Conservation Rooflight Installation

What a conservation rooflight actually is

Strip away the marketing and it is a roof window with three deliberate constraints. The frame is narrow, usually a steel or cast aluminium section rather than a thick timber or plastic surround. The glass is divided by a single vertical bar. And the whole assembly is designed to sit low in the roof plane, so that from the ground it reads as a dark rectangle in the slate rather than as an object bolted onto it.

Everything else is ordinary rooflight engineering. There is still a sealed glazing unit, still a lead or aluminium weathering detail, still a trimmed opening in the rafters underneath. The conservation part is a set of decisions about proportion and profile, made because a planning department or a listed building officer will look at the result and form a view.

The Victorian original, and what it was for

The units these copy were not architectural features. They were cast iron lights fitted to bring daylight into lofts, workshops, stables and servants’ rooms, in an era when glass came in modest sheet sizes and cast iron was the cheap way to make a weatherproof frame.

That history explains the shape. The frames were small because the roof structure was hand cut at close and irregular centres and because a large sheet of glass was expensive. The central bar existed because it let two smaller panes do the work of one large one. The frame was flat because iron could be cast thin and because nobody wanted a projecting object on a roof they still had to walk on.

Modern conservation units reproduce those constraints on purpose. The result looks right on an 1870s terrace in Moulsham Street for the same reason the original did: the proportions came from the same limits.

Steel, aluminium and the modern conservation frame

Two constructions dominate. Pressed or rolled steel frames, hot dip galvanised and then powder coated, give the thinnest sightline and the closest match to the original casting. Cast or extruded aluminium frames are lighter, will not rust if the coating is damaged, and are usually a millimetre or two thicker in the visible section.

Steel is the purist choice and it is heavier: a 550 by 980 unit in steel is a genuine two-person lift onto a roof. Aluminium is easier to handle and more forgiving in a coastal position, which matters on the Blackwater and around Maldon where salt-laden air reaches inland further than people expect.

Both are supplied with a thermal break between the outer and inner sections. Without one, the inner frame runs cold in January and collects condensation along the bottom edge, which then sits on the plaster of the reveal.

The central glazing bar, and why it survives

The bar is the single detail that distinguishes a conservation unit at fifty metres. It runs vertically down the centre of the glass, dividing the light into two tall rectangles, and it is the reason the unit reads as historic rather than modern.

On most current products the bar is not structural. The sealed unit is a single pane, and the bar is a bonded external section with a matching internal one, so the insulating cavity runs unbroken behind it. That is a better arrangement than a genuine divided unit, because two small sealed units in one frame double the perimeter seal length and therefore double the places that can eventually fail.

Some conservation officers ask for a bar of a particular width. Anything from 25 to 40mm is common, and the narrower the bar, the more the unit reads as Victorian rather than Edwardian.

Sitting flush, and the projection that gets measured

The defining performance figure is how far the frame stands above the surrounding roof covering. A standard roof window typically sits 60 to 90mm proud of a slate. A conservation unit is designed to sit at roughly 8 to 30mm, depending on the model and the covering.

That is achieved by recessing the frame into the rafters rather than fixing it to the top of them. The unit is dropped so its outer face is close to the plane of the slate, and the flashing then carries water over the top edge and away down each side.

Recessing has an internal consequence worth planning for. Dropping the frame between the rafters rather than onto them means the head of the unit sits lower in the roof build-up, so the internal reveal is deeper and the plaster line lands closer to the glass. On a 100mm rafter with insulation between and a plasterboard finish, that reveal can be 180mm or more, which is exactly why splaying it outwards towards the room is worth the extra plastering.

Article 4 directions across the Essex market towns

Permitted development normally allows a rooflight without a planning application, provided it projects no more than 150mm beyond the roof plane and sits no higher than the highest part of the roof. A conservation unit clears both conditions comfortably.

The complication is Article 4. A local authority can issue a direction removing permitted development rights across a defined area, and Essex authorities have used that power widely in historic centres. Uttlesford, Braintree, Colchester and Maldon all carry Article 4 directions over parts of their conservation areas. Where one applies to roof alterations, the rooflight needs planning permission even though it would have been permitted three streets away.

Checking takes one search of the authority’s map and it is done before anything is ordered, because the answer changes both the programme and, occasionally, the product.

Listed building consent, and how it differs from planning

These are two separate consents and confusing them causes real problems. Planning permission concerns the effect of development on the area. Listed building consent concerns works that affect the special architectural or historic interest of the building itself, inside and out.

