Roofing underlayment is often described by experts as “an essential component” for helping a roof “withstand the elements” and protect the home’s interior. In practice, it is a protective layer (sometimes called a breather membrane or roofing felt) that is installed on top of the roof decking before the tiles or slates go on. The underlay sits directly above the wood deck and below the tiles. Its job is to act as a second line of defence against wind-driven rain and snow.

If a tile cracks or blows off in a storm, the underlay will stop water from reaching the decking. Without this layer, moisture could soak straight into the roof structure, causing leaks, rot and mold. In addition to waterproofing, underlay provides a smooth, level base for the tiles and seals around any roof protrusions or flashing. Some modern underlays even include a thin insulating layer to improve thermal performance and can slightly dampen sound under the roof.

Types of Roofing Underlayment

Felt Underlayment (Traditional Option)

Felt underlay (aka roofing felt or tar paper) is the classic choice used for decades. It consists of fibrous mats (organic or fiberglass) fully saturated with asphalt for water resistance. Basic felt comes in grades – typically 15 lb or 30 lb felt – with the thicker 30 lb providing more durability. Felt is inexpensive and easy to install with standard nails. It naturally compresses around nails and often sags slightly between rafters (a drape that actually helps water run into the gutters).

The downside is that felt is heavy and can tear over time. When left exposed in cold weather, felt can absorb moisture and crack as its fibres swell. Very old felt may dry out under the tiles or even grow mold. For these reasons, while felt is still used on many UK roofs, it is increasingly being replaced by newer materials.

Synthetic Underlayment (Modern, High-Performance Choice)

Synthetic underlays are made from polymers such as polypropylene or polyethylene. They are very lightweight yet extremely strong and tear-resistant. Most synthetics have a woven or multi-layer construction that resists UV damage and won’t absorb water. High-quality synthetic membranes remain intact even if left exposed for months during construction, and they resist rot and mold. They usually include printed lines or markings to speed up installation and create a secure walking surface for roofers.

These advantages make synthetic underlay a favourite among professional roofers. Its main drawback is cost: a good synthetic membrane can be significantly more expensive than felt. Also, unlike felt, synthetic underlays do not self-seal around fasteners, so careful overlapping or taping of seams is essential. Always ensure that any synthetic underlay chosen meets British standards for roofing.

Rubberized Asphalt Underlayment (Premium Protection Option)

Rubberized asphalt underlays (often called self-adhesive or ice-and-water shields) are the top-of-the-line option. They consist of a thick, polymer-modified asphalt layer with a peel-off backing. When installed, they stick directly to the roof deck and form a fully waterproof seal – even around nails and seams. A rubberized underlay can handle extreme weather and remain effective if exposed to rain or UV for extended periods. It stays flexible in cold and won’t dry out.

The price of rubberized underlay is higher than other types, so it’s usually used where maximum protection is needed: for example along eaves, roof edges, valleys and dormers. In those spots, its 100% waterproofing can prevent leaks better than plain felt. Many builders recommend premium underlay in vulnerable areas, especially on low-slope roofs.

Pros and Cons of Each Type

Each underlay type has trade-offs:

• Felt: The traditional choice – least expensive and easy to work with, but heavier and prone to degradation over time (tears and UV damage). Good for straightforward pitched roofs if budget is tight.
• Synthetic: Modern and long-lasting – very tear-resistant, lightweight and often UV-stable. Costs more upfront but may never need replacing in a 20-30 year roof life. Requires correct overlaps and fasteners (no staples!).
• Rubberized Asphalt: The most protective – fully waterproof and self-adhesive, ideal for critical areas. Much higher cost, so usually used selectively (e.g. eaves, gutters, dormers). Provides insurance against leaks but at a premium price.

Your roofing contractor can recommend the right mix for your home and budget. Often you might use a quality synthetic or felt for most of the roof, and install rubberized underlay in the valleys and eaves as needed.

Why Roofing Underlayment Matters

Protection Against Leaks and Water Damage

Underlay’s greatest benefit is preventing leaks. If the outer roof covering ever fails (for example, a tile cracks, slips, or flashing leaks), the underlay stops water from reaching the wood below. In practice, that means a hole in the tiles won’t immediately drench your ceiling. As one roofing guide warns, without underlayment “water is able to infiltrate the roof deck,” which quickly leads to rot and mold. In Putney and Fulham, where heavy rains can occur, that secondary shield is critical. Underlay keeps rainwater from ever touching the deck, protecting your home’s interior.

