A Liquid Overlay, Step by Step: What Happens on the Roof

The short answer: a liquid overlay runs in a fixed order. The roof is inspected and found dry and sound, cleaned and prepared, primed with a base layer matched to its surface and reinforced at every detail, then given its field coats with a cure between each. Finally it is checked, photographed and handed over with its paperwork.

That order is the whole of this guide. Whether an overlay suits your roof, and whether liquid or felt should form it, are separate questions with their own pages. Here the decision has been made, and what you want to know is what the days on site involve, whether you own the house underneath or manage the block.

What must already be settled before work starts?

The existing roof has to be dry through its layers, well bonded to the surface beneath it and sound enough to take preparation without breaking up. Those three facts are established at the inspection that came before the quotation, and nothing that follows is worth doing if any of them is false.

Dryness matters most, because a liquid membrane seals whatever it covers. Moisture shut in beneath it has nowhere to go and works back against the new layer from below. Adhesion matters because an overlay is only as well fixed as the covering it bonds to, and soundness matters because preparation is physical: a surface that crumbles under a scraper will crumble under feet as well. The tests an old felt roof has to pass are covered separately. If any answer was doubtful, you should have been told in writing, with the alternative priced beside it.

What comes off the roof first?

Everything that would stop the new layer bonding, and very little else. An overlay keeps the covering that is there, so the first stage is cleaning and correcting rather than stripping:

• Moss, silt and plant growth, with outlets and gutters cleared so water leaves the roof while work is under way.
• Loose chippings and flaking mineral surface, which would lift away and take new material with them.
• Old patches, mastic smears and failed repairs that have parted from the surface beneath.
• Anything left up there that does not belong: redundant brackets, abandoned cable and the debris every roof collects.

Defects in the old covering are then put right, so the base is flat and continuous. Splits are opened and closed, soft spots are investigated rather than covered, and any lap that has lifted is laid back and fixed. The roof is brushed or washed clean and left to dry before anything wet touches it. On an occupied building this is the noisiest stage and the one worth warning neighbours about; the rest is far quieter.

Why is the base layer different on every roof?

Because the first material to touch the roof has to bond to that particular surface, and felt, asphalt, profiled metal and older single ply each want a different product. The base layer, often called a primer, is chosen from the manufacturer's specification for the substrate in front of it, not from habit or from what is in the van.

On the system we install, HYDRONYLON, the assessment behind the system lists the surfaces it is tested on and what goes first on each. Where a surface falls outside that list, a bond test on the roof itself decides, and the result is written into the job record rather than left in somebody's memory. The base layer is thin and quick to lay, but adhesion is won or lost there, so it goes onto a clean, dry surface and nowhere else.

Why are the details done before the rest of the roof?

Because upstands, outlets, kerbs and penetrations are where flat roofs leak, and they need reinforcement the open field does not. Doing them first means each junction is built properly and on its own terms before the larger areas are brought up to meet it.

At each one a reinforcing fleece is bedded into wet material and taken from the flat onto the vertical or down into the outlet, then saturated until it disappears into the membrane rather than sitting on its surface. Corners take shaped pieces rather than folds, and pipes and cable entries are wrapped. Where a detail caused the old leak, it gets the most time. At the end of this stage the roof looks oddly unfinished, finished details standing in a field of primed surface, and that is exactly how it should look.

Why is there a wait between coats?

Because each coat has to cure before the next goes over it. A liquid system builds its depth in layers, and fresh material laid onto a coat that is still soft traps moisture or solvent between them and weakens the bond that makes them one membrane.

The manufacturer sets the gap and the weather moves it: cool, damp air slows a cure and warm, dry air hastens it. So the interval is judged on the roof, by touch and by the conditions, inside the window the specification allows. Leave it too long and the surface may need preparing again before the next coat will take. The field always goes on in the same order, the base first, then the body coats that give the membrane its depth, then the top coat that finishes it and, where people will walk, carries a textured surface underfoot.

What does the weather have to be doing?

Staying dry, above the system's lowest working temperature and free of dew, for the whole of the cure and not only while people are on the roof. A forecast that looks settled at breakfast can still leave condensation on the surface after dark, and that matters as much as rain does.

When conditions turn, the work stops, and a careful contractor would rather lose the day than lay material that will not cure. Any edge left part-finished is closed off so the roof stays watertight, the next coat is rebooked and you are told why. It is normal for a programme to move with the weather more than once, and that is a sign the system is being respected rather than a sign the job is going wrong.

How do you know it is finished?

By looking closely and in order. The finished membrane should be one continuous surface: even in colour and texture across the field, with the reinforcement invisible at every detail and no pinholes, thin patches, bubbles or bare edges anywhere.

The checks follow the order of the work. Details come first: every upstand ended neatly under its flashing or trim, every outlet open and dressed, every penetration closed. The field is then walked methodically for thin areas and damage, and the drainage is watched, because a roof that still holds water was never given the falls it needed, whatever now sits on top. Anything that fails is put right before the job is called done, and the last thing to leave is the rubbish.

What should you receive at the end?

A record, not just a roof. At minimum that means a dated photograph of every stage, a note of the system and every layer used, the guarantee for the work, and the maintenance the system asks for in return.

The photographs matter more than people expect, because the base layer and the reinforcement vanish once the top coat is on and the pictures are the only proof they exist, and a job that ran smoothly is owed the same record as one that did not. The guarantee covering the work is ours, and the product guarantee from the system's British distributor, which we hold, sits behind it on condition that the roof is looked after. That is why the maintenance terms travel with the handover and why planned maintenance is offered alongside. What a complete close-out file should contain, on a job of any size and from any contractor, is the subject of the handover pack guide.