, flat weights evenly across the door and leave it for 24–48 hours.
- Seal all faces immediately once the door is flat.
This method works best on solid timber doors and is not suitable for engineered or hollow-core doors.
Step 5 — Improve Sun and Weather Protection
Prevention is far easier than cure. Consider these measures:
- Install a pergola, awning, or deep eave over the door opening to reduce direct sun exposure.
- Apply a UV-resistant exterior finish — products with a satin or semi-gloss sheen tend to reflect more UV than flat finishes.
- Inspect and touch up the finish every 1–2 years, especially on north- and west-facing doors.
- Ensure the door threshold and head flashings are sound so water doesn't penetrate the frame — refer to BRANZ guidance on external joinery weathertightness.
Step 6 — Consider Replacement Timber or Alternative Materials
If the door continues to bow season after season, the timber may be inherently prone to movement (for example, if it was not properly kiln-dried when manufactured). In this case, replacement is the most durable solution. Options include:
- Engineered timber door blanks, which are more dimensionally stable than solid timber.
- Aluminium-clad timber French doors, which protect the exterior face from UV and moisture while retaining the timber look inside.
- Discuss options with a qualified joiner who can advise on what suits your home and climate zone.
When to Call a Professional
Call a qualified joiner or carpenter if:
- The bow exceeds 8–10 mm and DIY methods haven't resolved it.
- The door frame itself is twisted or out of square — this can indicate structural movement and may require assessment under the NZ Building Code.
- You need to replace the doors entirely, especially if the opening requires any structural alteration (which may need a building consent — always check with your local council).
- The door is part of a weathertight system and you're concerned about water ingress.
A skilled joiner can re-machine a bowed door on a planer, re-hang it correctly, and advise on the best finishing system for your specific exposure. You can find qualified local joiners and carpenters on tradedirectory.co.nz.
