A cut roof is one built on site from individual timbers rather than from factory made trusses, and it is what almost every house in this area built before about 1965 has. It is also the roof type that accommodates a roof window most readily, because its members can be cut and trimmed in a way a truss cannot.
What a cut roof is made of
Rafters running from the wall plate at the eaves to the ridge board at the top. Purlins running horizontally partway up the slope, supporting the rafters at mid span. Ceiling joists across the bottom tying the feet of the rafters together. Struts from the purlins down to a loadbearing wall below, and collars tying opposing rafters together higher up.
Each member has a job and the assembly is a structure rather than a kit. Understanding which member is doing what is the whole of forming an opening safely in one.
Why the ceiling joists matter more than they look
The ceiling joists appear to be there to hold up the ceiling, and that is their smaller job.
A pitched roof pushes outward at its feet under load. The ceiling joists tie the two wall plates together and resist that spread. Remove or cut them without providing another tie and the roof spreads, the walls are pushed out, and the ridge drops.
That is not usually a rooflight problem, because a rooflight is in the slope rather than the ceiling. It matters because a loft conversion frequently involves both, and because anybody working in a roof space needs to know which timbers are doing that job.
Trimming between rafters
The standard method, and the one most roof window openings use.
Where the window is narrower than the clear space between two rafters, no rafter is cut at all. Trimmers go in horizontally at the head and the sill of the opening, spanning between the two rafters either side, and the window sits in the rectangle they form.
That is the cheapest and cleanest version of this job, and it is why standard roof window widths are sized to fit between common rafter spacings.
Cutting one rafter
Where the window is wider, one rafter is cut and the load it was carrying is transferred.
The cut rafter is supported at its cut ends by the trimmers. The trimmers span to the rafters either side, which are usually doubled to take the additional load. The load path is now longer but it is continuous, and it is ordinary structural carpentry.
Most standard roof window sizes are designed around exactly this: one rafter cut, two trimmers, the neighbouring rafters doubled.
Cutting more than one
| Opening | Structural work | Engineer? |
|---|---|---|
| Between two rafters, none cut | Trimmers at head and sill | No |
| One rafter cut | Trimmers, neighbours doubled | Not normally |
| Two rafters cut | Heavier trimmers, neighbours doubled or trebled | Frequently |
| Three or more cut | A designed beam at head and sill | Yes |
| A purlin cut | A designed solution, whatever the width | Always |
The purlin, which is the real question
A purlin runs horizontally across the rafters partway up the slope, and it is doing a great deal of work: it supports every rafter it crosses at mid span, which is what allows the rafters to be the size they are.
Cutting a purlin is a different order of intervention from cutting a rafter. Every rafter it was supporting loses its mid span support at once, and the load has to be carried by something else.
It can be done, and it needs designing. A trimming beam, or a new steel taking the purlin load round the opening, sized by an engineer. This is the single thing most likely to turn a simple roof window job into a structural one, and it is decided by where the purlin sits relative to where you want the window.
Working round the purlin instead
Usually the better answer, and it costs nothing but a conversation about position.
A roof window fits below the purlin, or above it, in the clear zone of the slope. On a typical roof there is a usable band below the purlin and another above it, and a standard sized window fits comfortably in one or the other.
What decides which is the room below and the head height available, and where the room genuinely needs the window at the purlin line, the designed solution is the honest answer rather than a compromise.
Struts and collars
Struts run from the purlin down to a loadbearing wall or a binder, and collars tie opposing rafters at high level.
Both get in the way of a loft room more than they get in the way of a rooflight, and both are structural. A strut that is inconvenient is not simply removed; its load has to go somewhere. A collar cut without replacement lets the roof spread.
Where a loft is being converted at the same time, these come into the design as a whole rather than one at a time, which is one of the reasons rooflight positions are best settled alongside the conversion rather than afterwards.
Rafter spacing here
Traditional cut roofs in this area are commonly at around 400mm centres, though it varies with the age of the building and with the span.
Older roofs are less regular. A Victorian or Edwardian roof was set out by a carpenter on site and the spacing wanders. A 1930s semi is generally more regular. On a genuinely old building the spacing may vary by a considerable amount across one slope.
Which means the spacing is measured in the roof space rather than assumed, and the window size is chosen from what is actually there.
Measuring from inside
Almost everything needed to plan the opening is available from inside the roof space, without touching the covering.
- Rafter spacing, measured across several bays rather than one.
- Rafter depth and width.
- Purlin position, measured up the slope from the wall plate.
