Accoya
Accoya is a brand of acetylated softwood, a timber modified so it absorbs far less water and resists rot better than ordinary softwood. It moves less with changes in moisture, which helps paint finishes and joints last longer on timber roof lanterns. More
Anodised finish
An anodised finish is a hard, metallic surface made by thickening the natural oxide layer on aluminium through an electrochemical process. It is harder to touch up than paint, so damaged anodised frames are usually cleaned and resprayed rather than patched. More
Argon fill
Argon fill is an inert gas used instead of air in the cavity of a sealed glass unit because it conducts heat less readily, which improves the unit's U-value. A small amount escapes over the life of the unit, and most of it is lost once the edge seal fails. More
Bottom gasket
A bottom gasket is the rubber seal on top of a glazing bar that the glass sits on, cushioning the unit and sealing its inner face. Gaskets that have shrunk, hardened or slipped out of their groove are a common cause of drips along the bars. More
Building control
Building control is the service that checks building work meets the Building Regulations, provided in England and Wales by the local authority or a registered building control approver. In Northern Ireland it is run by council Building Control, and in Scotland the equivalent role sits with the local authority verifier that issues building warrants. More
Building warrant (Scotland)
A building warrant is the approval a Scottish local authority issues before most building work covered by the Building (Scotland) Regulations 2004 can start. Some work listed in schedule 3 of the regulations does not need a warrant, but it must still meet the standards in the Technical Handbooks. More
Chain actuator
A chain actuator is the electric motor that opens and closes a roof lantern vent by pushing out and drawing back a rigid folding chain. Faults usually come from the motor, the chain, the wiring or the control switch, and many systems link to a rain sensor that closes the vent when it rains. More
Cold bridge
A cold bridge is a part of the building fabric that lets heat escape much faster than the area around it, such as an uninsulated timber kerb or a frame without a thermal break. Its inner surface stays colder, so condensation and mould tend to form there first. More
Competent person scheme
A competent person scheme is a government-authorised scheme in England and Wales that lets registered businesses self-certify that certain types of building work, such as some glazing replacement, meet the Building Regulations without a separate building control application. Scotland and Northern Ireland have their own arrangements, and like-for-like repairs are usually outside the regulations altogether. More
Condensation
Condensation is water that forms when warm, moist indoor air touches a surface colder than its dew point, such as the inner pane or frame of a roof lantern on a cold day. It can run down the bars and drip, so it is often mistaken for a leak. More
Desiccant
Desiccant is the drying material, usually small granules, held inside the spacer bar of a sealed unit to absorb moisture in the cavity. Once the edge seal fails and the desiccant becomes saturated, moisture shows as misting between the panes. More
Dew point
The dew point is the temperature at which air holds as much water vapour as it can, so any surface colder than it collects condensation. Warmer, more humid rooms such as kitchens have a higher dew point, which is why lanterns over them mist up more easily. More
Drip edge
A drip edge is a projecting lip or groove that makes water fall clear of a surface instead of running back underneath it. On a roof lantern it is found on eaves trims and cladding, and a missing or damaged drip can let water track onto the kerb. More
Eaves beam
The eaves beam is the perimeter frame at the base of a roof lantern that sits on the upstand and carries the lower ends of the glazing bars. It usually includes the gutter or drainage channel, so split joints and blockages here are a frequent source of leaks. More
Eaves gutter (drainage channel)
The eaves gutter is the channel built into the eaves beam that collects rain running off the glass and lets it out through weep slots or outlets onto the flat roof. When leaves, moss or silt block it, water can back up over the inner edge and drip into the room. More
End stop (glass stop)
An end stop, or glass stop, is the bracket or clip at the lower end of a glazing bar that stops the glass unit sliding down the slope. If it breaks, is left out or works loose, the pane can creep down and open a gap at the top. More
Engineered timber
Engineered timber is wood made by bonding several layers or laminations together with their grain arranged to resist twisting and warping. It is more stable than solid timber of the same size, which is why it is used for many timber lantern frames and orangery roofs. More
EPDM
EPDM (ethylene propylene diene monomer) is a synthetic rubber roofing membrane laid in large sheets on flat roofs and turned up the sides of the lantern upstand. Leaks often appear where it has shrunk, come unstuck or been poorly finished at the corners of the kerb. More
Flashing
Flashing is a strip of lead, aluminium or membrane that covers the joint between two surfaces so water runs over it rather than into it. On a roof lantern it usually seals the junction between the flat roof covering and the upstand or eaves beam. More
Flat rooflight
A flat rooflight is a low-profile glazed unit, usually a single sealed glass panel on an upstand, set into a flat roof with a slight fall to shed water. It suffers many of the same faults as a roof lantern, including failed seals, misted glass and leaking upstands. More
g-value
