Pairs and triples on one slope

Combination flashing

The technical point that matters. Where windows sit side by side, they need a combination flashing kit designed for that arrangement, which weathers the space between them properly.

Two single kits butted together is not the same thing. The gap between them becomes a channel with no designed way of shedding water, and it leaks. This is a defect we are called to correct.

Combination kits specify a fixed gap between units, typically 100mm or 250mm depending on the system, and that gap is not negotiable on site: the flashing is made for it.

Structure

Two windows side by side means the rafter between them either stays, if the units are sized to sit either side of it, or goes, in which case the opening is trimmed as one.

Sizing the pair to keep the rafter is far cheaper, and it usually suits the combination gap anyway.

Vertical pairs

One above another on a steep roof, occasionally used where the roof is tall. It works, needs the right kit, and is worth thinking about carefully: the upper window is often too high to see out of and functions as a light source.

Spacing for light

As a rule of thumb, spacing two windows at roughly a third and two thirds along the room gives more even coverage than putting them close together in the middle.

Close pairs read as an architectural feature and light one part of the room brightly. Spread pairs light the room. Both are legitimate; they are different intentions.

Cost

Two smaller windows are often similar in total cost to one large one and sometimes less, because each may fit between rafters where a single wide one would need trimming.

Why more than one is usually better

Given a choice between one large rooflight and two or three smaller ones of the same total area, the multiple units win in most rooms, and for four separate reasons.

Light distribution. The most important. One large unit gives a bright pool with the corners of the room darker by contrast, and contrast is what makes a room feel gloomy. Two or three spaced across a slope read as ambient daylight.

Structure. On a trussed rafter roof, units narrow enough to sit between trusses at 600mm centres need no structural work at all, where one wide unit needs a designed trimmer and an engineer. That is frequently the difference between a straightforward job and an engineered one.

Cost of glass. Price rises steeply rather than proportionally with pane size, so three moderate units are usually cheaper than one large one of the same total area.

Independent failure. A seal failure in twenty years means replacing one modest unit rather than a very large and expensive pane.

The argument the other way

Worth being honest about, because it is real.

Frame width is roughly constant regardless of unit size, so a small window loses proportionally more of its opening to frame. Two small units do not deliver the same glass area as one of twice the size, even though the structural openings add up.

And each opening needs its own flashing kit, its own underlay collar, its own insulation collar and its own making good. Three openings is three lots of that work rather than one.

So multiple units win on distribution and structure and lose slightly on glass area and on labour. In most rooms the distribution argument dominates.

Spacing them

Even spacing along the slope, set in from the walls rather than tight against them.

A unit hard against a wall throws light down that wall and does less for the room. One set a metre or so in lights the floor and the space people occupy.

On a long room, three units evenly spaced along its length give a consistent result. On a room with a defined working area, positioning one over that area is worth more than symmetry, and nobody notices afterwards that the spacing is not perfectly regular.

Alignment matters more than you expect

From outside, a row of rooflights on a slope is read as a set. Any inconsistency shows.

They should align horizontally with each other, sit at the same height on the slope, and be evenly spaced. On a slope where the structure forces uneven spacing, that is visible from the garden for the next twenty five years.

Where the structure will not permit even spacing, it is worth considering two rather than three, or shifting the whole set, rather than accepting an irregular row.

They should also be the same model and the same size. A row of three that includes one slightly different unit is more noticeable than three that are all slightly smaller than you wanted.

Coupled against spaced

Two different arrangements and worth distinguishing.

Coupled units use a manufacturer’s kit to join two or more windows side by side under a common flashing, with a slim structural section between them. From the ground it reads as one wide opening divided by a bar.

Spaced units sit apart, each with its own flashing, with roof covering between them.

Coupling is right where the effect wanted is a single generous opening. Spacing is right where the aim is an evenly lit room, which is most of the time.

