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.

Book a survey

Or call us on 07593 196505

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 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: a wet layer sealed under a new surface stays wet, and the roof then performs worse than it did while it leaked.

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 the recommendation can be checked against what was found.

The work

How a liquid overlay goes on over felt

Where the roof passes those tests, the usual specification is cold-applied liquid waterproofing, the HYDRONYLON system, worked over the felt already there, which stays put and keeps doing its share rather than going into a skip.

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 one of the surfaces that system's assessment names, with a base layer specified for it, which is why this one is among the straightforward ones. No flame is involved either, which matters where the roof abuts timber, a rooflight kerb or a boundary.

The alternative

When a felt overlay is specified instead

A fresh cap sheet bonded over the old roof is still right on plenty of buildings: a large open field with straightforward details, a surface that takes a sheet well, and a build-up (the layers under the covering) that can carry the weight. 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 comes down to the detail count, what is already up there and the weight calculation.

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. The honest route is taking the roof back and rebuilding it.

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, which is why the test is a probe rather than a photograph.

Can any felt roof be overlaid?

No, and that is the point of looking first. An overlay needs three things to be true: the layers underneath dry through, the felt still bonded to what it was laid on, 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 recovered 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.