A rooflight in a listed roof will almost always need listed building consent, even where the planning position is clear. There is no application fee, but the assessment is more searching: the officer is considering the historic fabric being cut, not just the appearance of the finished elevation.

Consent is not a formality and it is also not a closed door. Rear and secondary slopes are routinely approved. What causes refusals is a large modern unit on a principal elevation, or an application with no drawing showing the relationship between frame, slate course and ridge.

What a conservation officer is actually looking at

Four things, in roughly this order. Which slope the unit sits on, and whether that slope is visible from the public realm. How large the unit is relative to the roof and to the openings already there. How far it projects. And what the frame is made of and finished in.

Size is where most schemes are trimmed. A 780 by 1400 unit on a modest cottage roof will be asked to come down, because it competes with the chimney and the dormers for attention. Two units at 550 by 980, spaced to line up with the windows below, usually go through where one large one does not.

Alignment matters more than people expect. Setting the rooflight on the same vertical centre line as the window beneath it makes the elevation read as designed rather than as altered.

Which slope gets the rooflight

The default advice, and the one Historic England repeats, is to put new rooflights on the least visible slope. In practice that means rear elevations, inner slopes of an L-plan, and roofs facing a private garden rather than a lane.

This is not always where the light is needed, and that tension is the real design problem in heritage work. A north-facing rear slope gives even, shadowless light with almost no solar gain, which suits a studio or a landing very well. A south-facing rear slope over a bedroom in Saffron Walden will need solar control glass, which sits alongside the wider question of rooflight thermal performance.

Where the only sensible slope faces the street, the honest answer is a smaller unit set low in the roof, close to the eaves, where the pitch hides it from anyone standing at the boundary.

Welsh slate, and how the frame lands in it

Slate is the easiest covering to fit a conservation unit into, which is convenient because it is what most Victorian Essex terraces are roofed with. It is thin, flat and laid in regular courses, so the frame meets a predictable surface on all four sides.

The detail is straightforward. Slates are stripped back beyond the opening, the frame is fixed to the trimmed rafters at the correct height, the head flashing goes under the course above, side soakers are interleaved with the slates either side, and the apron dresses down over the course below. Every lap is a minimum of 100mm and each one carries water onto the next.

Slate roofs also have a working minimum pitch of around 20 to 22.5 degrees for a standard headlap, and most conservation rooflights are rated from 15 or 20 degrees, so the covering usually sets the limit before the unit does.

Essex peg tiles, and the harder detail

Hand-made clay plain tiles, pegged or nibbed, are the characteristic covering across the north Essex villages and much of the older stock around Braintree, Coggeshall and Great Bardfield. They are a harder proposition than slate for one reason: they are not flat.

A hand-made peg tile has a camber front to back and a slight variation in width tile to tile. Laid double lap, the surface has texture, and a rigid metal apron dressed over it will bridge the high points and leave voids. The answer is lead, dressed by hand into the tile profile with a bossing stick so that it follows the camber rather than spanning it.

Old peg tiles are also brittle. Stripping back around an opening on a hundred and fifty year old roof reliably produces breakages, so salvaged tiles of the same run are set aside before work starts.

Pantiles towards the Suffolk border

Single lap pantiles appear in the north-east of the county, on the Suffolk fringe around Sudbury and out towards Bures and Nayland, and occasionally on farm buildings across the Dengie.

The pantile is the most demanding covering for a flush rooflight, because its S-profile creates a deep trough and roll across the width of every tile. A flashing has to weather both the flat of the trough and the crown of the roll, and a pressed kit intended for a shallow interlocking tile will not reach.

The working method is a lead apron dressed into every trough, with the lead taken up the roll and bossed to shape. It is slow, skilled work and it is the reason a pantile installation carries more hours than the same unit in slate.

Lead flashings, and why they are dressed by hand

Conservation rooflights are usually supplied with a lead flashing kit rather than a pressed aluminium one, and the difference is not cosmetic. Lead is malleable at ambient temperature, so it can be worked into a tile profile and it stays where it is put.

Code 4 lead is the normal specification for soakers and aprons at this scale, with code 5 where a long run or an exposed position asks for more weight. Individual pieces are kept under 1.5m in length, because lead expands and contracts with temperature and a long unrestrained piece will eventually split at its fixing.

Fresh lead is also protected with patination oil on the day it is dressed. Without it, rainwater carries lead carbonate down the slates and leaves white streaks across the roof below, which on a listed elevation is a conversation nobody wants.

