How to install a roof window

Parts of the home like the space under the roof are often under used—simply because of the lack of light or ventilation. Installing a roof window gets around these problems and enables you to create an extra room with a minimum amount of work—without the need for major structural alterations to the house.

Although roof windows are most usually fitted in attics, their use is by no means restricted to the main roof of the house. They can just as easily be installed in garage, shed or extension roofs providing the pitch of the roof is somewhere between 20° and 85° and the timbers are sound.

Planning a roof window

You will almost certainly need building regulations approval for a roof window —this means drawing up plans of your proposals and submitting them to your local council. You may also need permission under the planning regulations, which are different from building regulations: see the panel opposite for details.

If you simply want to introduce more light and ventilation into an attic which is already used as a room, then obtaining the necessary permissions is relatively straightforward. But if you want to convert an ordinary attic into a habitable room the procedure is more extensive. From the planning point of view, you will be creating an extension, which may or may not require planning permission. From the building regulations point of view, you will have to make sure the attic will satisfy the regulations for a habitable room—for


Building regulations must be consulted and planning permission sought before any work is undertaken. Briefly, both sets of regulations can apply even if you are not carrying out any structural work, but are merely changing the use of a room—for example, converting a loft into a habitable room.

Building regulations are concerned with ensuring that buildings are constructionally sound. Regulations vary from country to country; contact your local authority to find out what applies to you.

Planning laws exist to ensure that what you do is environmentally acceptable. In both the UK and Canada, the work of converting a loft may not need formal approval under the planning laws—but again, check with your local authority first. example you must leave enough head room; and a strong enough floor Converting your loft in this way js a major undertaking and should not be undertaken lightly.

Since the weight of even the largest roof window is spread over a wide area, the ratters do not have to be particularly wide; but they should be at least 100mm deep. Check also that they are well seated at the eaves and at the apex of the roof by standing near the middle of each rafter and shaking it vigorously. If you detect excessive movement, the rafters concerned must be strengthened and reseated before the window is installed in the roof.

Choosing a roof window

Recent developments in the design of roof windows have greatly extended the range and effectiveness of the old-style ‘skylight’. This consisted of a simply constructed metal- or wood-framed window, hinged at the top and opened and closed by means of a long handle attached to the bottom of the fixed frame.

By contrast modern roof windows are usually double-glazed and hinged centrally on both sides, allowing the sash to pivot through 180° for easy cleaning and maximum ventilation. They can be fitted with locks, and also with integral ventilation flaps and blinds which open and close between the double glazing.

For roof windows which are out of reach, and therefore difficult to open and close, electrically-operated models are available. And for areas which are hemmed in, an emergency exit window can be fitted which pivots both sides— like the ordinary model—and along one edge to allow easy escape.

Once you have decided on the type of model most suited to your needs, take care to choose the size which gives optimum light and ventilation without looking unduly large compared to the room in which it is fitted. As a rough guide, the area of the window—including any air bricks— should be at least 10 percent of the floor area of the room. Another important point to bear in mind is that two small windows can often be used instead of one large one. Such an arrangement gives as much ventilation and light as one window but can avoid the need to cut through rafters during fitting.

Because a roof window is more exposed to the elements than a window mounted on a vertical wall, it is usually protected with flashing around the outside of the frame. This is supplied with the window but before purchasing, check that you have the flashing compatible with your particular type of roof. Two types are available: U-type flashing for profiled roofing such as tiles, and L-type for thin or flat roofing materials such as slates or bituminous felt.

Essential equipment

Very few specialized tools are necessary to fit a roof window successfully and you will probably have most of them already. To mark out the area to be removed you need a tape measure and a pencil or felt tipped pen as well as a spirit level and plumbline. A sharp handyman’s knife is necessary to cut away any felt which might be on the roof under the covering.

Many tiles or slates are difficult to prise loose—particularly if they are firmly nailed—and a small plasterers’ trowel or slate ripper will make this task easier. A panel saw is also needed to cut the rafters and battens once the tiles or slates are removed.

Once the window is set in place a screwdriver is needed to fit the side profiles securely. Then, to ensure that the flashing is flattened against the surrounding tiles or slates a lead dresser or soft-faced hammer is useful. Finally, to trim and shape the roofing material to fit around the window you need slate cutting tools and pincers, plus a supply of fixing nails.

