Felt Roofing Felt: Why the Exact Product You Choose Really Matters
When customers ask me about “felt roofing felt”, they’re usually stuck on the same problem: there are dozens of rolls in the merchant, all called some kind of felt, and nobody explains what’s actually suitable for a real roof in Brighton, Hove or Worthing weather.
In my 18 years on the roof, I’ve seen everything from cheap shed felt stuck on a house extension, to premium SBS torch-on systems wasted on a garden bar that only needed something basic. Here is my honest advice on how I choose felt, where each type belongs, and how you can avoid paying for the wrong material on your roof.
Not All Felt Is the Same: Three Main Categories I Work With
If you ask me, I’ll tell you straight: when someone just says “felt”, it’s meaningless. I split roofing felts into three broad categories:
- Lightweight shed felt – for temporary or non-critical roofs
- Traditional pour-and-roll felt – older style, usually nailed and bitumen-bedded
- Modern SBS torch-on felt systems – what I use on most proper flat roofs
Each has its place. The big problems happen when the wrong felt ends up on the wrong roof.
Shed Felt vs Proper Roofing Felt: Where I Draw the Line
Let me clear up one common mistake: the cheap roll of “roofing felt” from a DIY shop is not designed for your kitchen extension or bedroom dormer.
Where shed felt is acceptable
I only treat lightweight felt as suitable for:
- Garden sheds
- Small storage lean-tos
- Temporary bike or log stores
- Short-term patch jobs where the client is planning a full rebuild soon
Even then, I explain that it’s a short to medium-term solution. Brighton seafront winds and sideways rain will find every weakness in that kind of felt.
Where I would never use shed felt
On any roof directly over:
- Bedrooms or living rooms
- Kitchens and bathrooms
- Home offices with electrics and computers
- Garages with valuables or classic cars
For those, I use proper flat roof systems – usually an SBS torch-on felt build-up. If you want to see how I price that kind of work, have a look at my flat roof repair and replacement service.
How I Choose the Right Felt System for Each Type of Flat Roof
Instead of just saying “I’ll put some felt on it”, I match the felt system to the type of flat roof and what’s underneath it.
1. Garage flat roofs
Garages vary a lot in Sussex – some are cold, basic storage, others have electrics, insulation and gym gear. My approach:
- Basic storage garage: mid-range torch-on system, two or three layers, mineral cap sheet
- Insulated or attached garage: full felt system with proper ventilation detail and upgraded cap sheet
If you’re comparing options, you can use my garage flat roof cost calculator to get a realistic price before I even visit.
2. Rear extension roofs
On a kitchen or living room extension, I don’t cut corners. I look at:
- How warm the room gets (kitchens and south-facing rooms need better UV resistance)
- Whether you have rooflights
- How easy future access will be for maintenance
For these, I almost always go with a high-quality SBS felt system with good fire performance and a decent life expectancy. If you want to plan a project like this properly, I break it down on my rear extension flat roof page.
3. Porch and bay window roofs
Porches and bays are small, but they’re right in your eyeline, so appearance matters as much as waterproofing.
- I pick a mineral felt colour that works with your tiles and fascias
- I pay extra attention to drip edges and trims because they’re highly visible
- On bays, I also think about how the felt detail meets the brickwork and any lead flashing
If you’re comparing materials for these little roofs, have a look at how I price porch flat roof projects and bay window flat roofs.
4. Balconies, terraces and walk-on areas
This is where the phrase “felt roofing felt” can really mislead people. Not every felt is suitable under a balcony or deck.
On walk-on or decked roofs I look for felts and detail strips that:
- Cope well with trapped moisture under decking
- Have protection layers to avoid punctures from furniture feet
- Allow me to create reliable upstands around doors and railings
If you’re thinking of a roof terrace or composite deck over felt, it’s worth reading my approach to balcony and terrace waterproofing first so you understand the build-up.
5. Dormer flat roofs
Dormers are small but fussy. The felt has to tie into slates or tiles, fascias, gutters and sometimes lead cheeks.
- I choose felts that can be neatly detailed around upstands
- I allow for extra reinforcement where the dormer roof meets the main roof
- I consider how future re-roofing of the main roof will affect the felt detailing
If your dormer felt is ageing or cracked, you can see how I typically tackle these on my dormer flat roof service page.
Felt Roofing Felt on Pitched Roofs: Underlay vs Top Layer
Another area where the word “felt” confuses people is pitched roofs. The “felt” you see from the scaffold under your tiles these days is usually not old-style bitumen felt at all – it’s a breathable membrane.
What I use under tiles and slates now
On most new pitched roofs I replace:
- Old, brittle bitumen-based underfelt
- Rusty or rotten battens
with modern breathable membrane and treated battens. It still gets called “felt” on site, but the performance is completely different to the heavy, non-breathable felts from decades ago.
If your tiles are fine but the underfelt is torn or sagging into the gutters, I offer dedicated roof batten and felt membrane replacement.