The g-value is the share of the sun's heat that passes through glazing, given as a number between 0 and 1. A lower g-value means less solar heat gain, which is why solar control glass is specified for lanterns over rooms that overheat. More
Glazing bar (jack rafter)
A glazing bar is one of the sloping bars that run from the ridge or a hip down to the eaves and support the edges of the glass. Jack rafters are the shorter glazing bars on the hipped ends of a lantern, running from a hip bar down to the eaves. More
Glazing tape
Glazing tape is a double-sided adhesive sealing tape used to bed glass onto a frame or to seal laps and joints. Butyl tape is a common type that stays soft and tacky rather than curing, so it keeps sealing as parts move with temperature. More
GRP
GRP (glass reinforced plastic) is a fibreglass roof covering made by laying resin and glass matting over a deck and letting it cure into a hard, seamless finish. On roofs with lanterns, cracks often appear at the trims and corners where the GRP meets the upstand. More
Heat-soak test
A heat-soak test is a factory process in which toughened glass is held at a high temperature for a set time so that panes containing nickel sulphide inclusions break before they leave the factory. It reduces, but does not remove, the risk of toughened glass breaking spontaneously once installed. More
Hip bar
A hip bar is the sloping bar that runs from the end of the ridge down to a corner of the eaves, where two roof slopes meet. Hip caps and their seals cover it, and lifted or split hip caps are a common route for rain into the lantern. More
Insulated kerb
An insulated kerb is a factory-made upstand with an insulated core that raises the lantern above the flat roof. Compared with a site-built timber kerb, it reduces cold bridging at the base of the lantern and with it the risk of condensation. More
Laminated glass
Laminated glass is two or more panes bonded with a plastic interlayer, so if it breaks the pieces stay stuck to the interlayer instead of falling. Industry guidance recommends it as the inner pane of overhead glazing such as roof lanterns, and British Standards already call for a laminated inner pane where a rooflight is more than 5 m above floor level or over water. More
Lead flashing
Lead flashing is sheet lead dressed over a junction, for example where a lantern's upstand meets a wall or roof covering, to shed water. It lasts well when correctly sized and fixed but can split if laid in lengths that are too long. More
Light transmittance
Light transmittance is the percentage of visible daylight that passes through a glass unit. Solar control coatings usually lower it a little as well as cutting heat, so both figures are worth comparing when choosing replacement glass. More
Listed building consent
Listed building consent is permission from the local planning authority for works that affect the character of a listed building, which can include changes to rooflights and lanterns. It is separate from the Building Regulations, so check with the council's conservation officer before changing materials, glass or design. More
Low-E coating
A Low-E (low-emissivity) coating is a microscopically thin metal oxide layer on one pane of a sealed unit that reflects heat back into the room. It improves the unit's U-value and keeps the inner pane warmer, which helps reduce condensation. More
Low-modulus sealant
A low-modulus sealant is a soft, highly elastic sealant that stretches and compresses with joint movement without putting much stress on the materials either side. It suits joints that move a lot, such as the junction between a lantern frame and its kerb. More
Make-safe
A make-safe is temporary work to stop more water getting in, or to deal with a hazard such as cracked glass, until a permanent repair can be carried out. Depending on the fault, it might mean sheeting, temporary sealing, taping or boarding. More
Opening vent
An opening vent is a hinged section of a roof lantern that opens to let warm air and moisture escape, worked by hand, a winding pole or an electric actuator. Powered vents often have a rain sensor that closes them automatically when it starts to rain. More
Orangery
An orangery is an extension with solid walls or brick piers and a flat roof perimeter around a central glazed roof lantern. Because the lantern is often large and made of timber or aluminium, orangery roofs need the same seal, gutter and glass care as any other lantern. More
Powder coating
Powder coating is a factory finish in which dry paint powder is sprayed onto aluminium with an electrostatic charge and baked hard in an oven. It cannot be redone on site, so damaged powder-coated frames are touched up or resprayed with a matched liquid paint. More
RAL colour
RAL is a standard reference system for paint colours, such as RAL 7016 anthracite grey or RAL 9016 traffic white, used to specify and match frame finishes. Knowing the RAL number of your lantern makes matching cappings, trims and resprays more straightforward. More
Resin repair
A resin repair removes decayed timber and fills the cavity with a two-part resin filler that is shaped, sanded and painted to match. It suits small, localised rot on timber lanterns where the surrounding wood is still sound. More
Ridge
The ridge is the horizontal beam at the top of a roof lantern where the glazing bars from each side meet. It is covered by a ridge cap, and the seals along the ridge are a common source of leaks on older lanterns. More
Ridge end cap
A ridge end cap is the moulded cover that closes off each end of the ridge where it meets the hip bars. If it cracks, loosens or loses its seal, rain can get in at the top of the lantern and run down inside the bars. More
Roof lantern
A roof lantern is a raised, pitched glazed roof, usually set on a flat roof, made of glazing bars meeting at a ridge and sitting on an upstand or kerb. It brings daylight into the room below and is a common feature of kitchen extensions and orangeries. More
