Checking an older Northwich roof before solar panels go on

Why the roof matters more than the panels

A solar array is fixed to a roof by brackets screwed into the rafters, the sloping timbers that carry the roof covering. The brackets hold aluminium rails, and the panels clamp to the rails. That arrangement has two jobs. It has to carry the weight of the panels down into the structure, and it has to hold them on in strong wind, when air moving over and under the array tries to lift it. Both jobs depend on the timbers being sound, properly sized and still where the original builder put them.

On a newer house with a standard roof, most of that can be taken on trust. In Northwich it often cannot. In the Witton ward, 44.7% of households live in terraced houses, and in Winnington and Castle the figure is 31.3% (Census 2021). Older roofs are more likely to have been repaired, re-covered or altered at some point, and terraces share walls and roof lines with their neighbours. The town also sits inside the brine subsidence compensation district created by the Cheshire Brine Pumping (Compensation for Subsidence) Act 1952, which is why a careful surveyor here looks for evidence of movement as well as for rot. The background is on subsidence and solar; this page is about the inspection itself.

The rafters: size, spacing and condition

The rafters are the heart of the check. A surveyor who stays on the ground or works only from aerial photographs cannot see them, so the first test of a proper survey is simple: does somebody go into the loft with a torch and a tape measure?

What they should measure

• Size. The depth and thickness of the rafters. Older roofs were often built with timbers that would not be chosen today, and some were cut down when roofs were altered.
• Spacing. The distance between rafter centres. It decides where brackets can go and how many rafters will share the load from each rail.
• Span and support. How far each rafter runs between supports. In a traditional cut roof, the rafters rest on purlins, horizontal beams running along the roof, which are themselves propped by struts. Removing a strut, which sometimes happens during loft storage work or a conversion, changes how the whole roof behaves.

What they should look for

• Rot, especially at the foot of the rafters where they meet the wall plate, and around old leaks near chimneys and valleys.
• Woodworm holes and dust, which may be old and inactive or may not.
• Splits, shakes and notches cut for cables or pipes.
• Rafters that sag between supports, or that have twisted or pulled away from the purlin or the wall plate.

A useful survey records what was found, not just a tick in a box. Ask for photographs of the timbers where the array will go. They cost nothing extra to take, and they are the only evidence you will have later that the structure was looked at.

Signs of past movement

Ground movement leaves marks on a house, and many of them show in the roof first. None of the signs below proves that a house has subsided, and none rules out panels. They are reasons to look harder.

From the street or the garden

• A ridge that dips in the middle or drops towards one end.
• A roof slope that looks twisted, or rows of slates or tiles that wave rather than run straight.
• A gap or a fresh line of mortar where the roof meets a gable wall or a neighbour’s roof.
• Stepped cracks in brickwork, bulging walls, or windows and doors that have gone out of square.

From inside

• Rafter feet that have slid off the wall plate, or a wall plate that has moved.
• Cracks where the ceiling meets the walls on the top floor.
• Daylight around the edges of the roof space where there should be none.
• Packing pieces, wedges or new timber bolted to old, which may be the trace of an earlier repair to movement.

The important distinction is between movement that happened long ago and has stopped, and movement that may still be going on. A website cannot tell the two apart, and an installer’s surveyor may not be able to either. That is the moment to bring in someone qualified to judge it, which is covered further down.

The roof covering: slate or tile

Northwich roofs carry a mix of coverings. Historic England’s entry for the R.A.O.B. Hall on Witton Street, for instance, records a grey slate roof. Other houses in the town have concrete interlocking tiles or clay tiles, and each covering needs a different approach when brackets go in.

• Slate is thin and brittle. Brackets are fitted by removing or sliding individual slates, fixing to the rafter beneath and weathering the bracket with a flashing plate, a thin metal or rubber-backed sheet that keeps rain out where the bracket passes through. Breakages are normal, so the quote should say who supplies matching slates and who replaces them.
• Concrete tiles are thicker and easier to lift and replace, but the brackets still have to sit neatly under the tile above so it lies flat. A tile lifted by a badly placed hook lets in wind-driven rain.
• Clay tiles are smaller and can be fragile with age. They need the same care as slate.