Key Specs I Look At When I Pick a Felt for Your Roof
When I’m choosing a felt system for your particular roof, I’m not just guessing. I look at a few key details on the data sheet.
1. Thickness and weight
Heavier isn’t always better, but:
- Very thin felts puncture easily and don’t like foot traffic
- Good quality cap sheets usually feel robust and hold their shape when hot
2. Type of binder (SBS vs APP vs oxidised)
On Sussex homes I mostly lean towards SBS-modified felt because:
- It stays flexible even in cold weather
- It copes well with slight movement in timber decks
- It’s more forgiving around details and upstands
3. Fire performance
Especially on extensions and dormers, I always check the fire rating. Some felts are designed specifically to achieve certain fire classifications when laid as part of a full system. That matters when you’re close to boundaries or neighbours.
4. UV resistance and colour
The mineral finish on the top felt isn’t just for looks – it protects the bitumen from the sun. Lighter colours can also keep inside rooms a bit cooler in summer, which is handy on south-facing extensions.
How Cheap Felt Ends Up Costing You More
In my 18 years on the tools, I’ve noticed a pattern: whenever someone has gone for the absolute cheapest felt they could find, one of three things usually happens within a few years:
- The first strong wind starts to lift the edges
- Cracks appear wherever there’s movement in the boards
- The mineral surface wears away and the sun bakes the felt brittle
At that point, you might pay me to do an “emergency” patch, then later pay again for a full replacement. If the right felt had been used in the first place, it would likely have been a single, planned job instead of a series of small crises.
If you are already in that emergency stage, you can use my urgent roof leak booking service to get priority help.
Why Detailing Matters Just as Much as the Felt Roll Itself
Even the best felt roofing felt will fail if the details are poor. When I survey a leaking felt roof I always look closely at:
- Upstands: how far the felt runs up walls and parapets
- Drips and edges: whether water sheds cleanly into the gutter
- Pipes and vents: how they’re sealed and reinforced
- Joints between flat and pitched areas: especially around dormers and valleys
- Lead flashings: how they overlap and protect the felt
Often, the issue isn’t that the felt itself is terrible, it’s that it wasn’t installed as part of a thought-through system. For example, I regularly have to sort out leaks where the felt is fine, but the lead flashing was never properly chased into the brickwork, or the parapet felt capping was missed altogether.
Felt Roofing Felt and the Rest of Your Roof System
I never look at the felt in isolation. On a real roof I also have to think about:
- Decking boards: Are they rotten, swollen or poorly fixed?
- Insulation: Is there any? Is it correct for the type of roof?
- Ventilation: Will the felt and build-up trap condensation?
- Rainwater run-off: Can your gutters and downpipes cope?
- Perimeter trims and fascias: Will the new felt integrate cleanly?
On many flat roofs I also upgrade or replace the guttering and downpipes, or fit new uPVC fascia boards at the same time. Doing it together usually gives you a neater finish and better long-term performance.
How I Help You Choose the Right Felt Before Any Work Starts
If you’re unsure what type of felt roofing felt is right for your particular roof, here is how I normally guide customers through it:
- Quick conversation: You tell me what kind of roof you’ve got (garage, extension, dormer, porch, balcony, bay) and what problems you’re seeing.
- Rough cost first: You can use my calculators, for example for garage roofs or porch roofs, to see whether a full felt system fits your budget.
- Free inspection: If the estimate looks sensible, I come out, check the deck, falls, edges, and existing felt or membrane. You can book a free roof inspection online.
- Fixed-price quote: I specify the exact felt system, number of layers and detailing, then give you a fixed price – no vague “day rate” guesswork.
- Full transparency during the job: Through my client portal, you see daily photos, schedule updates and final warranty documents.
When to Repair Existing Felt and When to Start Again
People often ask me: “Can you just patch the felt?” My honest rule of thumb:
- Local damage but generally sound felt: a proper patch can be fine
- Multiple age-related cracks and blisters: usually better to plan a full overlay or replacement
- Wrong product used originally (e.g. shed felt on an extension): I recommend biting the bullet and converting to a proper system
The only way to be sure is for me to get up there, lift a few edges and see what’s happening underneath. That’s exactly what I do on a free roof inspection visit.
Felt Roofing Felt: My Straight Advice Before You Spend a Penny
To wrap it up, here’s my honest advice if you’re currently comparing “felt roofing felt” options:
- Decide if your roof is critical (rooms below) or non-critical (shed, short-term store)
- Don’t let anyone put shed felt on anything important
- Ask what type of felt system they’re using, not just “felt”
- Check that details like parapets, flashings and gutters are included
- Make sure you get a fixed price and a clear description of what’s being installed
If you’d like me to look at your roof and give you a straight, no-pressure opinion on the best felt option, you can book a free inspection in Brighton and Sussex here. I’ll tell you exactly what I’d do if it were my own home – and I’ll back it up with photos, a fixed quote, and full transparency from start to finish.