Fitting a Dormer Window

Filed Under: DIY Outdoor, Do it yourself, Home repair, Remodeling    by: ITC

In converting an attic the installation of one or more dormer windows will not only provide essential natural light but also increase the headroom over areas of floor that were previously unusable.

In addition to providing windows and extra floor space, with a dormer built up from eaves level, you can site a staircase immediately below it fixed to the external wall of the house, whereas normally it would have to run through the center for there to be enough headroom at the top.

Since several rafters will have to be cut through make the doliner, the framework must be strong enough to support the load previously borne by the cut rafters. It must also have support at floor level unless it is possible to set the framing directly on top of a loadbearing wall. The original attic joists would certainly not be strong enough. However, since a dormer will be put in as part of an attic conversion this is not really a problem. A strengthening framework and extra joists will have to be put in for the new attic floor and this can be made to support the claimer framework as well. This new framework is built directly onto the house load- bearing walls and is completely independent of the original attic joists and ceiling below.

Calculating the loads involved and designing the supporting framework is specialist work for an architect, engineer or a contractor.

The construction of the donuer framework would follow on from constructing the floor support structure. Once the dormer is complete, do the rest of the conversion work. Access to the roof will be needed so that bulky materials can be passed through from the outside after removing a few tiles or slates and cutting an opening in the roofing felt. An access tower and roof ladder are essential.

The first job is to build the framework of the dormer, making sure it is secure before cutting through the original rafters and removing them.

The first sections of framework to be erected are the two corner posts for the outer end of the dormer. There is no need to strip off all the roofing within the dormer area for this initial construction work; remove

only small sections of tiles and pass the framework through. In this way the roof can be kept reasonably weathertight for most of the time.

The corner posts stand on the supporting floor joists below and are linked immediately below rafter level by a horizontal beam. Short wooden studs are nailed between the purlin and the supporting floor beam. The purlin has two purposes: to tie the bottoms of the corner posts together and to support the lower ends of the original rafters when they are cut through. All the construction is toe-nailed to fit.

Next, nail a horizontal beam across the tops of the corner posts. The joists for the top of the dormer can then be fitted: nail their outer ends to the top of the header and pass them right through the roof and bolt them to the rafters on each side for stability.

If the roof of the dormer is to be flat, tapered wooden slats, called furring pieces, are nailed to the tops of the joists so that the roof will have a fall to the front for drainage. If the dormer is to have a pitched roof, a ridge board and additional rafters are installed above the joists.

Before removing the roof from within the dormer framework, fit additional trimmer rafters between the corner posts and the roof ridge bar (or hip rafter if the dormer is a wide one on a hipped roof). Cut shallow notches in the sides of the corner posts to take the ends of these trimmers, and nail in place.

To complete the framework, the roofing must be stripped off. Lift the tiles or slates from the battens, cut out the felt and saw through the battens to expose the joists: cut these off flush with the undersides of the dormer joists and level with the inner face of the new supporting purlin.

Nail vertical studs between the trimmer rafters and the joists above to provide support for the side “cheek” cladding of the dormer, spacing them to take account of the width of the cladding sheets so that their edges always fall on the centerline of a stud.

Complete the framework by adding a wooden subframe to support the window itself. This is usually a horizontal beam set between the corner posts and supported below by short studs nailed to the top of the new purlin, and possibly to the sides of the cut rafters as well. The window will be narrower than the distance between the corner posts, so nail additional studs between the horizontal beam and header to support it at the sides.

The roof is covered first and if it is flat, it is decked with exterior grade plywood, butting the sheets together and nailing them to the supporting joists. To provide support for the flashing, slip a narrow strip of board under the roof at the junction with the dormer and nail it to the original rafters. To provide a certain degree of protection until the job is completed, you can add the first two layers of felt at this stage, taking them up under the original roof and leaving overlaps at the sides and front for finishing off.

Next the sides and front of the framework on either side of the window opening can be paneled in: you can use foil-backed gypsum board for the cheeks. Nail it to the outside of the framework with the foil side outei inost to prevent moisture penetration.

Fit lead soakers beneath the tiles on each side of the dormer; it may actually be easier to do this before cladding the sides, since they will slip in from the ends of the courses without the removal of the tiles.

Before finishing off cladding the sides, add flashing to the front of the dormer below the window-opening, taking it over the top of the frame-supporting beam and down over the roof tiles or slates below.

If tiles are to be used for cladding, nail narrow battens horizontally around the dormer (the spacing being dictated by the tile size). If boarding is used, nail the battens on vertically. Overlapping tiles and boards are nailed on in pattern to keep out rainwater.

Fit window frames made from seasoned wood. A gutter is fixed along the front fascia board with a short down-pipe at one end which can be led down the corner of the dormer to discharge its contents over the roof below.

Having clad the outside of the dormer and glazed the window, finish off inside — this can be done at the same time as building the interior of the attic room.

While the framework is still exposed, however, glass fiber or polystyrene insulation can be fixed between the various frame members at the sides and in the roof before they are clad with drywall or whatever internal wall cladding is being used.

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