Dormer roof junction detail

By - , Build 170

A very tricky roof-to-wall junction detail occurs at the top and bottom junctions of a monopitch dormer roof. Here is one option that, if built correctly, will be weathertight and look good.

Figure 3 Construction sequence for detail B – top junctions for a dormer on a monopitch roof. The side walls are clad with a sheet material and installed over a drained and vented cavity.
Figure 1 Junctions at monopitch dormer roof.
Figure 2 Construction sequence for detail A – bottom junctions for the dormer on monopitch roof.

AS PART of the Build roof-to-wall junction detail series, we look at the flashing requirements for the top and bottom junctions of a monopitch dormer roof (Figure 1). Although detailing of the junctions is described separately, construction of each must occur concurrently.

The following sequences of 3D drawings are for a monopitch dormer with side walls clad with a sheet material installed over a drained and vented cavity.

Figure 1 Junctions at monopitch dormer roof.

Construction sequence for bottom corner

The first construction sequence works through the installation at the bottom corners of the dormer (detail A on Figure 1, see Figure 2).

Step 1: Install underlay to the main and dormer roofs, turning the roof underlay up the sides and face of dormer. Lap the turn-down of the dormer roof underlay over the side wall underlay. (Install in conjunction with steps 1–2 for detail B.)

Step 2: Cut and fit the roofing to the main roof along both sides and below the dormer. (Install in conjunction with step 3 for detail B.)

Step 3: Install fabricated flashings at each front corner of the dormer and main roof.

Step 4: Install apron across the bottom face of the dormer. Fit parallel apron flashings to sides of dormer. (Install in conjunction with step 4 for detail B.)

Step 5: Lap an additional layer of wall underlay over all apron flashing upstands. Install cavity closers and cavity battens to the side walls of the dormer. (Install in conjunction with steps 5–6 for detail B.)

Step 6: Install corner flashings in accordance with E2/AS1 over the battens. Then fix the cladding over the cavity battens.

Figure 2 Construction sequence for detail A – bottom junctions for the dormer on monopitch roof.

Construction sequence for top corner

The second construction sequence works through the installation at the top corners of the dormer (detail B on Figure 1, see Figure 3).

Steps 1–2: Install underlay to the main and dormer roofs, turning the roof underlay up the sides and face of the dormer. Lap the turn-down of the dormer roof underlay over the side wall underlay.

Steps 3–4: Cut and fit the roofing to the main roof along both sides of the dormer and to the dormer roof.

Step 4: Fit parallel apron flashings to the sides of the dormer.

Step 5: Lap an additional layer of wall underlay over all apron flashing upstands.

Step 6: Install cavity closers and cavity battens to the side walls of the dormer.

Step 7: Fix cladding and barge boards.

Step 8: Install barge flashings.

Step 9: Install timber packers to align with the top plane of the side apron flashing extensions. Install fabricated flashings at the junctions between the dormer and the main roof.

Steps 10–11: Install transverse apron flashings at the change of pitch between the dormer and main roof over the fabricated flashings. Extend the flashing beyond the dormer roof at both sides. Lap an additional layer of wall underlay over all apron flashing upstands.

Step 12: Install roofing to the main roof and over the transverse flashing upstand above the dormer. Extend past the side apron, pull profile down tight and fix. Note that there will always be a gap created at the main roof cladding due to the need to fit it over the transverse flashing, which prevents it from meshing into roofing installed in step 3.

Note: Apron flashings must comply with the minimum upstand height and roof cover required by Acceptable Solution E2/AS1 Table 7.

Figure 3 Construction sequence for detail B – top junctions for a dormer on a monopitch roof. The side walls are clad with a sheet material and installed over a drained and vented cavity.

Some specifics from E2/AS1

Specific requirements of Acceptable Solution E2/AS1 include that there must be:

  • a 75 mm minimum wall cladding cover over the upstand
  • a gap between the wall cladding and the roofing as given in E2/AS1 Table 7
  • cover over the roofing as given in E2/AS1 Table 7 depending on wind zone and roof pitch.

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Figure 3 Construction sequence for detail B – top junctions for a dormer on a monopitch roof. The side walls are clad with a sheet material and installed over a drained and vented cavity.
Figure 1 Junctions at monopitch dormer roof.
Figure 2 Construction sequence for detail A – bottom junctions for the dormer on monopitch roof.

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