Metal roofs

Metal Roof Waterproofing & Cut Edge Corrosion

Warehouses, workshops, trade counters and light industrial units are mostly roofed in profiled steel laid flat or at a pitch shallow enough that water barely moves. Those roofs behave like flat roofs and fail like them, at laps, fixings, gutters and edges. That is the work we take on: flat and low-pitch metal, treated and waterproofed in place wherever the metal still has enough left in it.

Book a survey

Or call us on 07593 196505

What sort of metal roof this page is about

Two constructions cover most of what we are called to. Profiled sheet is the familiar ribbed steel deck, coated at the factory, fixed through its crowns or troughs and lapped where one sheet meets the next. Standing seam is the smoother, flatter cousin, where sheets are joined by upstanding folded seams and held by clips rather than fixings driven through the surface.

On a genuinely steep roof, water leaves before it can do much. On the shallow pitches used across industrial estates it does not: it runs slowly, sits in laps, backs up behind debris and lingers in the gutters. Every failure below is water staying somewhere longer than the detail was designed to tolerate. Anything steeper than that, wall cladding, or re-roofing an entire building in new sheet, is somebody else's specialism and we will point you at one rather than pretend.

Cut edge corrosion, and why it is always the eaves

Coated steel is protected across its faces, but the moment a sheet is cut to length the bare metal at that cut is exposed. Stack two sheets into a lap, put that lap where water drains slowest, and you have the classic industrial roof defect: rust that begins as a brown line along an edge, then works backwards under the factory finish, lifting it in flakes and thinning the sheet as it advances.

Left alone it goes one way only. Treated in time it is a straightforward job: the affected edges are cleaned back to sound metal, treated, and then encapsulated so the edge is sealed and the lap is bridged rather than left open. That is done from above, in place, without the building being opened up or the sheet coming off. Set against re-sheeting a roof over a working unit, that is a different order of expense and interruption altogether.

Fixings, washers, laps and penetrations

The rest of the failures on these roofs come from the fittings rather than the sheet.

Fixings and washers. Every fastener is a hole through the waterproof surface, sealed by a washer that hardens and shrinks with age. Overtightened at installation, they dimple the sheet and hold a puddle around themselves. Failed washers are one of the commonest sources of the drip nobody can locate.
Side and end laps. Sealant in a lap has a working life; capillary action does not. Once the seal has gone, water is drawn back up between the sheets against gravity, especially on shallow pitches.
Penetrations. Extract fans, flues, cable trays, rooflights and mounted plant all pass through the covering, and each was detailed by whoever fitted it, sometimes years after the roof went on.
Gutters. Valley and eaves gutters hold water longest, corrode first and discharge internally, so a failure lands inside the unit rather than outside it.

Treatment before waterproofing, in that order

Nothing goes over rust. Corrosion left underneath a new surface continues quietly out of sight and pushes whatever was applied off the sheet from below, so the sequence is fixed: clean back, treat the exposed metal, then waterproof. Our specified system for this work is liquid waterproofing applied cold, HYDRONYLON, which goes on once that preparation is done and carries across the laps, around the fixings and into the details in a single continuous operation, which is the whole argument for using a wet-applied membrane on a surface with this many interruptions in it.

Where only the edges are the problem, the work stays confined to them. Where the sheet is generally tired but sound, treating the edges and details while resurfacing the whole area is usually the better buy: the access has been arranged either way, and access is a large part of what these jobs cost.

What the survey establishes first

The governing question is how much metal is left. Sheet thickness and the extent of corrosion decide whether treatment is a genuine intervention or a cosmetic one, and that has to be established before a specification exists, not discovered halfway through. The survey also records fixing condition, lap behaviour, gutter state, the detailing around every penetration, and whether the roof carries any insulation that may have taken on water.

Large roofs and awkward access are exactly where survey method matters, which is covered on the surveys and condition reports page, and the operational side of working over a running site is set out for industrial and warehouse buildings. Once a roof is in reasonable order, keeping gutters clear and checking edges on a schedule is what stops the cycle restarting, which belongs on a maintenance programme.

When we will tell you not to bother

Adhesion and substrate condition set the limit here as everywhere. Some factory finishes need specific preparation before anything will bond to them, some contaminated or weathered surfaces cannot be brought to a workable state, and a sheet that has already perforated cannot be rescued by anything applied to its surface. Where corrosion has run far enough, replacing those sheets is the answer and we will say so.

The same goes for what sits beneath. Where an insulated build-up has taken water on board, the covering is not the fault and sealing over it locks the moisture in. Where the structure itself is deflecting and holding water as a result, no surface treatment addresses the reason. The survey settles which of those you have, not the visit that quotes the job, and whatever guarantee comes with the specified route is put in writing with the price.

Frequently asked questions

What is cut edge corrosion?

Rust that starts at the exposed edge of a coated steel sheet. The sheet is protected on its faces, but wherever it was cut to length the bare steel is left open at the edge, usually along the overlap at the eaves. Water sits in that lap, the edge rusts, and the rust creeps back underneath the protection, lifting it as it goes.

Does the roof have to be stripped to deal with it?

Rarely, and that is the point of treating it. Where the sheet still has sound metal left, the affected edges are cleaned back, treated and encapsulated in place, at a small share of what re-sheeting would cost and interrupt. Where sections have perforated or thinned past the point of use, those sheets get replaced instead.

You only do flat roofs. Why metal?

Because a great deal of industrial roofing is flat or barely pitched, and the same waterproofing skills apply to it: falls, laps, penetrations, gutters and details. We work on flat and low-pitch metal roofs as part of that. Steeply pitched sheet roofing, cladding to walls and re-sheeting a whole building are not our trade and we will say so.

Why does the survey care how thick the sheet is?

Because it decides whether anything can be applied at all. A waterproofing system relies on the sheet beneath it to stay put and carry the loads it always did. If corrosion has eaten far enough into the metal, no surface treatment restores that, and encapsulating a sheet that is close to perforating just postpones a replacement while looking like a solution.

Can gutters be dealt with at the same time?

They usually should be. Valley and eaves gutters on these buildings are where water lingers longest, so they corrode first and leak into the units below at exactly the point everyone parks or stores stock. Lining a sound gutter in the same visit as the edge treatment saves a second access arrangement later.

Rust along the eaves, or a drip nobody can trace?

Tell us roughly how big the roof is, what it covers and how the access works. That is enough for us to plan a sensible first visit.