What People Usually Get Wrong About Felt for Flat Roofs

When customers ask me about felt for a flat roof, they’re usually really asking one of three things:

  • “Is the felt on my roof actually any good?”
  • “Has my roofer used the right felt, or have I been fobbed off?”
  • “How do I know if I’m getting proper felt, not cheap stuff, in a new quote?”

I already have a full guide on how I build a modern felt roof from scratch, so here I want to focus on something different: how you, as a homeowner, can judge the quality of the felt that’s already on your roof or being proposed in a quote.

In my 18 years on the roof, I’ve seen everything from solid, well-installed felt that’s still going strong after 25 years, to “new” felt roofs that fail in under five because the wrong materials were used, or installed badly.

If you ask me, I’ll tell you straight: the word “felt” on a quote tells you almost nothing. You need to know which felt, how many layers, and how it’s being installed.

How to Take a Quick “Health Check” of Your Existing Felt Roof

You don’t have to be a roofer to spot the early signs that your felt is either poor quality or past its best. Here are the things I tell customers to look for before they even call me out.

1. Look at the Surface: Does It Still Have Mineral Grit?

Most domestic felt flat roofs in Sussex use a mineral-finished cap sheet – the little grey, green or brown “stones” you can see on top.

  • Good sign: The mineral layer is mostly intact, fairly even in colour, and not bald in large patches.
  • Bad sign: You can see smooth black patches where the mineral has worn away, or the surface looks shiny and dried out.

If the mineral is missing, the sun is attacking the felt directly and it will crack much sooner. I often see this on cheap single-layer jobs.

2. Check the Joints and Seams

The overlaps between strips of felt are one of the first places to fail.

  • Good sign: Joints are well-bonded, tidy, and you can’t easily lift them with your fingers.
  • Bad sign: You can see gaps, splits, or you can peel the edge up. Sometimes people have tried to smear mastic along them as a “repair”.

If you can lift a lap, water can get under it. Once water is tracking under the felt, it can travel a long way before you see a drip inside.

3. Look Around the Edges and Upstands

Edges and upstands (where the felt goes up a wall, parapet or around a skylight) tell you a lot about the standard of work and the quality of the felt used.

  • Good sign: The felt is neatly formed up the wall, usually at least 150mm, and finished with proper metal edge trims or well-dressed lead flashings.
  • Bad sign: The felt just stops flush with the edge of the roof, or is barely turned up the wall with gaps, cracks or exposed timber.

When I’m called to deal with leaks, this is where I start – especially on older roofs with parapet walls or shared firewalls.

4. Feel Under Foot (Carefully)

If it’s safe to walk on the roof (and you’re comfortable doing so), pay attention to how it feels under your feet.

  • Firm and solid: Usually a good sign, assuming the roof has some give but no obvious soft spots.
  • Spongy or springy: This can mean the deck below the felt is wet or rotten, which no amount of new felt on top will fix properly.

When I inspect a felt roof, I’m not just looking at the felt – I’m thinking about what’s happening under it. Damp timber will eventually fail, no matter how good the top layer is.

Not All “Felt” Is the Same: How to Spot Cheap vs Proper Material

On a quote, “felt flat roof” might sound clear, but it covers a huge range of products. Some are fine for a garden shed. Some are built to last decades on a kitchen extension.

Here’s how I explain it to customers in simple terms when I’m on site.

Cheap Shed Felt vs Proper Torch-On Felt

If you’ve ever bought a roll of DIY felt from a builders’ merchant for a shed, that’s usually thin, often with a bitumen backing you nail down or stick with cold adhesive. On a house roof, that’s asking for trouble.

On a proper domestic flat roof – garages, extensions, dormers, bay windows – I use modern SBS torch-on felt systems. The key differences you should listen for when a roofer explains what they’re using are:

  • Layers: A proper system is usually two or three layers (underlay plus a cap sheet). A single layer is almost always a shortcut.
  • Reinforcement: Good felts have a polyester or composite reinforcement, not just a flimsy base.
  • Flexibility: SBS-modified felt stays flexible in cold weather and copes better with movement.

If someone tells you “it’s just standard felt mate, all the same really”, that’s a red flag for me.

What Your Quote Should Actually Say About the Felt

When I write a quote, I don’t just say “fit new felt roof”. I spell out the system I’m using – number of layers, type of cap sheet, and any manufacturer details that matter. This protects you and keeps me honest.

Here’s the sort of wording you should be looking for from any roofer:

  • “Two-layer SBS torch-on felt system, including underlay and mineral cap sheet”
  • “All laps fully torch-bonded, including vertical upstands and details”
  • “New edge trims / drips / flashings included”

If a quote just says “new felt flat roof” with no detail at all, you have no idea if you’re getting something that will last 25 years or five.

How Climate and Location in Sussex Affect Felt Flat Roofs

In Brighton, Hove, Worthing and along the coast, felt roofs take more of a beating than many people realise. Sea air, wind exposure and the way the sun hits certain aspects all play a part in how long your felt will really last.

Sun and UV Exposure

South-facing and west-facing flat roofs get baked in summer and hammered by winter storms. Cheap felts dry out, craze and crack much faster under this kind of exposure.

On very exposed roofs, I tend to specify heavier-duty cap sheets and pay particular attention to lap bonding and edge trims, because the wind likes to get under anything it can lift.

