Guide · Skylight & Rooflight Installation & Replacement Chelmsford
Replacing an old dome with flush glazing
There is a particular moment in a lot of Essex houses when somebody stands in a back bedroom or a rear extension, looks up at a yellowed plastic bubble, and realises it has been there since before they moved in. Domes went onto flat roofs across the county in enormous numbers from the late nineteen sixties onwards, and the ones still in service are all now well past the age they were built for.
Swapping one for a flush glazed unit is one of the most satisfying jobs in this trade, because the change in the room below is dramatic and the work is usually a day. It is also not a straight exchange, and this page sets out exactly what changes between the two. The wider subject sits on the skylight replacement page.
What a dome was actually doing up there
A dome is a formed plastic shell on a frame, and its shape is not decorative. Vacuum forming a sheet of acrylic or polycarbonate over a mould produces a curved skin that is stiff enough to span an opening without any framework across it, and the curve sheds water and debris by geometry alone.
That was a sensible piece of engineering for its time. Flat glass units of any size were expensive, heavy and thermally poor, and a plastic dome was light, cheap, quick to fit and needed no structural thought at all. What nobody designed for was the material spending fifty English summers under ultraviolet light.
The three kinds still found on roofs here
Almost every dome taken off a roof in this part of Essex falls into one of three families, and each behaves differently on the day.
Single skin acrylic, the earliest and thinnest, is usually clear or lightly tinted and by now has yellowed and gone brittle enough to crack under a thumb. Twin skin acrylic or polycarbonate, from the later seventies onwards, has an air gap between two shells and is usually opal white, and it typically crazes into a fine network of surface cracks rather than yellowing. Triple skin polycarbonate, from the nineties, holds up better and is the one occasionally still worth leaving alone for another few years. All three are usually sitting on a plastic or aluminium kerb frame that was bedded on mastic and screwed through the roof covering.
A dome that gives underfoot or sounds hollow when tapped has gone brittle and will not take any weight at all.
Why this is not an exchange of one lid for another
People reasonably assume that because both products cover the same hole, one comes off and the other goes on. Four things stop that being true.
The glass unit is several times heavier. It sits on a different sort of upstand, at a different height, with a different fixing arrangement. It sheds water in a different way, so what happens on the roof surface around it matters more. And the size grid it comes from is metric and modern, while the hole was formed to whatever the dome supplier of 1978 happened to sell. Any one of those on its own would make the job a proper piece of work. Together they make the survey the important part.
Kerb frames of the dome era, and how tall they are not
The upstand under an old dome is often a plastic or aluminium frame that came with the unit, sitting perhaps seventy five or a hundred millimetres above the deck, with the felt dressed up it and turned over the top.
That was accepted practice at the time, and it worked because the dome overhung the frame considerably and water was thrown clear. A flush unit does not overhang in the same way and does not need to, but it does need the upstand to hold water back on its own account. Current practice puts the top of the kerb at a hundred and fifty millimetres above the finished roof surface, derived from the depth water can pond combined with how far wind driven rain travels up a vertical face. A great many dome kerbs are half of that.
The upstand a flush unit expects to land on
The replacement usually involves building a proper timber kerb, and it is the half day that decides the next twenty years.
A kerb is built off the roof structure, not off the covering, in treated softwood, insulated on the outside face so that it is not a cold ring around the glass. The top is checked level and square, and out of wind, because a flush unit lands on a flange and a twisted kerb means either packing or a frame under stress. The roof covering is then dressed up the outside face and terminated under the unit. Whether an existing kerb can be kept at all is worth reading up on separately at reusing an existing kerb, because it is the single biggest variable in what the day costs.
Weight, and what the deck now carries
A twin skin acrylic dome of a metre square weighs very little. A double glazed flush unit of the same size is a different proposition, and a triple glazed one more so again.
In practice this is not usually a structural problem, because a flat roof deck is designed for imposed loads far greater than a glazed unit imposes. What it does change is handling. Anything much above a metre square is a two person lift onto a roof, and above about one and a half metres square it becomes a mechanical lift or a genuinely awkward morning. That is a programme and access question, and it belongs in the survey rather than in a surprise on the day. Where the deck itself is soft or the joists have taken water around the old opening, that is found by probing at survey.
Water on a flat pane, and how it gets off
The one genuine advantage a dome had is that its shape sheds water and dirt without being asked. A flat pane does not, and modern flush units answer that in one of two ways.
Some are built with a fall inside the frame itself, so the glass sits at around three to five degrees while the frame remains level on a level kerb. Others are level units laid on a kerb built with a fall, which means the kerb is tapered during construction. The first is neater and is the usual answer on a straightforward replacement. The second is used where the whole roof already falls in a known direction and the glass should follow it. What does not work is a level unit on a level kerb on a roof with no fall, because dirt settles on the glass, stays there, and the room below goes grey within two years.
The fall of the roof itself, and where the water goes next
A flat roof is laid to a fall, conventionally a minimum of one in eighty as designed, to allow for deflection and construction tolerance.
Around a rooflight that fall does two jobs. It moves water away from the upstand rather than letting it stand against it, and it keeps the area upslope of the kerb from becoming a shelf where water and leaves collect. Where a survey finds standing water beside the old dome kerb after rain, the sensible order of work is to sort the fall before the new glass goes in, because a new unit set into a roof that ponds simply moves the conversation to a later winter.
How much it stands above the roof, and how it reads
From the garden the visual change is larger than people expect, and it is the reason a great many of these jobs get done.