Acts as Backup Shield When Tiles or Shingles Fail

In effect, the underlay is your roof’s backup plan. InterNACHI notes that if the top roofing layer leaks, the underlayment “keeps your roof decking dry and safe”. This means any moisture or snow that gets past a damaged tile will be caught by the underlay. For example, if wind blows off a slate or a vent pipe gasket fails, a good underlay should block that entry and prevent a drip into the loft. Homeowners in Fulham or Putney benefit from this reliability: underlay ensures that even if a tile is breached, the leak is (hopefully) confined to outside.

Roof Longevity and Durability

By keeping the deck dry, underlay greatly extends a roof’s life. A dry timber deck can last for decades, whereas a wet deck will quickly rot. In the words of roofing inspectors, underlayment is “essential” for prolonging the roof’s service life. In other words, a roof with quality underlay will generally reach or exceed its expected lifespan without hidden damage. Neglecting underlay can mean your roof ages much faster – you may replace tiles only to find the deck has deteriorated underneath. Underlay also helps maintain a dry attic, so insulation works better and your home stays cozy longer.

Helps with Energy Efficiency and Temperature Control

A continuous underlay layer can improve your roof’s thermal performance. By sealing the roof deck, underlay reduces drafts and heat loss through any gaps. Some modern membranes even have reflective surfaces to bounce heat away. While it’s not a substitute for actual insulation, underlay does add a thin extra barrier. In winter, a well-sealed roof deck helps keep your loft insulation dry and effective. In summer, reflective or ventilated membranes can lower heat gain. Over time, a properly installed underlay can contribute to modest savings in heating and cooling bills. It also reduces noise from heavy rain on the roof, making your home quieter.

How Roofing Underlayment Works During Harsh Weather

Storm and Wind Resistance

Underlay plays a key role when storms arrive. Modern underlays must meet strict wind-uplift standards. In fact, the UK’s roof code (BS 5534:2014) now requires all underlay membranes to be tested and labelled by wind zone. After recent extreme wind events (such as Storms Bert and Darragh in 2024), experts have emphasised using only underlays tested for high wind. A robust underlay helps prevent “ballooning” in high winds – it stays fixed to the deck and holds down the tiles. The Roof Tile Association notes that underlay must bear a “significant proportion of the wind load” on a roof. In practical terms, this means the outer tiles and underlay share the force of wind, rather than the tiles alone being ripped off. By choosing an underlay rated for London’s wind zone, your contractor ensures the roof remains secure in gales.

Moisture and Ice Barrier Protection

Cold weather and melting snow can also test a roof. In very wet or freezing conditions, areas like eaves, valleys and dormers are vulnerable. For these spots, builders often use a fully waterproof underlay (sometimes called an ice-and-water shield) under the tiles. Industry guidance is to install such waterproof underlay wherever water can pool or back-up. Even though deep ice dams are rare in London, following this advice is prudent. For example, we recommend laying a rubberized membrane along the eaves and below slates at the bottom rows to block any meltwater. This means that if snow melts or heavy rain hits, the water cannot penetrate under the tiles at the weakest points.

UV Protection Before Final Covering

Underlay also protects the deck when the roof is being built. A quality synthetic membrane can be left exposed for some time before the tiles go on – in fact, certain rubberized underlays can survive months of exposure without damage. This is useful if bad weather delays the project. In contrast, standard felt should be covered as soon as possible: if left in the sun, the asphalt will dry out and become brittle. For this reason, professional roofers plan their work so that the underlay is quickly overlapped by tiles, minimizing any UV damage. In short, a tough underlay gives flexibility during construction and keeps the roof deck protected until the final covering is in place.

