What I Mean by “Torch On Underlay” – Plain English

When customers ask me about torch-on felt, they’re usually thinking about the top, mineral-finished layer they can see. But if you ask me, I’ll tell you straight: the real workhorse of a flat roof is the bit you don’t see – the torch on underlay.

On a modern felt flat roof, I normally install a two- or three-layer system:

  • Base / Vapour Control Layer (sometimes combined with underlay on warm roofs)
  • Torch on underlay – the main waterproofing layer
  • Mineral cap sheet – the UV-protected top layer you can see

This article is about that middle bit – the torch on underlay – how it actually works, where it goes wrong, and how I decide what to use on your specific roof in Brighton, Hove, Worthing or nearby.

What Torch On Underlay Actually Does for Your Roof

In my 18 years on the roof, I’ve noticed most leaks on “felt roofs” are blamed on the visible top sheet. But very often, the real problem is hidden in the underlay layer beneath.

Here’s what the underlay is really responsible for:

  • Primary waterproofing – the cap sheet protects from UV and weather, but the underlay often carries most of the actual watertight function.
  • Bonding the system together – it’s the layer that fuses to the deck or vapour layer and ties the whole build-up into one solid membrane.
  • Taking movement – your roof deck moves, flexes and expands; the underlay absorbs a lot of that stress.
  • Backing up the cap sheet – if the top mineral layer gets damaged or scuffed, the underlay is your second line of defence.

When I’m called out to a “mystery leak”, the most common story is this: someone only patched the top felt and ignored the state of the underlay. It looks fine from above, but water is already tracking through hairline splits or poorly welded laps underneath.

Where Torch On Underlay Sits in the Roof Build-Up

Let me walk you through how I build a typical flat roof from the deck upwards, so you can see exactly where the underlay fits. I’ll keep it focused on the underlay decisions rather than going into the full torch-on process (I cover that in my other guides).

Cold Flat Roof with Torch On Underlay

This is what I still see a lot on older garages and small extensions.

  • Timber deck (often OSB or plywood)
  • Torch on underlay bonded to the deck
  • Mineral torch-on cap sheet on top

On this type of roof, the underlay has to stick well to the deck and cope with more temperature swings because there’s no insulation above to stabilise things. If the wrong underlay is used or it’s not bonded properly, you get blisters, hollow spots and cracks over time.

Warm Roof with Torch On Underlay

On a modern insulated “warm roof”, the build-up typically looks like this:

  • Timber deck
  • Vapour control layer (VCL)
  • Rigid insulation boards
  • Torch on underlay
  • Mineral cap sheet

Here, the underlay has to bond firmly to the insulation or separation layer without melting it, and still give you a proper weld to the cap sheet above. This is where choosing the right underlay really matters – especially on coastal homes around Brighton and Hove where wind uplift is harsher.

Different Types of Torch On Underlay – And When I Use Them

If you just ask for “felt”, some roofers will simply pick whatever is cheapest on the merchant’s shelf that week. I don’t work like that. I pick my underlay based on what your roof actually needs.

SBS-Modified Torch On Underlay

This is what I use on the majority of flat roofs I install locally.

  • What it is: Bitumen combined with SBS (styrene-butadiene-styrene) to make it more flexible and resilient.
  • Where I use it: Most residential flat roofs – garages, extensions, dormers, bay windows and porches.
  • Why I like it: It copes well with movement, temperature swings and our coastal winds.

If you’ve already read my guides on Tecnatorch mineral felt, the underlay I pair with it will usually also be an SBS-modified torch on underlay to match performance.

Standard Bitumen Underlay

On older roofs I strip off, I still find basic, unmodified bitumen underlays.

  • What I see: They tend to crack more with age and don’t deal as well with movement.
  • Where I’d still consider it: Very small, low-risk areas where budget is tight and movement is minimal – but even then, I’ll talk you through the pros and cons.

Generally, if I’m putting my name to a full new roof, I prefer an SBS system throughout.

Self-Adhesive Underlay (For Low-Flame or No-Flame Areas)

There are situations where open flame just isn’t a good idea – close to timber cladding, plastic fascias, windows, or where building regs or insurers are strict.

  • Self-adhesive underlay: Heated with a hot air gun or activated by pressure instead of an open flame.
  • Partially bonded underlay: Used over sensitive substrates where I don’t want full torch heat.

On these jobs, I might use a self-adhesive underlay at edges and upstands, then transition to fully torched underlay where it’s safe. You’d be surprised how many “melted window frame” stories I’ve heard because someone insisted on torching right up to uPVC.

Where Torch On Underlay Commonly Fails (From What I See on Site)

When homeowners in Brighton and Hove call me out to diagnose a flat roof leak, the same underlay-related problems come up again and again. Here’s what I really look for under the surface.

1. Poor Lap Joints and Detailing

The most important areas on any underlay are:

  • Side laps (long joints)
  • End laps (short joints)
  • Upstands and corners
  • Pipe penetrations and rooflights

If those aren’t fully welded, you’ll get tiny capillary gaps where water slowly tracks under the sheet. From above, the roof can still look “fine” for years, while the decking rots underneath.

When I inspect, I check for cold joints, poor bleed-out (the little bitumen edge that tells me it’s fully welded) and lazily cut corners and upstands.

2. Wrong Underlay on the Wrong Substrate

I often see this on DIY jobs and quick fixes:

  • Underlay scorched onto insulation that can’t take the heat.
  • Non-compatible underlay on top of old felt or asphalt.
  • Underlay pumped straight on to damp timber or dirty surfaces.

If the bond fails, the underlay blisters, lifts, or slides – and once water finds those hollows, leaks are only a matter of time.

