Rooflight and skylight installation in Langham

Whether a loft is worth converting

Two checks before anything else, and you can do both yourself.

Height. Measure from the top of the existing ceiling joists to the underside of the ridge. A floor structure, insulation and plasterboard will consume a great deal of that, and the working rule is that you want comfortably over 2.4 metres to end up with a usable room. Below about 2.2 metres the answer is usually no without raising the roof, which is a different project entirely.

Structure. Rafters running eaves to ridge with a horizontal purlin partway up and clear space between them is a traditional cut roof, which converts normally. A repeating pattern of triangular frames with diagonal webs crossing the space, joined by flat steel plates, is trussed rafters, and those do not convert without replacing the roof structure.

Most of the older Langham housing is cut roof, which is why conversions work well here.

Escape windows

In a habitable loft room the roof window is usually doing two jobs, and only one of them is about daylight.

Building Regulations require a means of escape from a habitable room in a loft, and in most conversions that duty falls on a roof window. It brings a minimum clear openable area, minimum opening dimensions, and a maximum height of the bottom of the opening above the finished floor.

The practical consequences catch people out. A centre pivot window is elegant and frequently unsuitable as an escape window, because the sash sits in the middle of the opening and obstructs it. A top hung unit that swings clear gives the full opening and is usually what gets specified. And setting a window high in the slope for a better view of the sky can put the opening out of reach for escape purposes.

All of that is settled at design stage rather than at order stage, because reversing it after the unit has arrived is expensive. Where an alternative escape route exists the requirement may be met elsewhere, and the roof window is then free to be chosen purely for light.

Getting the light right in a converted loft

Two smaller units spaced along the slope beat one large one in almost every case. One large unit gives a bright pool and leaves the rest of the room darker by contrast; two read as ambient daylight. On a cut roof with a purlin, two narrower units are frequently easier to fit as well, because each can sit clear of the timber without trimming.

Orientation decides the glass. North facing gives even, cool, glare free light all day with no solar gain at all, and should be specified for thermal performance alone. South and west want solar control glass as standard, and a loft room under an uninsulated south facing slope is the classic overheating case.

Splay the reveal rather than boxing it square. Cutting it back at an angle at the head and sill opens the beam out and makes a modest window feel considerably larger. It costs a little more in plastering and it is the best value decision available on any installation.

Consent

Part of the parish falls within the Dedham Vale National Landscape and there are listed buildings scattered through it.

Listed building consent is required for a rooflight on a listed building on any slope, visible or not. Elsewhere, permitted development normally applies to houses: no more than 150mm projection beyond the plane of the roof and not above the highest part of it. A loft conversion is notifiable under Building Regulations whether or not planning permission is needed, and we handle Building Control where the work requires it.

Access

Good. Space around most properties for a tower or scaffold on private land, no highway licences, and generally room for a delivery vehicle. Larger jobs suit the way we plan work out here, and access is a fixed cost shared across everything done on the same visit.

Number and position

Two smaller units beat one large one in almost every room. One large rooflight gives a bright area and leaves the rest of the space darker by contrast; two spaced along the slope read as ambient daylight. On a cut roof with a purlin, two narrower units are frequently easier to fit as well, because each can sit clear of the timber without a trimmer.

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

Position in the slope changes the character of the light. Lower gives a view out and lets light reach further into the room. Higher gives more sky, more even light and better privacy. In a room with a low eaves height, lower is usually right; in a loft room with a knee wall, higher.

The reveal, which is half the light

A rooflight in a thick roof sits at the end of a tunnel, and the shape of that tunnel decides how much of the collected light reaches the room.

A square boxed reveal throws a narrow beam straight down. A splayed reveal, cut back at an angle at the head and the sill, opens the beam out and can make a modest window feel considerably larger. It costs a little more in plastering and it is the best value upgrade available on any installation, particularly on a shallower pitch where the reveal is longer.

Exposure on open ground

A house standing in fields takes more wind than one in a village street, and driving rain matters more for a roof window than total rainfall does, because wind driven water is pushed under a covering rather than running down it.

That puts the weathering job on the underlay collar and the flashing kit rather than on gravity. Specify the kit correctly for the covering and generously lapped rather than accepting a universal one, and use stainless fixings rather than plated, which cost very little more and do not stain an exposed roof.

What the quotation should say

The unit with manufacturer, model, size and glazing specification. The flashing kit and which profile. The insulation collar as a named line. Structural work where a trimmer is needed and whether an engineer’s fee is included. Access. Making good and whether decoration is included. Disposal. Building Control where notifiable. And a stated contingency for decayed timber on an older property.

Where ours is higher than another quotation, the itemisation will usually show you why. Where somebody is a long way below everyone else, the usual explanations are a universal flashing kit, no insulation collar, and making good that stops at the plasterboard.