Sealed unit (IGU)
A sealed unit, also called an insulated glass unit or IGU, is two or three panes held apart by a spacer bar and sealed around the edge, with dry air or gas in the cavity. When the seal fails, misting appears between the panes and the unit needs replacing because the inside cannot be cleaned. More
Self-cleaning glass
Self-cleaning glass has a coating on its outer face that breaks down organic dirt in daylight and lets rain wash it away in a sheet rather than droplets. It cuts down how often the glass needs cleaning but does not remove the need entirely, especially on shallow pitches. More
Setting block
A setting block is a small, hard plastic block placed under the lower edge of a glass unit to carry its weight and keep it off the frame. Missing or misplaced blocks can let the glass slip, sit on metal or put stress on the edge seal. More
Silicone sealant
Silicone sealant is a flexible sealant that cures to a rubbery seal and is used to weatherproof joints between glass, frames and trims. A neutral-cure silicone is normally used near sealed units, because some other types can damage their edge seals. More
Single-ply membrane
A single-ply membrane is a flat roof covering laid as one layer of synthetic sheet, such as PVC, TPO or EPDM, with welded or bonded seams. It is dressed up the lantern upstand, and poorly sealed corners at this junction often cause leaks. More
Skylight
Skylight is a general term for any glazed opening in a roof that lets daylight in, including roof lanterns, flat rooflights and roof windows. Repair approaches vary with the type, so it helps to send photos of the outside as well as the inside. More
Solar control glass
Solar control glass has a coating or tint that reduces how much of the sun's heat passes through, giving it a lower g-value. Fitting it in place of standard units is a common way to reduce overheating under a roof lantern without replacing the frame. More
Spacer bar
A spacer bar is the strip around the edge of a sealed unit that holds the panes apart and contains desiccant to keep the cavity dry. Traditional spacers are aluminium, while warm edge spacers use materials that conduct less heat. More
Splice repair
A splice repair cuts out a rotten section of a timber member and joins in a new piece of matching profile, glued and fixed into sound wood. It saves replacing a whole bar, beam or sill when the decay is confined to one area. More
Thermal break
A thermal break is a strip of low-conductivity material, usually polyamide, built into an aluminium profile to separate the inner and outer metal. It reduces heat loss through the frame and keeps the inner face warmer, which helps limit condensation. More
Tie bar
A tie bar is a horizontal rod or bar fixed across a roof lantern or orangery roof to stop the eaves frame spreading outwards under the weight of the roof. Loose or missing tie bars can let the frame move, which strains the seals and glass.
Top capping
A top capping is the external cover strip clipped or screwed over a glazing bar to hold the glass down and shed water. Cappings that have lifted, cracked or lost their wing gaskets are among the most common causes of lantern leaks. More
Torch-on felt
Torch-on felt is a modified bitumen roofing membrane laid in layers and bonded by heating its underside with a gas torch. It is turned up the lantern upstand, and because it uses a naked flame, hot works near timber kerbs need careful fire precautions. More
Toughened glass
Toughened glass is heat-treated glass that is several times stronger than ordinary glass and breaks into small, blunt pieces. In a roof lantern it is commonly used as the outer pane, with laminated glass recommended for the inner pane. More
Trickle vent
A trickle vent is a small, closable vent that provides background ventilation while the main windows or vents are shut. Some roof lanterns have them in the ridge or eaves to help clear moist air and reduce condensation. More
U-value
A U-value measures how much heat passes through a building element, in watts per square metre per degree of temperature difference (W/m²K). The lower the figure, the better the insulation, and the Building Regulations set limits for new and replacement rooflights. More
Upstand
An upstand, also called a kerb, is the raised frame or wall around the roof opening that lifts the lantern above the flat roof surface. It needs to be high enough for the roof covering to be turned up and sealed well clear of standing water, and rot or poor detailing here causes many leaks. More
uPVC
uPVC (unplasticised polyvinyl chloride) is the rigid plastic used for many lantern cappings, trims and frames, often over an aluminium structure. It needs little maintenance, but parts can become brittle, discoloured or warped with age and sun. More
Warm edge spacer
A warm edge spacer is a spacer bar made from a low-conductivity material, such as a plastic composite or stainless steel, rather than aluminium. It keeps the edges of the glass warmer, reducing heat loss and the line of condensation that often forms around the edge of the pane. More
Water test
A water test is a controlled wetting of one part of a lantern at a time, usually working upwards from the lowest point, while someone watches inside for water. It helps pinpoint where a leak gets in so the right part is repaired. More
Weep slot
A weep slot is a small drainage opening in a gutter, channel or frame chamber that lets collected water escape outside. Blocked weep slots cause water to build up inside the frame until it overflows into the room. More
Wing gasket
A wing gasket is the flexible rubber fin along the edge of a top capping that presses down on the glass to make a weathertight seal. When wing gaskets shrink, curl or pull away, rain can run under the capping and into the lantern. More