Preparatory work

Although all of the work involved in installing a roof window can be safely carried out from inside, it is very easy to drop tiles and timber on unsuspecting passers-by. So, before you start work, cordon off the area immediately below where you are working with clearly marked signs and improvized barriers.

If fitting the window involves cutting away rafters, prepare for this before you start work. To support the ends of each severed rafter, make up two pieces of timber long enough to stretch from the floor and cut the top ends at an angle to fit the rafter slope at top and bottom.

Once the rafters are cut through, timber noggins—called trimmers—of the same dimensions as the existing rafters are fixed at top and bottom across the gap to strengthen the frame. Prepare for this by measuring the distance between rafters that will accommodate the width of the window.

Next, unpack the window and remove the wooden sash by rotating it through 180° as if you were opening it. This will expose the hinges so that you can fully tighten the retaining screws and lift the frame away.

Strip the sash free of all metal components except for the two aluminium profiles near the hinge— most can be screwed loose then pulled free. Check that all the woodwork is clean and free of defects then give it a protective coat of polyurethane lacquer, allowing this to dry before starting work on the roof; further coats can be added later.

Positioning the frame

You may already have a good idea where you want to instal the window but check this exactly at this stage by lifting the frame into position against the roof. Take into account that you will want the control bar within easy reach at the top of the window and perhaps also a view from where you are likely to be sitting.

Once you have worked out the exact position in which you want to fit the window, measure its outside dimensions and carefully transfer these on to the rafters with the help of a spirit level. Align the base of the marked-out area with the bottom of a course of slates or tiles—this will help to fix the flashing more neatly once the window is in place

Next examine the installation instructions supplied with the window. These specify the clearance required between the frame and the roofing material to ensure a neat fit. Take careful note of the dimensions, marking them onto the roof timbers in a different colour if necessary, so that you know how many tiles or slates to remove.

Note also that the trimmers which need to be inserted above and below the frame to give it support are not tight against the frame itself: they are set further back to enable the internal window linings to be splayed and so allow a greater spread of light.

First check the thickness of your trimmers and measure and mark this distance above and below the frame position to allow for their width.

Then allow for the inner splay, using a spirit level to draw horizontal and vertical lines from your new top and bottom marks respectively to the inside of the rafter. It is at these points that the rafter will be cut and the trimmers nailed into position.

Cutting an opening

Once you have rechecked that the inside of the roof is correctly marked out, start to remove the roofing materials. How you proceed depends on both the structure of the roof and its covering:

Tiles and slates: Start by cutting away any internal roofing felt with a sharp handyman’s knife so that you expose the tiles or slates themselves.

Remove these one at a time, starting from the centre and working slowly towards the outside. If they are nailed to the battens, work each tile or slate free with a slate ripper or wedge a trowel under the top edge to break it loose. Continue in this way until you have a space well clear of the area to be occupied by the window. Felt roofs: Using the marks you have made on the inside on the roof as a guide, drill a 20mm diameter hole right through both the wooden decking and the covering felt. Then insert a padsaw and carefully cut around the outside of the marked-out area. When the cut is completed, the inside of the area will drop away and can be lifted clear of the working space.

Once you have removed the roof covering, the next step is to cut away any rafters running across the area to be occupied by the window. But before you do so, make sure that they are supported at top and bottom with timber wedges, placed well away from the area in which you intend to work.

Saw each rafter along the two lines previously marked, trying to keep the cut at the correct angle to the roof. Then prise the freed section away from the roof, taking care not to break or damage any of the battens or roof timbers.

To take the place of the missing rafters fix your trimmers across the top and bottom of the frame. Check carefully that these are correctly aligned with the existing roof timbers and square at each corner before nailing them securely through the uprights and on to the ends of the cut rafter.

With the trimmers set above and below where the frame is to go remove the temporary support and fix a false rafter down one side. Fit this according to the installation instructions so that there is a gap on each side of the frame to allow for the roofing material. Cut the rafter to length, check carefully for alignment and square, then nail it firmly to the trimmers top and bottom.