Coupling must use the manufacturer’s kit. Two windows butted together with an improvised flashing between them is where water gets in: the junction between two units is the hardest detail on the roof and water arrives at it from both sides.

Mixing fixed and opening

Frequently the sensible specification on a pair or a triple.

One opening unit gives you the ventilation, and where required the means of escape, while the others are fixed. You get the glazed area without paying for three mechanisms, three actuators to maintain and three future actuator replacements at ten to fifteen years.

Position the opening one at the highest point of the slope, because warm air collects at the top of a room and that is where a vent does most good.

The visual difference between a fixed unit and an opening one of the same model is slight, so a mixed row still reads as a set.

Specifying glass across a set

Here the units are not necessarily identical.

On a single slope they all face the same way, so orientation gives the same answer for all of them: solar control if the slope faces south or west, nothing extra if it faces north.

Where they serve different parts of a room with different uses, other specifications may differ. Acoustic laminate over the bed in a loft bedroom and standard glazing over the rest. Obscure glazing over a shower and clear elsewhere.

Those differences are invisible from outside and worth making.

Doing them at the same time

Strongly worth it, and the reason is access.

Scaffold, a tower, a highway licence where one is needed, setting up, protection and clearing away are a fixed cost shared across everything done in the same visit. Two windows fitted together cost meaningfully less than two fitted a year apart.

Two or three in one visit is two to three days rather than three separate days, because the second and third go faster than the first.

On a replacement it matters more still, because units fitted at the same time fail at about the same time. If two of three have misted, the third is not far behind, and it is coming whether it is planned or not.

What to send us

A photograph of the roof from outside showing the whole slope, one looking up inside the loft, and the room dimensions.

The loft photograph settles cut roof against trussed rafters and the clear spans available, which is what decides how many units and what width. The slope photograph shows us the alignment constraints.

From those we will suggest a number and a spacing rather than a single option, and we will quote the whole slope itemised so you can see what each unit costs and what the shared access is worth.

Why more small units beat one large one

The structural argument decides it on most roofs.

A unit narrow enough to sit between two rafters, touching neither, needs no structural intervention. Two or three such units in adjacent bays, with the rafters left standing between them, cut nothing at all.

One wide opening covering the same glass area means cutting two rafters, trimming, and on a trussed rafter roof an engineer and a designed repair.

The cost difference is substantial, and the glass area is comparable.

The combination flashing

What makes the group read as one element rather than three separate windows.

A combination kit provides a single continuous weathering system across all the openings, with purpose made sections between them. It is not several individual kits butted together, and the difference matters.

Where two individual kits meet there is a junction neither was designed to make, and between them a narrow strip of covering usually too small to lay properly. That strip cannot shed water reliably and it is where the group leaks.

The dimension that has to be right first

Combination kits specify a fixed gap between the units, and it is not adjustable. It corresponds to the rafter left standing between the openings.

Which means the opening positions are determined by the kit before any carpentry starts. Setting the openings by eye and ordering the kit afterwards leaves gaps no kit fits, and the result is lead dressed by hand at additional cost.

Order the kit, then set out the openings to it.

Alignment, which is what people notice

Units in a row should align at head and sill, and the spacing between them should be equal.

On a slate or plain tile roof with fine regular coursing, any misalignment is obvious from the ground. On a large format profiled tile it is less so, but the eye still reads a row that is not level.

Where existing features sit on the same slope, a chimney or an earlier opening, aligning to them matters more than aligning to the centre of the roof.

Units stacked up the slope

Combination kits also exist for units one above the other, which suits a tall slope and gives a dramatic result in a loft room: light high for depth, and a lower unit for the view.

Same principles apply. Fixed gap, ordered first, carpentry set out to it.

Ordering the kit first

A combination kit fixes the gap between openings, which means the carpentry is set out to the kit rather than the kit ordered to the carpentry.

Order it, confirm the dimension in writing, and give it to whoever is forming the openings before they start.