Blinds Above or Below the Roof? Heat, Glare and Which Stays Cleaner

Blinds for a glazed roof come in two forms, and they are sold as though they were the same product at two prices. They are not. One goes above the glazing, on the weather side, and the other goes below it, inside the space. They solve different problems, they perform very differently, and they need looking after in completely different ways.

Choosing between them is not a budget decision. It is a decision about whether your complaint is heat or glare — and those are not the same complaint, even though people describe both as "it gets too hot in there".

Heat and Glare Are Different Problems

Solar energy arriving at a glazed roof does two things. Some of it passes through as visible light, which is what makes the space bright and, at the wrong angle, makes you squint. Some of it is absorbed and re-radiated as heat.

The crucial physics is where that absorption happens. If you stop the sunlight before it reaches the glass, the heat never gets into the space. If you stop it after it has passed through, the energy is already inside, and whatever you have used to stop it — a fabric blind, a curtain, anything — absorbs that energy and re-radiates it as warmth into the room below.

That single fact is the whole argument. External shading is dramatically more effective at controlling temperature than internal shading, and it is not a small difference. Internal shading is very effective at controlling glare, and only modestly effective at controlling heat.

Above the Roof

An above-roof blind runs over the glazing on the outside, so the sun is intercepted before it ever reaches the glass. The energy that would have become heat is reflected and dissipated in the open air.

If the problem you are solving is that the space is unusable at two o'clock on a July afternoon, this is the answer, and nothing fitted underneath will match it. It also keeps the glass itself cooler, which matters on a structure that is already accumulating a lot of solar gain across a large plane.

The costs are real. It lives outside, so it takes weather, ultraviolet and dirt, and it will need cleaning and will not last as long as something protected. It has to be retracted in high wind, which means either remembering or specifying a wind sensor. And it collects leaves and debris on top of the fabric, particularly under trees, which then need shaking off rather than being left to sit.

Below the Roof

A below-roof blind runs on the inside face of the glazing. It is protected from the weather entirely, which changes everything about how it lives.

It stays clean, because it is out of the rain and out of the wind. It lasts longer for the same reason. It is not affected by gusts and does not need retracting before a storm. And it is very good indeed at glare — the low evening sun that comes in flat and makes it impossible to look at anything, and the harsh overhead light that makes a laptop screen useless.

It also does something people undervalue: it softens the quality of light in the space, turning hard shadows into something even, in the same way that opal glazing does but on demand rather than permanently.

What it will not do is keep the space cool in the way an external blind does, because by the time the light reaches it the energy is already inside.

The Honest Way to Choose

One question: is the complaint about temperature or about looking at things?

If the veranda is south or west facing, if you use it in the middle of summer, and if the phrase you keep using is "too hot", go above the roof. Nothing else touches it.

If the veranda is used for sitting, reading, working or eating, if the phrase is "you can't see anything at six o'clock", and if the space is otherwise pleasant, go below the roof. It is cheaper to own over its life, it needs less attention, and it is solving the problem you actually have.

If you are north or east facing, be honest about whether you need either. A great many blinds are fitted to structures that were never going to overheat, and the money is better spent closing the windward end.

Having Both

On a structure used seriously — an outdoor kitchen, a garden room, somewhere that gets used from March to November — both is a legitimate specification rather than an indulgence.

The external blind handles high summer, dropping the temperature before it becomes a problem. The internal one handles low sun in spring and autumn, when you want the warmth but not the glare in your eyes. They operate independently and they are rarely both deployed at once.

Add glass sliding doors to that and you have genuine control over the space rather than a covered area you tolerate: roof shading for heat, internal shading for glare, and glazing you can open completely on a good day or close entirely on a bad one.

Decide Before You Build

Both types need fixing points, and the fixing points are part of the structure rather than accessories bolted on afterwards.

An above-roof system needs its rails and its cassette accounted for in the roof design, and its position affects where beams sit. A below-roof system runs in guides that relate to the beam spacing, so the bay widths determine what blind widths are available. Retrofitting either is usually possible and is always easier and neater when it was planned for.

Fabric Choice Changes the Result

The mounting position decides heat versus glare. The fabric decides how much of either, and it is the part that gets chosen from a small swatch and lived with for a decade.

Openness factor is the number that matters — how much of the fabric is gaps. A more open weave keeps the view through and lets air move, at the cost of less shade. A closed fabric blocks more and turns the space darker. Colour works counter-intuitively on external blinds: a lighter fabric reflects more energy away and performs better thermally, while a darker one cuts glare better and keeps the view crisper, which is why dark fabrics are common on internal blinds and lighter ones outside. Ask to see the fabric at the size you are ordering rather than as a swatch, because openness reads completely differently over four metres.

One practical note on operation. Whichever position you choose, think about how it gets controlled and where the switch or remote lives, because a blind that is awkward to deploy is a blind that stays retracted through the exact afternoon it was bought for. On a wide structure with two or three separate blinds, being able to run them together rather than individually makes a genuine difference to whether they get used.

And be realistic about maintenance. An external blind under a tree needs the debris shaking off it before it retracts, or the fabric rolls up with the leaves inside it and stains. That is a two-minute job when you think of it and a fabric replacement when you do not.

If there is any chance you will want blinds later, say so at design stage. It costs nothing on a drawing to leave the option open, and a good deal on a finished structure to create it.