Sizing to the slate course rather than to the room

On a modern roof the rooflight size is chosen for the room and the covering is cut to suit. On a heritage roof the logic runs the other way, and it is worth understanding why.

Slates and plain tiles are laid to a set gauge, which is the vertical spacing between courses. Welsh slate at a 500 by 250 size with a 75mm headlap gives a gauge of around 212mm. If the rooflight height lands mid-course, the fitter has to cut slates horizontally along the head and the apron, which weakens them and shows.

Choosing a unit whose external height works out close to a whole number of gauges means the flashings land on natural course lines. It is a detail that costs nothing at order and shows for the life of the roof.

The standard conservation sizes, and the module behind them

The trade has settled on a group of sizes that recur across manufacturers, and they are narrow by modern standards because they are meant to fit between rafters rather than through them.

Bespoke sizes are available from most makers and add several weeks to the lead time. Where the design wants more glass, two standard units in a considered rhythm usually beat one special, both in appearance and in programme.

Rafters in an old roof, and what the spacing allows

A Victorian cut roof has rafters that were sized by eye and spaced by the carpenter’s rule of thumb. Common sections are 100 by 50 or 100 by 38, at anything from 350 to 450mm centres, and the spacing is rarely consistent across a whole slope.

That has a practical consequence. The opening cannot be set out from a drawing. It is set out from the actual rafters, after the ceiling or the slates have been opened up and the true positions measured. A unit chosen on the assumption of 400mm centres will occasionally find 365mm on one side and 430mm on the other.

Where a rafter is cut, the two either side are doubled and trimmers are fixed above and below the opening with joist hangers or traditional housed joints. On an old roof the timber is dry, hard and often shrunk, so pre-drilling is standard.

Roof planes that are not planes

The timber frame and pargeted buildings around Thaxted, Great Bardfield and Finchingfield have roofs that have moved for three or four hundred years. Ridges dip, rafters bow, and the surface a rooflight has to sit in is a gentle curve rather than a flat plane.

A steel or aluminium frame is rigid and will not follow that. Forcing it into line by packing one corner puts the frame out of square, which then binds the opening sash and loads one corner of the sealed unit.

The correct approach is to level the trimmed opening itself. The trimmers and doubled rafters are set to a true plane using firring pieces cut to the measured deflection, and the frame then lands on flat timber. The surrounding slates are eased into the transition over two or three courses so the change is not visible from the ground.

Slim double glazed units, and what fits inside the frame

A slim frame constrains the glass. A standard roof window takes a 24 to 28mm sealed unit. A conservation frame is usually limited to somewhere between 14 and 24mm depending on the manufacturer, and everything about the thermal specification follows from that.

A typical build-up is a 4mm toughened outer, a cavity of 8 to 14mm, and a 4mm laminated or low-emissivity inner. The outer pane is toughened because a rooflight in direct sun develops thermal stress across the pane where the edge stays shaded by the frame. The inner is often laminated so it stays in place overhead if anything strikes it.

Overall U-values from that build-up land around 1.3 to 1.6 W/m²K in a good product, which is respectable given the constraint but is not what a deep-frame unit achieves.

Krypton, argon and narrow cavities

Cavity gas is where a slim unit claws performance back, and the physics is specific enough to be worth stating.

Argon reaches its best insulating performance at a cavity of about 16mm. Squeeze it to 8mm and its contribution falls away, because the gap becomes narrow enough that conduction across it dominates. Krypton has a lower thermal conductivity and a smaller molecule, and it performs at its best at around 8 to 10mm, which is precisely the cavity a conservation frame allows.

The practical upshot is that a slim unit filled with krypton can match a wider argon unit, at a meaningful premium on the glass. On a north-facing landing rooflight that premium is hard to justify. On a large unit over a bedroom that is heated every evening, it usually pays for itself in comfort long before it pays for itself in fuel.

Part L in a protected building

Fitting or replacing a rooflight is notifiable work under Building Regulations in England, and Part L sets a maximum U-value for the replacement unit. Rooflights are assessed in the vertical position, which is a technicality worth knowing because it is the number that appears on the certificate.

Regulation 21 allows a relaxation where the building is listed, is in a conservation area, or is of architectural or historic interest, and where full compliance would unacceptably alter its character or appearance. That is the mechanism under which a slim-framed unit with a modest U-value is accepted in a heritage roof.