A dome typically stands two hundred to three hundred millimetres proud, in white or opal plastic, and reads as equipment. A flush unit sits low on its kerb with a slim frame in a dark grey or black finish, and from a normal viewing angle it reads as a dark rectangle or as nothing at all. Where the flat roof is overlooked from an upstairs window, that difference matters. Where the extension is single storey behind a house, nobody sees either, and the case rests entirely on what happens underneath.
The daylight, once the plastic has gone
This is the part people notice within an hour of the job finishing.
Opal or twin skin plastic diffuses light. It spreads it evenly, which was the intention, but it also removes any view and gives the light a flat, slightly grey quality. A fresh dome might transmit around three quarters of the light landing on it. A forty year old one, crazed and yellowed, is frequently down near half that, and the decline is slow enough that nobody registers it happening. Clear glass transmits around seventy eight per cent and, more to the point, gives a view of moving sky. The room stops being lit and starts being connected to the weather.
Winter warmth and summer gain, both at once
The thermal jump on a dome swap is the largest available anywhere in this trade, and it goes both ways.
| Unit | Approximate U-value | Solar behaviour | What the room feels |
|---|---|---|---|
| Single skin acrylic dome | 5.0 to 5.5 | No control, high gain | Cold in winter, hot in summer |
| Twin skin dome | Around 3.0 | Diffused but uncontrolled | Better in winter, still hot in August |
| Current flush glazed unit | 1.0 to 1.4 | Solar control available | Warm inner surface, managed gain |
Going from five to a little over one on a square metre and a half of roof is a change that is felt standing under it in January. The inner surface is warmer, so it condenses far less often, so the marks on the reveal stop appearing. On a south or west facing roof, specifying solar control glass at the same time deals with the other half of the problem, and that decision is covered on upgrading the glass while the roof is open.
Ventilators built into old domes
A fair number of domes on bathrooms, stairwells and utility rooms were fitted with a fixed vent or a hinged opening section, and the room has been relying on it quietly for decades.
That function has to be replaced deliberately rather than lost by accident. The options are a hinged flush unit operated by pole or actuator, a fixed unit alongside a separate roof ventilator, or a mechanical extract discharging properly to outside. Which one suits depends on reach and on what the room does. In a bathroom, removing a passive vent and fitting a sealed unit without addressing the air movement produces condensation on the new glass and a reasonable belief that the new unit is at fault.
Where condensation inside a frame is meant to drain
Modern flush rooflights carry a drainage channel inside the frame perimeter, with small weep openings through to the outside face.
Its purpose is to collect the small amount of surface condensation that forms on the frame and the pane edge on a cold night and take it out of the building rather than onto the ceiling. It only works if the weeps are clear and if the unit is fitted the right way round on a kerb with a fall, because water in that channel moves by gravity like everything else. A unit fitted level on a level kerb has a channel that fills and does nothing. This is one of the reasons the kerb and the unit are considered together rather than separately.
Domes that were never really rooflights
Two things get called domes and only one of them is a rooflight.
A daylight dome is a glazing unit. A roof access hatch is a hinged dome on a deeper kerb, fitted with a stay and a catch, put there so somebody could get onto the roof from a landing or a plant space. They look similar from below and they are not interchangeable, because an access unit has a different kerb height, a different structural duty and a locking arrangement. Where the survey finds a hatch rather than a rooflight, the honest answer is either a hinged glazed unit rated for access or a fixed unit with the access route reconsidered, and that is a conversation before an order rather than after one.
Sizes, and where a 1978 opening lands on a modern grid
Dome kerbs were made in the sizes their makers sold, often in imperial or in odd metric numbers, and current flush rooflight ranges are built around their own standard sizes that do not land on those.
Three outcomes follow. The nearest standard unit fits over a rebuilt kerb sized to suit it, which is straightforward because the kerb is being rebuilt anyway and the structural opening beneath usually has tolerance. The opening is genuinely fixed by structure, in which case a made to measure unit is ordered with a longer lead time. Or the opening is enlarged, which brings structural work into what was a day. Because the kerb is a new build in most dome swaps, this is far less of an obstacle than it is on a roof window, and the detail is set out on matching a replacement to an obsolete size.
The internal reveal, and the chance to open it up
Below the old dome there is usually a plasterboard shaft from the ceiling up to the kerb, square sided, often lined in whatever was to hand in 1978.
Because the whole perimeter is being disturbed anyway, this is the cheapest opportunity there will be to splay it. A square reveal drops a hard edged rectangle of light onto the floor and leaves the surrounding ceiling dark. A splayed one, opening outwards towards the room, spreads light across the ceiling and roughly doubles the apparent size of the opening. It costs a little plastering and nothing at all in glass, and on a deep flat roof build-up the difference is the single most noticeable improvement on the job.
The order of work on a dome swap
For a single unit with a kerb rebuild, one day to a day and a half in ordinary weather.
The new unit arrives, is unwrapped and checked against the measured kerb before anything is stripped. The room below is protected, because brittle acrylic and forty year old mastic make a mess. The dome is lifted off from above and the old kerb frame is removed. The covering is cut back cleanly and the deck is checked and probed. The new kerb is built, insulated and squared. The covering is dressed up the kerb and terminated. The unit is set on its seal and mechanically fixed through the flange. Then the reveal is finished inside. The roof is closed and watertight before anybody leaves, whatever the forecast is doing.
Keep reading
More on skylight & rooflight installation & replacement chelmsford
Sizing a rooflight for the room
Read the guide →Roof window or rooflight, and the difference
Read the guide →Flat roof against pitched roof rooflights
Read the guide →What happens on installation day
Read the guide →Cleaning and maintaining a rooflight
Read the guide →All about skylight & rooflight installation & replacement chelmsford →
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