Roof Underlayment Installation Hampshire

Daylight visible through tears, splits or detached sections of felt in the loft indicates underlayment that is nearing the end of its lifespan. Most underlayment damage of this kind is detectable from inside the loft rather than from outside the property.

Damp patches or staining on the underside of the felt or boarding in the loft indicate that water is penetrating through the deck. Underlayment failure of this kind can lead to broader roof issues that often go unnoticed until the damage becomes significant.

Brittle, cracked or flaking traditional 1F felt at lap joints is a sign of bituminous felt that has exceeded its useful life. Felt over 50 years old is typically friable, sheds granules into the gutter and tears at lap joints under normal wind conditions.

Felt sagging between rafters indicates that the fixings have failed and the underlayment is no longer properly supported. This is a clear warning sign that the underlayment requires replacing.

When tiles appear intact but leaks persist in the same area of the ceiling, the underlayment is often the source of the problem. Granules from the felt collecting in the gutters are a further indication that the felt is shedding and deteriorating.

During the strip and inspect stage, we remove the old underlay and clear the deck of debris and nail residue. A proper underlayment installation requires a clean, dry and structurally sound deck; without this preparation the longevity of the new membrane cannot be assured.

At the timber check stage, we probe battens for rot and take moisture meter readings across the deck. Readings above 18% indicate that replacement is required before the membrane is laid.

A low-cost underlay installed over wet timber will cause both the timber and the membrane to deteriorate within five years. We do not seal a roof over timber with moisture readings above 18%, and proper lap seating demands dry conditions — we avoid sealing joints during active rain.

During the membrane laying stage, we lay the eaves course first and then proceed with horizontal and vertical laps in accordance with BS 5534. There are three principal types of roof underlayment, each suited to different pitched-roof applications depending on the property’s age, ventilation strategy and site exposure.

Breathable HR underlay is the standard choice for modern pitched roofs, allowing moisture vapour to escape while shedding bulk water. The membrane type is confirmed during the survey, before any tiles are removed.

Where required, we install counter-batten over breathable membranes to create a drained air gap. Vapour-permeable membrane with integral counter-batten is a premium option for low-pitch or exposed applications and is BBA-certified for temporary covering during installations.

For pitches below 17.5°, specialist underlay with integral counter-batten is often necessary, as not all tile or slate manufacturers warrant their products below this pitch. We confirm the roof pitch and manufacturer specifications before providing a quote.

At the ridge dressing stage, we carry the membrane over the ridge and seal it accordingly. Breathable membranes serve an additional function here: they allow water vapour generated inside the home to escape through the roof, preventing condensation on the cooler deck side. The result is drier timber, longer-lasting insulation and a warmer loft.

Underlayment typically fails before the overlying tiles or slates because it is thinner and more exposed to the conditions that act on the underside of the roof. Common causes include UV exposure during a delayed installation, sustained moisture from leaks, mechanical damage from foot traffic, fastener corrosion and the natural ageing of older bituminous felt.

The fixing pattern has a significant bearing on durability. Wind uplift can cause underlay tears at nail perforations — one of the most frequent failure modes — and correct lap dimensions and underlay weight specifications help prevent such failures.

Roof underlayment provides a secondary weatherproofing layer beneath the tiles or slates. It intercepts wind-driven rain, water displaced by missing ridge tiles, water entering through tiles lost in a storm, and condensation — all before these reach the structural timber.

Breathable membranes perform an additional function: they allow water vapour generated inside the home to escape through the roof, preventing condensation on the cooler deck side. The result is drier timber, longer-lasting insulation and a warmer loft.

There are three primary underlayment types for standard applications. Traditional 1F bituminous felt is heavy and non-breathable, and is preferred for conservation-area work where original materials must be matched. Breathable HR membranes are the modern standard: lighter, breathable and suitable for most applications. Vapour-permeable membranes with integral counter-batten are the premium choice for coastal sites and low-pitch roofs.

These types can be compared on the basis of weight per square metre, breathability, UV tolerance and compliance with BS 5534 for lap dimensions and fixings. We discuss these trade-offs with you during the survey.

Underlayment can be installed on any pitched roof with a slope of 12.5° or more. For low pitches under 25°, lap dimensions are increased to ensure adequate weatherproofing.

For pitches below 17.5°, specialist underlay with integral counter-batten is often necessary, as not all tile or slate manufacturers warrant their products below this pitch. We confirm the roof pitch and manufacturer specifications before providing a quote.

Roof underlayment requires no routine maintenance after installation. However, a loft inspection every two to three years can identify unexpected damp patches or tears, and we recommend carrying this out alongside annual gutter cleaning.