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close this bookAttic Completion Work - Course: Timberwork Techniques. Trainees' Handbook of Lessons
close this folder3. Building-in of Roof Superstructures
View the document3.1. Building-in of Roof Superstructures in a Couple Roof
View the document3.2. Partition of Rooms in the Attic
View the document3.3. Covering of Visible Roof Rafters in the Room and of the Roof Superstructure

3.1. Building-in of Roof Superstructures in a Couple Roof

If the attics shall be used for housing purposes, roof windows are to be built in for natural lighting and ventilation of the living space.

The width of roof windows is limited by the rafter distance.

Figure 6 Roof window in the clearance between the roof rafters (rafter distance)

1 rafter of main roof, 2 parapet trimming, 3 lintel transom, 4 window post, 5 string transom, 6 rafter of lean-to dormer roof of window

This is of special importance for couple roofs because the couples support each other at the ridge and the acting supporting forces in the roof rafter are carried off to the end cogging.

Too low supporting forces in the roof rafters involve the danger of collapse!

In this respect, it does not matter whether the roof structure is supported by a wooden beam or solid ceiling.

Figure 7 Direction of forces in a couple roof

(1) Solid ceiling
(2) Wooden beam ceiling

1 rafter, 2 ridge, 3 wooden beam, 4 solid ceiling, 5 cornice, 6 inferior runner, 7 end cogging, 8 ceiling support

In the couple roof, the width of the roof window is dictated by the clearance between the roof rafters.

If roof windows of greater width than the clearance between the rafters are to be built into a purlin roof, the rafters must be interrupted by a rafter trimming to be inserted.

Figure 8 Roof window with interrupted rafters

1 trimmer rafter, 2 parapet trimming, 3 window post, 4 string transom, 5 lintel transom, 6 parapet transom, 7 lower tail rafter, 8 upper tail rafter, 9 rafter trimming, 10 rafter for gable roof of window, 11 valley board

Insertion of a rafter trimming, however, means to change the direction of forces of the supporting structure!

Figure 9 Change of direction of forces

F - acting force
1 trimmer rafter, 2 rafter trimming, 3 parapet trimming,
4 upper tail rafter, 5 lower tail rafter

In a couple roof it is not possible to interrupt the rafters for windows of greater width!

Why is it not possible to interrupt the rafter in a couple roof for windows of greater width?


Without static verification it is not allowed to interrupt more than one rafter in a purlin roof.

If several roof windows are required for natural lighting of the living space, it is to be made sure that one trimmer rafter must only support one lower and one upper rafter trimming.

Figure 10 Lower and upper rafter trimming

1 trimmer rafter, 2 parapet trimming, 3 rafter trimming, 4 upper tail rafter, 5 lower tail rafter, 6 ridge

Why must a trimmer rafter not support a rafter trimming at both sides?


In order to support the two tail rafters, rafter trimmings are inserted.

The parapet trimming should be inserted at a height allowing a parapet transom to be built in.

The surface of the parapet transom should not exceed the perpendicular size of 900 mm.

Figure 11 Roof window height limitation

1 trimmer rafter, 2 parapet trimming, 3 window post, 4 string transom, 5 inferior purlin, 6 parapet transom, 7 trussed purlin, 8 king post, 9 attic beam, 10 parapet height, 11 lower tail rafter, 12 lintel transom, 13 upper tail rafter, 14 surface of trussed purlin

The upper rafter trimming can be omitted if the upper tail rafter rests on a trussed purlin.

The height of windows in purlin roofs with double upright truss is limited by the trussed purlin.

It is advantageous when the surfaces of the trussed purlin and lintel transom are located at equal height which provides for a uniform room height when fixing the subceiling.

Figure 12 Limitation of roof window height

1 trussed purlin, 2 lintel transom, 3 king post, 4 valley beam, 5 string transom, 6 roof boards (roofing), 7 subceiling, 8 parapet transom

The visible part of the trussed purlin in the subceiling can be ground and painted or covered (faced).


- For purlin roofs it is recommended to insert windows fitting into the clearance of the rafters.

- If a window of greater width is necessary, no more than one rafter must be interrupted unless static verification is available.

- It is a must that one trimmer rafter only supports one rafter trimming at one side.

- The number of adjacent rafters that can be interrupted is limited.

- Constructional details.

The opening in the roofing must only be as big as to allow the subceiling or protection against trickling matter to be provided in the roof superstructure (see also Fig. 12).

The timberwork of the roof superstructure to be built in is to be joined.

Though, normally, rafter trimmings and tail rafters are joined by mortise-and-tenon joints, for attic completion work for housing purposes it is recommended to use screw-on squares to provide greater stability.

Figure 13 Joining of structural parts

1 trimmer rafter, 2 tail rafter, 3 steel square, 4 borehole, 5 hexagon-head wood screw


Because of the change of direction of acting forces, there is greater load on the trimmer rafter than before. The cross section of the rafter would be additionally weakened by the mortise.

What effect would the weakened cross section of the trimmer rafter have on its bearing capacity?


The window post has not necessarily to be the corner post but it may be placed inwards depending on the width of the window.

Figure 14 Arrangement of window and corner posts

1 corner post, 2 window post

A profiled batten for fixing the window frame can be fixed to the window post and lintel transom.

The roof rafters of the lean-to dormer roof of the window are supported on the roof rafters of the main roof with oblique notch. The roof rafters of the gable roof of the window are joined with the string transom by a triangular notch (birdsmouth).

The parapet transom and string transom are supported on the roof rafter or trimmer rafter and not sunk in into the roof rafter.

Why should the parapet transom not be sunk in into the roof rafter?


For hip roofs of windows, a packing piece is to be placed on the lintel transom to birdsmouth the jack rafters.

Figure 15 Hip roof of window

1 trimmer rafter, 2 lintel transom, 3 window post, 4 hip rafter of window roof, 5 packing piece, 6 jack rafter, 7 rafter of window roof, 8 triangular notch (birdsmouth)

The valley boards are to be nailed onto the roof rafters. The width and height of the rafter trimmings are to be suited to the existing roof rafters.

A cross section of 100 · 100 mm2 or 120 · 120 mm2 is recommended for posts and transoms.

The rafter cross section of gable or hip roofs of windows should be 60 · 120 mm2 and of lean-to dormer roofs of windows 80 · 140 mm2 .

Before interrupting a roof rafter, the rafter to be interrupted is to be underpropped. The prop is to be placed on the attic beam and to be secured against displacement or slipping off!

The diameter of the prop should be big enough to avoid collapse of the prop and accidents caused thereby!

The top end of the prop should at least have a diameter of 100 mm.