Underlayment Installation Process

Key Steps Professionals Follow

Installing underlay correctly is a precision job. Experienced roofing teams follow these basic steps:

1. Prepare the deck: Repair or replace any rotten roof boards or sheathing first, and clear off debris.
2. Lay the first course: Start at the lowest point (eaves) and roll out the first strip of underlay, making sure it extends over the drip edge or gutter flashing.
3. Overlap carefully: Roll out each subsequent strip up the roof, overlapping the previous strip by the manufacturer’s recommended amount (typically ~100 mm or 4 inches) so water always sheds down onto an underlay below. Fasten each strip with cap-headed nails or staples at the spacing specified by the supplier.
4. Secure seams: At horizontal laps or edges, apply approved tape or install a “fly” batten as required. This ensures strong bonds in high winds.
5. Detail around features: When reaching chimneys, vents or skylights, cut the underlay precisely around the opening. Tuck it under any flashing or step flashing. Often extra sealing (e.g. roofing cement or tape) is used at these protrusions to maintain the waterproof barrier.

By following the manufacturer’s instructions and British standards, professionals ensure the underlay is airtight and watertight.

Common Installation Mistakes to Avoid

Mistakes in underlay installation can lead to leaks or void warranties. Common errors include:

• Insufficient overlap: Every underlay strip must overlap enough (as specified) to shed water. Failing to do so leaves gaps.
• Wrong fasteners: Synthetic underlays should be fixed with cap nails or screws, not ordinary staples (which don’t seal). Using the wrong nails, or too few fasteners, can allow strips to lift in wind.
• Improper orientation: Some underlays have a top side. Installing them upside-down or starting from the top of the roof (instead of the eaves) will trap water under the sheets.
• Unsealed laps or edges: Not taping or battening down horizontal laps, hips or ridges as required by code can let wind or rain get under the underlay.
• Wrinkles and debris: Letting the underlay lie wrinkled or leaving nails sticking out creates points of weakness. Also, do not install underlay over dust and debris – the deck must be clean.

Roofers with experience know to avoid these pitfalls. Each supplier has specific overlap and nailing guidelines, and skipping them “can compromise the integrity” of the system. (For example, one source insists on a taped joint or extra batten to secure each seam to meet wind load requirements.)

Why DIY Installation Is Risky

Because of these details, underlay installation is generally best left to professionals. A homeowner might save on labour costs, but if something is done incorrectly, leaks can start months later with little warning. A DIYer is unlikely to test for the small shrinkage or lapping differences that professionals check. Moreover, incorrect installation can void both manufacturer and insurance warranties. In Putney and Fulham, local roofers carry insurance and guarantees – if an issue does occur, you’re covered. By contrast, an uninspected job can leave you paying for damage out of pocket. For safety and peace of mind, it’s usually worth hiring a qualified roofing contractor to handle underlay and tile work together.

When Should You Replace Roofing Underlayment?

Signs of Worn or Damaged Underlayment

Because underlay is hidden, it can degrade quietly. You should inspect or consider replacing it if you notice unexplained leaks. Inside the loft, look for stains on the rafters, damp insulation, or mold on the undersides of roof boards. If you see daylight or feel a breeze through the roof deck, that’s a sure sign of failure. Sometimes you can spot tears or missing granules if you lift a loose tile. Other clues: moss or algae on the roof (trapping moisture) or sagging sections of deck. In many cases, catching a worn underlay early requires a professional roof inspection. When in doubt, it’s safest to replace the underlay along with any new tiles.

How Often Underlayment Needs Replacement

Underlay life varies by material and climate, but in UK roofs it’s common practice to install new underlay anytime the roof covering is replaced. A quality felt or synthetic underlay on a pitched roof can last 20–30 years under good conditions. In fact, Felt roofing typically endures 20–30 years on a sloped roof. However, because underlays wear invisibly, many roofers will fit all-new underlay whenever doing a re-roof. In Putney and Fulham homes, this means roughly every few decades (often when doing a tile renewal). Synthetic membranes may last even longer than felt, but since it’s hard to inspect, crews usually err on the side of replacement. If your roof was installed without underlay (common on very old houses), or if it has had multiple repairs, consider replacing it next.

