Flat felt roofs provide a continuous weather-resistant covering for garages, extensions and other flat-roofed structures. Although roofing felt is durable, it must cope with sunlight, rain, temperature changes and natural movement throughout its service life.
Cracks, splits and weak areas often develop gradually rather than following one obvious event. Once the covering loses its integrity, moisture can reach the layers beneath and affect the roof deck. Understanding the common causes can help property owners recognise when professional attention is needed.
What Causes Flat Felt Roofing to Deteriorate?
Felt roof damage can result from ageing, movement, trapped moisture, poor drainage or physical impact. In many cases, several factors combine to weaken the covering.
Ageing Makes the Felt Less Flexible
Roofing felt naturally changes as it becomes older. Exposure to ultraviolet light and changing weather conditions can gradually dry the material, reducing its flexibility.
A less flexible covering cannot accommodate movement as effectively. Fine surface cracks may begin to form, particularly around joints, edges and places where the felt has been folded. These small defects can widen over time and eventually allow water beneath the surface.
Temperature Changes Cause Expansion and Contraction
A flat roof can become warm in direct sunlight before cooling considerably later in the day. These temperature changes cause both the felt and the supporting roof deck to expand and contract.
Correctly installed roofing materials can tolerate a degree of movement. However, repeated cycles place stress on the covering, especially if the felt has been fixed too tightly or laid without allowing for natural movement. The result may be cracking, splitting or separation at the joints.
Movement in the Roof Deck Can Damage the Covering
The performance of a felt roof depends heavily on the stability of the deck beneath it. If boards flex, swell or move at their joints, the felt must move with them.
Cracks may appear above board joints, while larger areas can weaken if the deck has deteriorated because of moisture. Repairing only the visible surface will not provide a dependable result if the supporting structure remains unstable.
A professional assessment should therefore consider both the felt and the condition beneath it.
Poor Drainage Allows Water to Remain on the Roof
Flat roofs are normally constructed with a slight fall to direct rainwater towards an outlet. If the gradient is inadequate, the deck settles or an outlet becomes obstructed, water may remain in low areas.
Persistent ponding keeps sections of the felt wet for extended periods. It can place additional strain on weakened joints and encourage moisture to enter through existing cracks. The weight of standing water may also increase depressions in an already vulnerable roof deck.
Removing the water does not resolve the underlying issue. The reason it is collecting must also be identified.
Trapped Moisture Can Create Blisters and Weak Spots
Moisture or air trapped between layers can create raised blisters in the felt. As the roof warms, the trapped contents expand and push the material upwards.
A blister may initially remain watertight, but the stretched felt is more vulnerable to cracking and puncture. Foot traffic or falling debris can break the raised surface, allowing rainwater into the roof system.
Blisters should not be cut open or flattened without establishing what lies beneath them.
Joints and Seams Can Begin to Separate
Felt roofing is commonly installed in overlapping sections. These laps must remain fully bonded to prevent water from travelling between the layers.
Age, poor adhesion and repeated thermal movement can cause seams to lift or separate. Even a narrow opening may admit wind-driven rain, particularly where water naturally flows towards the joint.
Because the damage may extend beneath the visible surface, simply sealing the edge does not always address the full extent of the problem.
Edges and Roof Junctions Are Vulnerable Areas
Perimeters, outlets, upstands and junctions with adjoining walls require careful detailing. The felt is often folded, shaped or fixed differently in these locations, making them more susceptible to stress.
Loose edges can allow wind to get underneath the covering, while poorly sealed corners may direct water behind the felt. These defects can remain unnoticed from ground level until dampness appears inside the building.
Physical Damage Can Puncture the Surface
Flat roofs are sometimes accessed for maintenance to gutters, walls or nearby equipment. Foot traffic can compress, scuff or puncture the felt, particularly when the material is already brittle or when someone steps on a blister.
Falling branches, tools and sharp debris can also damage the surface. Some punctures are immediately visible, while others remain concealed beneath dirt or standing water.
Previous Repairs May Become New Weak Points
A localised patch can be effective when the surrounding felt and roof deck remain sound. However, older repairs may begin to lift, crack or separate as the materials move at different rates.
Repeated patching can also conceal widespread deterioration. If new defects continue to appear around previous repairs, a broader inspection is needed to determine whether another local repair remains appropriate.
Warning Signs That Should Be Investigated
Possible signs of a deteriorating felt roof include:
- Visible cracks or open splits
- Raised blisters and bubbles
- Loose laps or lifting edges
- Soft or spongy areas
- Water remaining after rainfall
- Damp patches on ceilings or walls
- Discolouration within the roof space
- Leaks that return after previous repairs
- Moss or debris obstructing drainage points
Not every defect means that the complete roof covering has failed. Early assessment can establish whether the problem is localised or more extensive.
What an Effective Repair Should Address
A lasting repair should deal with both the visible damage and its cause. This may involve treating a localised split, renewing a failed joint, correcting an edge detail or repairing part of the roof deck.
Drainage should also be checked so that water leaves the roof effectively. Applying new material over an unstable or damp surface may hide the defect temporarily without restoring dependable performance.
Conclusion
Flat felt roofs develop cracks, splits and weak areas because of ageing, temperature movement, poor drainage, trapped moisture, an unstable roof deck or physical damage. These defects can allow water beneath the covering and may worsen if the underlying cause remains untreated.
If you have noticed signs of felt roof deterioration in Clifton, Nottingham, Clifton Roofing Repairs can inspect the covering, assess the structure beneath and recommend an appropriate course of action. Contact the team to arrange a professional roof assessment before a small weakness develops into a more extensive problem.
Call us on: 0115 647 1193
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