Sea Air and Wind

Along the seafront and on high roofs in Brighton and Hove, wind uplift is a real concern. If the felt hasn’t been properly bonded and detailed, it’ll start to lift at the corners and edges first.

If you live in a very exposed spot and your roof felt is already flapping, creased, or lifting at the edges, it needs attention sooner rather than later – that sort of damage rarely stays “just cosmetic”.

When Felt for a Flat Roof Is the Right Choice – and When It Isn’t

People sometimes ask me if felt is old-fashioned, or if they should “go for rubber or fibreglass instead”. My answer is always the same: the material has to match the roof and how you use it.

Good Uses for a Proper Felt Flat Roof

  • Standard garage roofs where you want solid waterproofing without spending a fortune.
  • Rear extensions and kitchens that need reliable drainage and good detailing around doors and rooflights.
  • Bays, porches and dormers where neat edges and good flashing work really show.

On these, a well-specified felt system is often the most practical balance of cost, performance and appearance.

Situations Where Felt Needs Extra Thought

There are some roofs where I’ll still use felt, but I think a lot harder about the exact system and what’s going on underneath.

  • Balconies and roof terraces that take a lot of foot traffic – you need protection layers and a proper build-up, not just bare felt.
  • Very low-pitch roofs where water can sit – here, detailing and drainage are just as important as the felt itself.
  • Conversions over heated rooms where insulation and condensation control matter as much as the waterproofing.

This is where a quick look at the roof and a chat about how you use the space makes a big difference. The same roll of felt won’t be right for every job.

Red Flags I See All the Time on “New” Felt Roofs

Sometimes I’m called out to a leak on what the owner believes is a fairly new felt roof. When I get up there, I can usually tell within a few minutes why it’s failing.

1. Single-Layer Systems on Lived-In Rooms

I see this a lot on kitchen extensions and bedrooms. Someone has thrown down a single layer of felt, often straight over old material, and called it a “new roof”. It might look tidy on day one, but it won’t cope with movement or weather over time.

2. No Proper Upstands or Flashings

Where the roof meets the house wall, you should see the felt taken up the wall and neatly finished, usually under lead or a suitable flashing detail. If the felt just dies into a bead of mastic, that’s a temporary fix at best.

3. Felt Over Rotten Decks

Covering rotten timber with new felt is like painting over damp plaster. It might look okay for a year or two, but the problem underneath gets worse. If the deck flexes, the felt will soon crack or split at joints.

4. No Thought Given to Drainage

Flat roofs are never truly flat – or at least, they shouldn’t be. If water sits in big ponds for days after rain, the felt is under constant stress. Sometimes it’s a deck issue, sometimes the outlets or gutters are wrong.

My Honest Advice Before You Spend Money on New Felt

If your felt flat roof is leaking, or you’re being told you “need a whole new felt roof”, here’s how I’d suggest you approach it before you sign anything.

Step 1: Decide What the Roof Is Protecting

Be clear with yourself – is this just a basic garage storing tools, or is it a main room you live in? I’m a lot more relaxed about a cheap temporary solution on a shed than I am over your kitchen ceiling.

Step 2: Get the Roof Properly Looked At

You can pick up a lot from the simple checks I’ve described, but at some point a proper inspection from someone who’s actually on the roof is worth doing. I’m always happy to climb up, take photos and talk you through what I see in plain English.

Step 3: Ask Exactly What Felt System Is Being Used

Don’t be shy about asking a roofer:

  • How many layers are you putting down?
  • Is it an SBS torch-on system or something else?
  • How are you dealing with edges, walls, parapets and any rooflights?

A good roofer won’t dodge these questions, and the answers should make sense to you once they’re explained.

When a Felt Roof Repair Is Enough – and When It Isn’t

People often ask me if I can “just patch it” rather than replace the whole thing. Sometimes that’s perfectly reasonable. Sometimes it’s throwing good money after bad.

When a Felt Patch or Local Repair Can Work

  • The roof is generally sound, with good mineral cover and firm decking.
  • There’s one obvious split, puncture or local problem area.
  • The felt is not extremely old or brittle all over.

In those cases, a properly bonded patch in the right material can add years of life and buy you time until you’re ready for a full replacement.

When You’re Better Off Biting the Bullet

  • The felt is crazed, cracked and bald in lots of places.
  • There are multiple leaks or damp patches inside.
  • The deck feels spongy or there’s clear evidence of rot.

On those roofs, you can patch and patch, but you’re still living with a failing system underneath. If it’s over a room you care about – especially a kitchen or bedroom – I usually advise doing the job properly once.

How I Personally Approach Felt Flat Roof Jobs

When you ask me to look at your felt flat roof, I don’t start with a sales pitch – I start with a diagnosis. I want to understand what’s going on with the existing felt, the deck underneath, and how the roof actually drains.

I take clear photos, talk you through what I’ve found in normal language, and then we discuss realistic options – from targeted repairs to full replacement with a modern felt system. I price the work on a fixed basis, so you know exactly what you’re paying for and what materials are being used.

If you’re staring at a tired felt roof now and you’re not sure whether it’s worth repairing or replacing, the best next step is simply to have an experienced pair of eyes on it. From there, the decisions usually become much clearer.