Replacement Costs and What Influences Price

Underlay cost is a small part of a full re-roof estimate. Basic felt sheets might add only a few tens of pounds for an average house, while synthetic underlay can run roughly 50–100% more per square meter. Rubberized underlays are pricier still. Labor is often bundled into the overall roofing job rather than charged separately. Key cost factors include roof size, access difficulty (steep slopes add labor), and underlay type chosen. A larger or steeper roof needs more material and time. In quotes for roof repairs or replacements in Putney and Fulham, contractors typically include underlay replacement in their total. If you get multiple quotes, ensure each is clear on what underlay (if any) is provided. Remember: reusing old underlay can save money up front but may lead to bigger repair bills later.

Choosing the Right Underlayment for Your Roof

Climate-Based Recommendations

Putney and Fulham enjoy a temperate maritime climate – winters are cool with frequent rain, summers are mild. For most London roofs, a breathable membrane (also known as a breather felt) rated to BS5534 Type 1A is ideal. Such membranes keep out water yet allow any trapped moisture to escape. In our region, ice dams are uncommon, but heavy rain or wind-driven rain are always possible. Therefore, use an underlay with good wind-uplift resistance (Class 1A or higher). On roof sections that face north or get shade, consider an underlay marked W1 (tightly waterproof) to prevent moisture soaking. In short, choose an underlay made for UK weather. Local suppliers often specify materials that meet national codes; following their guidance ensures your roof can handle our seasonal changes.

Best Underlayment for Asphalt, Metal, Tile Roofs

The roof covering also affects your choice. For standard UK slate or tile roofs, traditional felt or a permeable breather membrane is common and works well. For modern metal roofing, a high-temperature synthetic membrane is often recommended, since metal can heat up and trap heat. For example, one roofing guide advises that metal roofs do well with synthetic underlay, while asphalt shingle roofs pair fine with felt. (The same idea applies to our local concrete tiles: they behave more like shingle.) In any case, ensure the underlay matches your roof pitch and covering. When Old English Contractors reroofs a property, we pick the underlay to suit the material – a brittle felt for some patios with ceramic tiles might be replaced by a polymer sheet, for instance. If you’re switching your roof type (say from slate to metal), discuss underlay options with your contractor.

Budget vs. Performance Comparison

In general, you get what you pay for. Basic felt is cheapest upfront but may need replacing sooner. Synthetics cost more but often last the life of the roof. Rubberized underlays provide premium protection (especially on low slopes) but at the highest cost. If budget is a concern, a practical approach is using a mid-range synthetic or heavy felt for most of the roof, and reserving the expensive self-adhesive underlay for the eaves, valleys and gullies. One roofing source notes: skipping underlay saves money now, but “for the safest, most water-resistant roof… be sure to include the underlayment in the installation”. A small extra investment in a quality underlay often prevents far more costly leak repairs later.

Final Thoughts: The Role of Underlayment in a Strong Roof

Summary of Key Benefits

Quality underlayment is a small part of your roof, but it plays an outsized role in keeping your home safe and dry. It provides a second waterproof barrier under the tiles and helps lock down the roof in high winds. It protects the wood deck from moisture and extends the whole roof’s lifespan. It even improves energy efficiency and sound insulation under the roof. In short, a good underlay prevents hidden leaks and ensures your roof delivers on its 20–30 year promise.

Why It’s Worth the Investment

The cost of quality underlay is usually just a few percent of a full roof replacement. In return, you avoid water damage (worth thousands in repairs) and get peace of mind. As one roofing expert points out, solid underlayment will “prevent costly repairs from major damage in the future”. Homeowners often find that a dry, well-sealed roof is priceless. At Old English Contractors, we always use premium underlay in our installations for precisely that reason: it protects your home and adds value. Skimping on underlay is one of those false economies that can turn a cheap roof job into an expensive lesson.

How to Get an Expert Roof Inspection

If you suspect your roof’s underlay might be old or if you’re planning repairs, it’s wise to call in professionals. Our roofing contractors in Putney and Fulham can inspect every layer of your roof (deck, underlay, tiles and flashings) and advise on repairs or replacement. We provide free roof surveys and quotations, so you’ll know exactly what work is needed. Whether it’s a small repair or a full re-roof, investing in a proper underlay will protect your home for years.

Old English Contractors is a family-run London company specialising in roofing and exterior improvements. We proudly serve Putney, Fulham and surrounding areas with top-quality workmanship and materials. Feel free to call us today for a free quote or advice on roofing underlayment and repairs in Putney or Fulham – your home’s protection is our priority.