Look beneath the covering too. The battens, the thin timbers the slates or tiles hang on, and the underlay, the sheet beneath them, wear out. If the survey finds them perished, it may make sense to re-roof first. Panels on a roof that needs replacing soon will have to be lifted off, stored and refitted, with scaffolding twice.

Previous repairs and alterations

Old repairs are part of the history of many Northwich roofs, and they are not a problem in themselves. What matters is knowing about them before brackets go in. The surveyor should note:

• Doubled or strengthened rafters, where new timber has been fixed alongside old. They may be stronger than the original, or they may be a patch over a weakness that still needs attention.
• Steel straps, plates or ties added to hold parts of the roof or walls together.
• A partial re-roof, where one slope or part of a slope has a different covering or newer battens.
• Loft conversions and dormers, which often change how loads travel through the roof.
• Removed chimney breasts in the rooms below, which can affect what supports the structure above.

Your own paperwork can help. Building control completion certificates, records of a past subsidence claim, or a homebuyer’s survey from when you bought the house may all describe work that is now hidden behind plaster. If you bought the property, the conveyancing file is the first place to look.

How the fixing method spreads the load

The fixing design is where a good survey turns into a sound installation. Each bracket must go into the centre of a rafter, not into a batten or the edge of a timber. The rails then spread the weight of each panel across several brackets, and so across several rafters. On a roof with closely spaced, healthy rafters that is straightforward. On a roof with wider spacing, slender timbers or old repairs, the installer may need more brackets, a different rail layout, or a strengthening plan before any fixing goes in.

Ask the firm to show you how the bracket layout was worked out for your roof, and how it allows for wind. You do not need to understand the calculation, but you should see that one exists and that it refers to your rafters rather than to a standard drawing. Separately, check on the MCS installer directory that the firm is certified for solar PV before the survey is booked.

In-roof systems, which replace part of the roof covering with a tray that the panels sit in, spread the load differently and change how the roof is weathered. They are sometimes chosen for appearance. If one is proposed for an older roof, the survey needs to say how the tray is supported and how it joins the existing slates or tiles.

When to bring in a structural engineer

An installer’s surveyor is not a structural engineer, and a good one knows the difference. A structural engineer, ideally chartered, assesses whether the roof can safely take the panels and, if not, what strengthening is needed. Expect a recommendation for one when:

1. the ridge, slopes or walls show signs of movement and the cause is not obvious;
2. rafters are undersized, damaged or have been cut, and the surveyor cannot say they are adequate;
3. earlier structural repairs are visible but there is no paperwork explaining them;
4. the roof has been altered by a conversion, dormer or removed supports;
5. the building is one of Northwich’s timber-framed or listed buildings (see liftable timber buildings);
6. the installer is uneasy and says so, which is a mark of a careful firm, not an awkward one.

The engineer’s report should name the timbers inspected, give a view on whether they can carry the array, and describe any strengthening needed. If strengthening is needed, it is done before the panels go on. Ask whether the installer will work to the report and keep a copy with your house documents; a future buyer may want to see it.

What a written survey should give you

• Confirmation that the roof space was inspected, by whom and when.
• Rafter size, spacing and condition where the array will go, with photographs.
• The roof covering type and condition, including battens and underlay.
• Any signs of past movement or old repairs, and what the surveyor made of them.
• The fixing method and bracket layout, and how it allows for wind.
• A clear statement on whether a structural engineer is recommended, and why.

Quotes that skip these points are not necessarily dishonest, but they leave the risk with you. Two quotes for the same roof that differ sharply in price often differ in exactly this part of the work. Choosing an installer lists the other checks worth making, and the cost guide explains how structural work and slate roofs show up on a quote. If you are buying a house that already has panels, the same roof questions apply in reverse: see buying a Northwich home with solar.