Deteriorated Roof Underfelt or Membrane on Your Building Survey – What Should You Do?

What Underlay Does

The underlay — sometimes called sarking felt, roofing felt, or membrane — sits beneath the tiles or slates and provides a secondary barrier against wind-driven rain. The primary weatherproofing is the covering itself; the underlay acts as a backup that catches any water that penetrates past the covering, directing it down into the gutters rather than into the roof structure.

Underlay also helps manage wind uplift — the pressure of wind on the covering that can lift tiles or slates — and, in the case of modern breathable membranes, allows moisture vapour to escape from the loft while resisting water entry.

For terminology reference, see our roof survey wording glossary.

Traditional Bitumen Felt vs Modern Membranes

Two main types of underlay are found on UK roofs:

• Traditional bitumen felt — the older type, common on roofs built before the late twentieth century. It can become brittle, tear, and sag between rafters over time. It is less breathable than modern membranes, which can contribute to condensation in poorly ventilated lofts.
• Modern breathable membranes — lighter, more durable, and allow moisture vapour to escape from the loft while resisting water entry. These are standard on newer roofs and re-roofs.

Both serve the same basic function as a secondary barrier, but they have different ageing characteristics. A specialist inspection can identify the underlay type and assess its condition.

What the Surveyor May Have Seen

If the surveyor accessed the loft, they may have observed:

• Deteriorated or perished underlay — the felt or membrane is brittle, torn, or disintegrating
• Sagging underlay — the felt has sagged between rafters, forming pockets that can trap water
• Tears or holes — visible damage to the underlay
• Missing underlay — sections where the underlay is absent, particularly at eaves
• Daylight visible through the underlay — gaps where the underlay has failed or was never installed

If the loft was inaccessible, the surveyor would not have been able to assess underlay condition — which is why underlay defects are often noted as limitations rather than confirmed findings.

Why Old Felt Alone Does Not Automatically Mean Replacement

This is one of the most important points for a buyer to understand: deteriorated underlay alone does not automatically mean the roof needs replacing. The underlay is a secondary barrier — the primary weatherproofing is the covering itself. If the covering is sound, well-fixed, and there is no evidence of water ingress, deteriorated underlay may not require immediate action.

Many older roofs have perished or sagging underlay that has been in this condition for years without causing problems, because the covering continues to perform its primary function. The underlay is a backup — its deterioration reduces the margin of protection but does not automatically mean the roof is failing.

When Underlay Condition Becomes More Significant

Underlay condition becomes more significant when:

• It is accompanied by covering defects — if tiles or slates are slipping, cracked, or missing, the underlay is doing more work as a secondary barrier, and its deterioration is more concerning
• There is evidence of water ingress — water staining on the underlay, rafters, or insulation indicates that water is penetrating past the covering and the underlay is not containing it
• The underlay is sagging significantly — sagging underlay can trap water in pockets, directing it towards joints and gaps rather than into the gutters
• The underlay is absent at the eaves — missing underlay at the eaves can allow wind-driven rain to enter the loft directly

In these situations, underlay deterioration is part of a broader pattern of evidence that may justify further investigation or remedial work.

The Relationship Between Underlay and Covering Defects

Underlay condition and covering condition are related but distinct. A roof with a sound covering and deteriorated underlay may perform adequately for years. A roof with both covering defects and deteriorated underlay is more concerning, because the secondary barrier is compromised at the same time that the primary barrier is failing.

This is why a specialist inspection assesses both elements together — the covering from outside and the underlay from the loft — rather than assessing either in isolation. For covering defects, see our guide on slipped, broken and missing roof tiles and slates.

Battens and Timber Condition

The battens that support the tiles or slates are visible from the loft, alongside the underlay. Common issues include:

• Rotten or decayed battens — timber decay, often caused by moisture from failed underlay or covering defects
• Stained rafters — water staining on rafters can indicate current or historic water ingress
• Timber movement — deflection or distortion of roof timbers, which may indicate structural movement (see our guide on roof distortion and sagging)

Localised batten decay is repairable by replacing the affected sections. Widespread batten decay, particularly if accompanied by underlay failure and covering defects, may indicate a broader problem.

Does Deteriorated Underlay Automatically Mean Replacement?

No. Deteriorated underlay alone does not automatically mean the roof needs replacing. The decision depends on the overall evidence — the condition of the covering, the extent of underlay deterioration, and whether there is evidence of water ingress.

Replacing underlay typically requires stripping the covering, which is why it is associated with re-roofing rather than localised repair. But the need for this work is determined by the overall condition of the roof, not by underlay deterioration alone. A roof with sound covering, no water ingress, and deteriorated underlay may continue to perform adequately without immediate action. For guidance on the financial scale of roof repairs and replacement, see our guide on roof repair and replacement costs.

What a Specialist Roof Inspection Can Add

Where a building survey notes underlay deterioration, a specialist inspection can provide evidence the general survey could not. Depending on access and conditions, this may include:

• Detailed loft inspection to assess underlay type, condition, and extent of deterioration
• Assessment of batten and timber condition from the loft
• Assessment of whether underlay deterioration is accompanied by covering defects or evidence of water ingress
• Moisture assessment where visual evidence suggests concern
• Identification of whether the underlay is absent at critical areas such as the eaves
• Photographic documentation of significant findings

For a full explanation of our evidence-led process, see our inspection methodology.

What Cannot Be Determined Without Further Evidence

If the loft was inaccessible during the general survey, underlay condition cannot be established at all. Even with loft access, it is not possible to establish:

• The condition of the underlay beneath the battens and covering
• Whether water is actively penetrating through underlay defects into the roof structure
• The full extent of underlay deterioration across the whole roof from a brief loft inspection

What a Homebuyer Should Understand Before Exchange

Before exchange, a buyer should understand:

• Whether the surveyor was able to access the loft — if not, underlay condition is unknown
• Whether underlay deterioration is isolated or widespread
• Whether underlay deterioration is accompanied by covering defects or evidence of water ingress
• Whether further investigation is warranted to establish the overall condition of the roof

Frequently Asked Questions