The relaxation is agreed with Building Control rather than assumed. Sunspire submits the notification and the product data as part of the job, so the file that ends up with the property is complete when a buyer’s solicitor eventually asks for it.

Heritage glass, and when it is worth specifying

Ordinary float glass is optically perfect and slightly wrong for a building of this age. Original glazing was cylinder blown or drawn, and it carries faint waves and a scatter of small bubbles that catch light unevenly across the pane.

Restoration glass reproduces that. It is available as a drawn or lightly textured outer leaf, laminated for overhead use, and it changes the character of the reflection when you look at the roof from the garden. On a Grade II listed cottage in Dedham or Wivenhoe it is often the detail that settles an application.

It is not universally right. On a Victorian workshop conversion or a rear slope with no public view, plain toughened glass with a good low-emissivity coating is the better use of the budget, and the difference is invisible from anywhere anyone stands.

Frame finishes, and the black that is expected

The default is a black or very dark grey finish, and there is a reason it is nearly universal. A dark frame recedes against slate and against the shadow of an opening, so the eye reads the glass rather than the surround.

Powder coating to RAL 9005 jet black is the common specification, usually in a matt or satin gloss level rather than a high gloss, because a shiny frame catches sun and announces itself. Some officers ask for a specific finish, and a few prefer the softer look of a semi-matt anthracite such as RAL 7016 on a tile roof rather than slate.

Internally the frame can be a different colour, since it is not part of the historic elevation. White or a light grey inner section reflects daylight into the reveal and makes a small unit feel larger from below.

Fixed, top hung and centre pivot in a conservation frame

Three configurations cover almost every job. A fixed light is the cleanest and the cheapest, and it suits a landing, a stairwell or a high-level unit that nobody could reach anyway. A top hung sash opens outwards from the head, which keeps the opening clear and lets a person lean out, and it is what an escape unit uses.

A centre pivot rotates about its middle. It takes less effort to operate at height and the outer face can be turned inwards for cleaning, which matters when the roof is three storeys up in a terrace with no rear access.

Escape duty has its own rules. Where a loft room needs a means of escape, the opening must give a clear area of at least 0.33m² with a minimum clear width and height of 450mm, and the bottom of the opening must sit between 600 and 1100mm above the floor.

Underlay, torching and what sits behind the slates

Open up a Victorian roof and there is frequently no underlay at all. Slates were nailed direct to battens, and the underside was sometimes torched, which means bedded in a haired lime mortar to close the gaps against wind-driven rain and snow.

That has two consequences at a rooflight opening. The old torching around the new frame is disturbed and needs making good with a compatible lime mix rather than a cement one, because cement is harder than the surrounding fabric and will crack the slate tails as the roof moves.

The second is ventilation. Introducing a modern breather membrane locally around a rooflight, in a roof that has always ventilated through its own gaps, can create a cold spot where moisture then collects. Where an old roof is opened, the detail is designed for how that specific roof breathes, not to a standard drawing.

Installation on an old roof, and what the days look like

A single conservation rooflight into an existing slate slope is generally one to two days. Two units on the same slope run to two or three, because the setting out has to hold across both. A unit in hand-made peg tile or pantile adds time for the lead work.

The order is consistent. Protect the room below and the floor, strip the covering back beyond the opening and set the salvaged slates or tiles aside, open the ceiling if the reveal is being formed, cut and trim the rafters, set the frame to a true plane and to the slate gauge, dress the lead, re-lay the covering, then finish the reveal internally.

The roof is closed and weathertight at the end of every working day, whatever the forecast says. On a listed building that discipline is not optional, because the fabric underneath has no tolerance for a wet night.

Where to start on a protected building

Two photographs and one fact will get you a sensible range by email: the roof slope from outside, the ceiling from inside, and whether the building is listed or in a conservation area. If you know the covering, say so, because slate, peg tile and pantile carry genuinely different hours.

After that the sequence is a survey, then a drawing showing the unit against the slate courses and the openings below, then the consent application if one is needed, then the order. Lead times on conservation units run longer than on a standard VELUX roof window, so the order is placed once consent is in hand rather than in hope of it.

Where an existing unit has misted between the panes or the frame has gone brittle, a like-for-like replacement into the same opening is often quicker than the first installation was. Sunspire fits new conservation rooflights and replaces units that have reached the end of their working life, across Chelmsford and thirty miles of Essex around it, and the workmanship carries a ten year guarantee. If you want the cost picture first, the costs page sets out what moves the figure, and the areas page covers the ground.