3. Overheating or Underheating with the Torch

Torch work is all about balance. Too cold, you don’t get a proper bond. Too hot, you burn the reinforcement or damage the substrate.

On underlay, I’m watching for:

  • A consistent melt pattern (not dry patches and not black, burnt areas)
  • Even bleed at laps
  • No sagging or sliding once it cools

This is one of those things that only comes with experience. In 18 years on the roof, I’ve learned that neat, even underlay work usually tells me the rest of the roof has been done with care too.

How I Decide If Your Existing Underlay Can Be Reused

Sometimes homeowners ask me, “Can you just re-torch a new cap sheet over the old felt to save costs?” The honest answer: only if the underlay and what’s below it are still sound.

When I look at a tired flat roof, here’s my thought process:

Step 1: Check for Movement and Soft Spots

If the deck feels spongy underfoot or I can see sagging, that usually means the underlay has already let water into the timber. Re-covering over that is just burying a problem.

Step 2: Inspect the Underlay Once the Top Sheet Is Stripped

On some roofs, I’ll strip just the cap sheet first and leave the underlay in place to inspect it.

  • If the underlay is flat, well-bonded, and the laps are still solid, it might be reusable as part of a refurbishment system.
  • If I find cracks, blisters, or hollow sounds when I tap it, it needs to come off with the top sheet.

Step 3: Weigh Up Cost vs. Lifespan

Here is my honest advice: if your roof is already 15–20 years old and I’ve got the top sheet off, it’s often false economy to leave an old, tired underlay in place. The saving now can cost you in a few years when the underlay starts failing underneath your new surface.

Why the Right Underlay Matters Even More on Coastal Roofs

Homes around Brighton, Hove and Worthing face a tougher environment than those inland:

  • Stronger, more turbulent winds
  • Salt-laden air that shortens material life
  • More driving rain coming in at an angle

On these roofs, the torch on underlay does a lot of hidden heavy lifting:

  • It helps resist wind uplift by bonding firmly to the deck or insulation.
  • It adds thickness and redundancy to the waterproof layer for when storms hit sideways.
  • It spreads the load around tricky areas like parapet walls, bay roofs and balconies.

This is one of the reasons I pair quality SBS underlays with my flat roof repair and replacement work in Brighton and Hove – cutting corners on unseen layers doesn’t make sense in this climate.

Torch On Underlay on Different Roof Types Around Your Home

Most homeowners only think about “the flat roof” as one thing, but your property might actually have several small flat areas, each with slightly different underlay needs.

Garage Flat Roofs

Garages are where I most often see thin, cheap underlays and patchwork repairs. If you’re pricing up a new garage roof, you can use my garage flat roof cost calculator for Brighton, Hove and Worthing to get an instant idea of budget, then I can advise what underlay system makes the most sense for you.

Rear Extension Roofs

Rear extensions tend to carry more heat from the room below and sometimes have rooflights. I usually choose an SBS underlay that copes well with this movement and works as part of a warm roof build-up. If you’re planning or refurbishing one, have a look at how I handle rear extension flat roofs and I can talk you through the underlay options on a site visit.

Dormer and Bay Window Roofs

Dormers and bay roofs are fiddly: lots of edges, small areas, and junctions with pitched tiles. The underlay detailing at these joints is critical because they’re such common leak points. I often combine torched underlay on the flat with carefully formed upstands and lead flashing to keep everything watertight.

If your bay or dormer is giving you trouble, my flat dormer roof repair and replacement service explains how I approach these details properly rather than just throwing another patch on top.

Questions I Usually Get About Torch On Underlay

“Isn’t the Underlay Just a Back-Up Layer?”

No – in many systems, the underlay is doing at least as much waterproofing as the top sheet. Think of the mineral cap as the armour and the underlay as the actual raincoat underneath.

“Can You Just Add Another Underlay Over the Old One?”

Sometimes, yes – but only if:

  • The old deck is sound and dry.
  • The existing felt is stable and compatible.
  • We’re following the manufacturer’s refurbishment specification.

I’ll never stack multiple layers just to hide problems. If I think your roof needs stripping back to timber, I’ll say so and explain why.

“Why Is My Roof Blistering?”

Blisters are often a sign that moisture or trapped air has expanded between the underlay and the deck or between layers. Sometimes they’re just cosmetic; sometimes they’re a sign the bond has failed. I normally cut a few test sections to see what’s going on underneath before advising whether you need local repairs or a full replacement.

How I Keep You Involved With the ‘Hidden’ Layers

One thing I hear a lot is, “I never really knew what they did under there last time – I just saw the finished roof.” I try to work differently.

  • Before I start: I’ll talk you through the exact underlay system I’m using and why.
  • During the job: I take clear photos of the deck preparation, underlay, and final cap sheet so you can see how each layer went down.
  • After completion: You’ll have a visual record alongside your invoice and any warranties.

If you like the idea of that level of transparency, you can start by using my rough scaffolding price calculator to see how access might affect your project. Then, when I visit, I’ll walk you through the underlay choices in person.

What to Do If You’re Worried About Your Existing Underlay

If you’ve got a flat roof that’s nearing the end of its life – or you’ve already had a couple of patch repairs – the real question is what’s going on underneath that top layer.

Here’s my honest advice:

  • If you’re seeing stains inside or suspect a leak, don’t ignore it. Water can track through underlay for a long time before you notice visible damage.
  • If you’re planning an extension or upgrade, it’s worth getting the underlay and build-up specified properly from the start so you’re not paying twice later.

If you’d like me to have a proper look and explain what your underlay is doing – or failing to do – I’m happy to come out, lift sections where needed, and give you a clear, fixed-price quote for putting it right.