Existing felt
Overlaying and Refurbishing an Existing Felt Roof
Most flat roofs in London are felt, and most of the ones we are called to are tired rather than finished. Somebody has usually been told it all needs stripping. Sometimes true; more often the covering has years left and wants a new weathering layer over the top. What decides is under the surface, not what it looks like from the ground.
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The short answer: An overlay is possible when the existing covering is dry, still bonded to the deck and sound enough to build on. Trapped moisture, boarding that moves underfoot, or too many layers already in place each rule it out.
Ageing
How a felt roof ages, in the order it happens
Felt does not fail suddenly, and knowing where a roof sits in the sequence is most of the diagnosis. What the material is and how a new one is built up is set out on the felt and torch-on roofing page; this is what twenty summers do to one. The sequence below assumes a roof over a heated space; on an unheated garage it runs in a different order, from the perimeter inward, because nothing beneath it ever dries the boards out.
- The mineral goes first. The granules take the ultraviolet light so the bitumen underneath does not. They wash into the outlets (the roof's drains) over the years, and the bare patches left behind are where ageing speeds up.
- The bitumen hardens. Stripped of its shield, the compound stiffens. A roof that could absorb the daily expansion of the deck (the boarding underneath) begins to resist it instead.
- Movement finds the weakest line. Never the middle of the field: a lap (where two sheets overlap), an upstand (the covering turned up a wall), or the change of plane at an edge, where the sheet already works hardest.
The test
What the survey checks before an overlay is possible
Three questions, and all three have to answer yes.
Dry through the layers
A covering can be dry on top and saturated below, and only opening it settles which. Where there is insulation, moisture in it is decisive: trap a wet layer beneath fresh waterproofing and it stays wet, so the finished roof ends up in a worse state than the leaking one it replaced.
Still bonded, details still sound
A new layer inherits the grip of the old one. Felt lifted from the deck gives it nothing to hold on to, and blisters show within a season. Details are checked in the same pass: an upstand too low to build from, or a rotten edge trim, is rebuilt rather than covered as it stands.
All of it goes into the written condition report, so what we advise can be read back against what was found.
The work
How a liquid overlay goes on over felt
Where the roof passes those tests, this is our recommendation: the HYDRONYLON cold-applied liquid system, worked across the felt already there, which stays put and keeps doing its share rather than going into a skip. On an ageing roof the case for it is easy to state. A roof that failed at its seams the first time does not want a second set of them, and a building already holding one bitumen covering does not want the load of another. Liquid goes on as a single cured thickness, joint-free, with no flame near anything that can burn.
The sequence on felt is specific. The roof is cleaned back to something sound and loose mineral taken off, because granules that come away take the new layer with them. Blisters are cut, dried and made flat rather than rolled over, and open laps dressed down and reinforced. Upstands, outlets and penetrations are detailed first and separately, with reinforcement worked into the wet system at every change of plane, since those are where the old roof failed and where a new one will if left to last. Only then does the field get its coats, so what cures is one continuous membrane with no joints in it.
Felt is a surface the assessment behind that system covers, and it carries its own specified base layer, which puts this among the straightforward substrates. No flame is involved either, which matters where the roof abuts timber, a rooflight kerb or a boundary.
The exception
When a felt overlay is specified instead
Not often, but genuinely sometimes, and when it is the report says why. A fresh cap sheet bonded over the old roof is the right answer on a large open field with straightforward details, on a surface that takes a sheet well, and on a build-up (the layers under the covering) with the capacity to carry it.
The calendar decides it more often than people expect, since a liquid system needs temperature and a dry surface held through the cure, and a roof that has to be closed in a cold wet week is a felt job. It suits a garage or a long shop roof better than one that is mostly upstands. Which of the two a roof gets is a question the refurbishment options guide answers in general; on site it turns on how many details there are, what the existing build-up will take and what it can carry.
Limits
When neither is honest and the roof comes off
There is a point past which an overlay hides a bill rather than avoiding one. Water already into the deck, moisture across the build-up, boards that give underfoot, or a third covering already there: at that stage the surface is not the problem and adding another will not make it one. What is honest at that stage is stripping it off and building the roof again.
We would rather price that and lose the cheaper job than sell an overlay asked to do something it cannot, and you are told before you have spent anything: pricing work costs nothing either way.
Questions
Frequently asked questions
How can I tell whether my felt roof is worn or finished?
Worn shows on the surface: lost mineral, faded colour, a chalky feel. Finished shows in the layers: laps opening, felt brittle enough to crack when lifted, softness underfoot, damp in the build-up. A roof can look poor and be sound, or look tidy over layers that have given up, so the answer comes from a probe rather than from a photograph.
Can any felt roof be overlaid?
No, which is precisely why anybody looks first. An overlay needs three things to be true: the layers underneath dry through, the felt still gripping whatever it was laid over, and the details sound enough to build from. Fail any one and an overlay seals a problem in rather than out.
How many layers of felt can a roof carry?
Fewer than most roofs already have. Every previous covering adds weight and another interface for water to travel along, and by the third the build-up is working against itself. A liquid overlay adds almost nothing by comparison, which is often why it is specified on a roof re-covered once already.
How long before the roof can be walked on again?
Longer than felt, and not a figure we will invent for you. Liquid cures by chemical reaction rather than by cooling off, so the wait is set by the build-up specified and by what the weather does meanwhile: it wants a dry surface and warmth kept up around the clock, nights included, not only while the crew is standing there. Where more than one coat is specified there is a day in between them by design. What you get is dates in a programme agreed before anyone starts, said out loud again if a cold or wet spell moves them, and the roof stays ours until it is genuinely ready to hand back.
Will an overlay stop a leak I cannot find?
Only if the survey has found it first. Water entering at one point and appearing several metres away is normal on a layered roof, so an overlay over an undiagnosed entry point leaves the leak running underneath the new surface.
Tired felt roof, and nobody has actually looked at it?
A photograph of the surface and one of where it meets the wall tells us a surprising amount. Send those and we will arrange a proper look.
We need a little more before we can send that: a name, the postcode of the roof, what is happening with it, and either a phone number or an email address.
Sorry, that did not send. What you wrote is still here, so press send again when you have a signal. If it will not go, ring 07593 196505 or email office@pluvian.uk.
That has come through a few times already. If the first one did not arrive, ring us instead or email office@pluvian.uk and we will pick it up from there.
Coming out and pricing the work is free; a written condition survey carries a fee, agreed before the visit and taken off the price of the work if you go ahead.