Most house extension owners do very little maintenance until something goes wrong. By then, a problem that could have been fixed for a few hundred pounds has become a repair costing several thousand. The pattern is almost always the same: a blocked gutter causes water to run down the wall, which saturates the masonry, which leads to penetrating damp inside, which damages plaster and decoration, and eventually the structure.
The good news is that most extension maintenance is straightforward and inexpensive if done regularly. This guide sets out what to inspect, how often, and what action to take when you find a problem. It is based on the maintenance issues TCM most commonly diagnoses in extensions built by other contractors — the same problems appear repeatedly.
Applies to all extension types
This guide covers single-storey and double-storey extensions, rear and side extensions, and wrap-around extensions. Flat-roof extensions have specific maintenance requirements that are covered in detail below. For loft conversion maintenance, see our loft conversion maintenance guide.
Seasonal Maintenance Schedule
The most effective approach to extension maintenance is a seasonal schedule — a set of tasks carried out twice a year (spring and autumn) that catch problems before they develop. The table below sets out the key tasks by season.
Spring Tasks
- ✓Clear gutters and downpipes of winter debris
- ✓Inspect flat roof membrane for splits, blistering, or ponding
- ✓Check external render and pointing for frost damage
- ✓Inspect windows and doors for failed seals or sticking
- ✓Check expansion joint sealant between extension and main house
- ✓Inspect internal walls for damp staining after winter rains
- ✓Test drainage by running water from downpipes
Summer Tasks
- ✓Inspect external walls for cracking caused by thermal movement
- ✓Check roof lights and skylights for failed seals
- ✓Treat any external timber (fascias, soffits, window frames) with preservative
- ✓Inspect the ridge and verge of any pitched roof sections
- ✓Check that air bricks and ventilation openings are clear
- ✓Inspect the damp proof course for bridging (soil or render covering it)
Autumn Tasks
- ✓Clear gutters and downpipes before the wet season
- ✓Check flat roof outlets are clear and free-draining
- ✓Inspect all external sealants (windows, doors, expansion joints)
- ✓Check roof covering for loose or missing tiles / membrane damage
- ✓Inspect external drainage channels and gullies
- ✓Test condensation ventilation — check trickle vents are open
Winter Tasks
- ✓Lag exposed pipework in unheated spaces (utility rooms, garages)
- ✓Check that heating is adequate to prevent condensation
- ✓Monitor internal walls for new damp staining after heavy rain
- ✓Inspect flat roof after heavy snow — clear if accumulation is significant
- ✓Check that overflow pipes are not blocked or frozen
Common Extension Maintenance Problems
These are the six problems TCM most commonly diagnoses in house extensions. Each entry covers the symptoms, the typical cause, and the recommended action.
Flat Roof Failure
HighSymptoms
Ponding water, interior ceiling stains, blistering or cracking of the roof surface
Typical Cause
UV degradation of the membrane, thermal movement, blocked outlets, or inadequate falls
Recommended Action
Annual inspection by a roofing contractor. Membrane replacement every 15–25 years depending on the system (GRP, EPDM, felt). Do not walk on a flat roof without proper access equipment.
Penetrating Damp
HighSymptoms
Damp patches on internal walls following or after rain, often at specific points (below a window, at a corner, below a parapet)
Typical Cause
Failed pointing, cracked render, blocked gutters, failed window seals, or a defective DPC
Recommended Action
Identify and fix the external defect first. Allow the wall to dry before redecorating. If the source is unclear, commission a damp survey.
Cracking at the Junction with the Main House
Medium — monitor firstSymptoms
Vertical or diagonal cracks at the point where the extension meets the original building
Typical Cause
Differential settlement between the new extension and the existing structure — normal in the first 1–2 years
Recommended Action
Monitor the crack with tell-tales over 3–6 months. If it is stable, fill and redecorate. If it is growing, commission a structural engineer's report.
Failed Expansion Joint
MediumSymptoms
Visible gap or cracking in the sealant between the extension and the main house, water staining at the junction
Typical Cause
Degradation of the flexible mastic sealant over time
Recommended Action
Rake out the old sealant, clean the joint, and re-seal with a suitable flexible mastic. A straightforward DIY job if accessible; use a builder if the joint is at height.
Condensation and Mould
MediumSymptoms
Black mould in corners, on cold surfaces, or around windows; musty smell; condensation on glass
Typical Cause
Inadequate ventilation, insufficient insulation, or cold bridges in the building fabric
Recommended Action
Improve ventilation (trickle vents, extractor fans, MVHR if appropriate). Check insulation continuity. Treat mould with a fungicidal wash before redecorating.
Blocked or Overflowing Gutters
Low — but causes serious damage if ignoredSymptoms
Water running down the external wall, damp patches at high level on internal walls, staining of external render
Typical Cause
Leaf and debris accumulation in gutters and downpipes
Recommended Action
Clear gutters twice a year (spring and autumn). Check that all joints are sealed and that downpipes discharge correctly to drainage.