The window frame should now be placed on the outside of the roof and checked for fit. Do this by sawing down through the centre of the battens or sarking so that you have a slot wide enough to allow the frame to be pushed through sideways. When the frame is on the roof adjust its position exactly—remembering that you will have to replace all the roofing materials around the outside once it is fixed. When you are satisfied that it is set exactly as you want it, mark this position on the roof battens underneath.

While the frame is still in place, locate the four angle brackets and screw them to the outside of the frame—two on each side. Use the line marked on the side of the frame as a guide and adjust the position of the brackets accordingly to ensure a watertight seal once the window is in place. The line should coincide exactly with the top surface of the battens or sarking.

Once you are satisfied with these adjustments, take the frame back inside and replace the roof covering up to the bottom of the marks made for the frame. Make sure you do this exactly so that the frame can be set back into position without gaps between it and the surrounding roofing material. Use pincers to nibble away sharp corners along the top of the tiles or slates so that the flashing is not damaged once it is applied— especially along the bottom edge.

Next, you must cut back the battens using the marks you have made on the outside of the roof as a guide. But study the installation instructions before doing so since the exact position of the cuts varies according to whether you are using U- or L-type flashing.

Set the frame back in position and check the clearance at all sides, adjusting as necessary by cutting away more of the battening or adding small battens where there is too large a gap. Afterwards, check that the frame is square by measuring both the diagonals and along the outside edges. Then secure the frame by screwing the angle brackets into the rafters at each side.

With the frame firmly held, fit the bottom flashing section into place and secure it by slotting the lower frame profile over the bottom of the window and screwing it tight. The rest of the flashing should then be fitted.

Fixing the flashing

The technique used to fit the flashing and ensure a weathertight seal around the window, depends on whether you are using U-type flashing for a profiled roof or L-type flashing for a flat or slated roof:

U-type flashing: Fix the side flashings first. They have a raised lip along the edge furthest from the window to hold the tiles at a correct distance from the frame, so check you have them the right way round. The side sections are secured with clips ready-fitted to them. Locate these and nail them to the battens below , then fix the bottom section in the same way.

The gap above the frame must now be filled to provide good support for the top flashing. Do this by nailing a number of battens across the rafters spaced at about 20mm intervals. Then replace the top and side profiles by sliding them into place and screwing them to the frame. Finally, clip the top flashing into position, making sure that it is firmly attached along its bottom edge to the top profile you have just fitted.

With all the flashing in place, double-check that it is fitted correctly and that all of the screws are tightened around its outer edges. Then, using a lead dresser or a soft-faced hammer, flatten the bottom flashing—which is made of soft pliable lead—against the roof coverings.

When you are satisfied that the flashing is fitted so as to give an all-round weatherproof finish, replace the tiles or slates around the outside. Start with those on each side of the frame, trimming them to size so that they fit neatly just under the edge of the flashing. Then replace the top ones, leaving a gap of 60-100mm above the window depending on where the bottom course finishes. If you have to cut across the tiles or slates to establish this gap—and so end up with a bottom course of shortened tiles— fit a tilting fillet of wood below the bottom course before you fix them in place.

L-type flashing: Secure the bottom flashing section—which should already be in place—by fitting the lower frame profile and screwing it tightly to the frame. Then fill the gap above the window with wooden battens, spaced at roughly 20mm intervals and nailed into the rafters, so as to provide a solid base for the flashing and roof covering above the window.

The roof on each side of the window is then made watertight by fixing the short sections of flashing—called soakers—to the edge of the frame with nails and alternating these with slates or pieces of roofing felt. Start at the bottom edge of the frame and fit a soaker. Lay a slate on top of this and then another soaker and continue in this way until you reach the top. Depending on the size of the window, the top soaker may have to be cut to length in order to fit below the top slate.

Once the sides of the window are made watertight, fit the top frame cover and side profiles and screw them down firmly. Finally, replace the slates or felt above the window so that the top frame is slightly overlapped. The window and its surrounds should then be checked to ensure that they are fitted correctly and screwed tightly in place.

Replacing the sash

Refit the sash into the frame and make sure that it is firmly held. If you have fitted it correctly and the frame is square, the window should open and close easily. But if it sticks, check that all screws are tightened and that the sash is correctly aligned before trying again.

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