- Positions of struts and collars.
- Any services running in the roof: cables, pipes, a water tank.
- The condition of the timber, particularly near the eaves and at any historic leak.
Half an hour in the loft settles the position and the size before anybody goes on the roof.
What we find in older roofs
Roofs are altered more than people realise, and the alterations are frequently invisible from the room below.
A previous opening trimmed badly or not trimmed at all. A purlin cut for a water tank and never made good. Rafters notched deeply for cables or a soil pipe. Beetle damage or historic rot at the eaves where the roof has leaked. A DIY conversion where struts were removed to make headroom.
None of this is unusual. All of it changes the work, and finding it at survey rather than on the day is the difference between a fixed price and a conversation.
Head height, and the constraint from below
In a loft room the window position is constrained from both sides. The structure sets what is possible on the roof and the room sets what is useful.
A window set too high sits above the eye line of somebody sitting in the room, so it gives light and no view. Set too low, the head is below standing eye level and the window is awkward to look through. On a shallow pitch, a tall window occupies a great deal of slope for very little vertical rise inside.
The sill height and the head height are both worth setting out on site with somebody standing in the room, rather than deciding from a plan.
Two windows, and where the rafter goes
Where two windows go into one slope, the rafter between them matters.
Two openings each fitting between rafters, with a rafter between them left uncut, is the tidiest arrangement structurally and the easiest to weather. Two openings that each require a rafter cut, close together, leave a short length of doubled rafter carrying a great deal, and that is when an engineer becomes worth involving.
The spacing between the units is also set by the flashing, since a combination kit has a defined gap. Structure and flashing therefore have to agree, and both are decided before ordering.
Insulation, and the depth you have
A cut roof being converted has to carry insulation between and usually under the rafters, and the rafter depth is frequently less than the insulation depth needed.
That is dealt with by deepening the rafters, by insulating below them, or by using a higher performance product in less thickness. All three reduce the head height in the room, which is why loft conversions in older houses are frequently tight.
Around a rooflight, the insulation has to be continuous up to the frame, which is what the insulation collar does. Leaving a gap there is the same cold bridge that appears at a flat roof kerb, with the same result on the reveal.
Propping while the work is done
A rafter about to be cut is carrying load right up to the moment it is cut, and that load does not pause while the trimmers go in.
The sequence is to prop the rafters either side of the opening and the one being cut, take the load onto the props, form the opening and fit the trimmers, then release the props onto the completed structure. It sounds obvious and it is regularly skipped on small openings, which is how a roof takes a set that shows as a dip in the slope afterwards.
On a roof where a purlin is being altered, the propping is a designed part of the job rather than a site decision, and the engineer will specify it alongside the permanent works.
Where the opening falls on the slope
The vertical position matters structurally as well as visually.
An opening close to the eaves sits where the rafters are least loaded but where the timber is most likely to have suffered from historic leaks and where the wall plate and the roof structure meet in a way that is worth not disturbing. An opening close to the ridge sits where the rafters are shortest and stiffest. An opening at mid span, which is where a purlin usually is, sits where the rafters are working hardest.
Given a free choice, the band of slope between the purlin and the ridge is frequently the easiest structurally, and the band below the purlin is frequently the better one for the room. Which wins is a conversation, and it is worth having before a size is chosen.
Ventilation of the roof space
A cut roof that is not converted is a cold roof space, ventilated at the eaves, and that ventilation keeps the timber dry.
Putting a rooflight into an unconverted roof space is unusual but it happens, over a stairwell or a landing. Where it does, the ventilation path must not be blocked by the trimming or by insulation pushed up against the opening.
Where the roof is being converted, the arrangement changes completely and a ventilated void above the insulation, or a fully filled warm roof detail, is designed as part of the conversion.
Building Control
Forming a new opening in a roof, cutting a rafter and trimming an opening are all notifiable under the Building Regulations, and a loft conversion is notifiable in its own right.
We notify Building Control where the work is notifiable and the completion certificate comes to you. On structural work that certificate is the document a buyer’s solicitor asks for, and its absence is a genuine problem at the point of sale rather than a paperwork detail.
How the measuring is done on a pitched roof
We measure the rafter spacing, the purlin position and the struts from inside the roof space before quoting, and choose a window size that fits the structure rather than one that forces work on it. Where a purlin is in the way we say so and set out the options, including moving the window. Where an engineer is needed we say so before you have chosen a unit.
A cut roof will take a window almost anywhere. It is much cheaper where it already had room for one.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.