Flat Roof Extensions: A Closer Look
Flat-roof extensions are the most common type built in Hertfordshire and North London, and they require more active maintenance than pitched roofs. The flat roof is the element most likely to cause problems — and the most expensive to repair if neglected.
Modern flat roofs use one of three main systems: GRP (glass-reinforced polyester, often called fibreglass), EPDM (rubber membrane), or warm-deck felt. Each has a different expected lifespan and maintenance requirement.
| System | Expected Lifespan | Key Maintenance | Common Failure Points |
|---|---|---|---|
| GRP (fibreglass) | 25–40 years | Annual inspection, keep outlets clear | Upstands, trim details, impact damage |
| EPDM (rubber) | 20–30 years | Annual inspection, check seams and flashings | Seam separation, flashing failure, punctures |
| Warm-deck felt | 15–20 years | Annual inspection, re-felt when blistering | Blistering, splits, blocked outlets |
The most important flat roof maintenance task
Keep the roof outlets clear. Flat roofs are designed with a slight fall towards the outlet. If the outlet is blocked, water ponds on the roof surface, increasing the load and accelerating membrane degradation. Check the outlet every spring and autumn and clear any debris. If you notice ponding after rain that does not drain within 24 hours, the outlet is likely blocked or the falls are inadequate.
When to Call a Professional
Many extension maintenance tasks are straightforward enough for a competent homeowner. Others require a qualified builder or specialist. Here is a guide to when to call in a professional.
Always use a professional for:
- ⚠Structural crack investigation
- ⚠Flat roof replacement
- ⚠Damp proofing and waterproofing
- ⚠Any work at height (gutters above 3m, roof access)
- ⚠Electrical work (extensions to circuits, new sockets)
- ⚠Gas work (boiler, heating extensions)
DIY is usually appropriate for:
- ✓Clearing gutters and downpipes (with proper ladder safety)
- ✓Replacing expansion joint sealant at ground level
- ✓Filling hairline cracks in plaster
- ✓Treating condensation mould with fungicidal wash
- ✓Repainting external timber (fascias, window frames)
- ✓Clearing flat roof outlets (if safely accessible)
Frequently Asked Questions
How often should I have my house extension inspected?+
A visual inspection of the key elements — roof, gutters, external walls, windows, and drainage — should be carried out twice a year: once in autumn before the wet season and once in spring after it. A more thorough inspection by a qualified builder or surveyor is recommended every 5–7 years, or sooner if you notice any signs of movement, water ingress, or cracking.
What are the most common maintenance problems with flat-roof extensions?+
Flat-roof extensions are more maintenance-intensive than pitched roofs. The most common problems are: ponding water caused by inadequate falls or blocked outlets; membrane splits or blistering caused by UV degradation or thermal movement; failed upstands where the membrane meets the parapet wall; and blocked or poorly positioned roof outlets. A flat roof should be inspected annually and the membrane replaced every 15–25 years depending on the system.
How do I know if my extension has a damp problem?+
The early signs of damp in a house extension include: tide marks or staining on internal walls, particularly at low level; peeling paint or wallpaper; a musty smell; salt crystallisation (efflorescence) on masonry; and cold or wet patches on walls. Rising damp typically affects the lower 1 metre of a wall, while penetrating damp follows the pattern of the external defect (failed pointing, blocked gutter, cracked render). Condensation damp is more diffuse and often worst in corners and on cold surfaces.
What causes cracking in house extensions and when is it serious?+
Most cracking in house extensions is cosmetic — caused by thermal movement, shrinkage of new materials, or minor settlement. Hairline cracks (less than 0.5mm wide) in plaster or render are normal and can be filled and redecorated. Cracks that are wider than 3mm, run diagonally through masonry, or are accompanied by visible movement of the structure require investigation by a structural engineer. Cracks that appear suddenly or grow quickly are always a cause for concern.
Do I need to maintain the expansion joint between my extension and the main house?+
Yes. The expansion joint (sometimes called a movement joint) between a new extension and the existing house is a deliberate gap that allows the two structures to move independently. It should be filled with a flexible mastic sealant, not mortar. Over time, the sealant degrades and cracks, allowing water to enter. Check the joint annually and re-seal with a suitable flexible sealant when the existing sealant shows signs of cracking or separation.
Related Guides and Services
House Extension Services
Single-storey, double-storey, and wrap-around extensions across Hertfordshire
Loft Conversion Maintenance Guide
Maintenance guide for loft conversion owners
How to Tell If You Have Rising Damp
Identifying rising damp vs penetrating damp vs condensation
Damp Proofing & Waterproofing
Professional damp proofing services across Hertfordshire
Property Maintenance Services
Ongoing maintenance contracts for residential properties
Commercial Maintenance Checklist
For commercial property owners and managers
Extension Maintenance or Repair?
TCM Building & Maintenance provides property maintenance services across Hertfordshire and North London. From flat roof repairs to damp proofing — get a free assessment.