Build gable extensions and A-frame dormer floors

Released on 14 September 2026. Building extension remains an experimental dormer mode. These updates improve its model geometry; member sizing and structural connections still need project-specific design.

Start with a saved gable building

In Dormer, choose Gable, Wall Dormer and Flush Wall. Enable Building extension, enter Extension Length, position the preview and use Set roof opening. The host needs roof sheathing. Simplified roofing follows the opening; detailed tiles retain the limitations described in Dormer Framer.

Rafter W/H starts from the main roof’s rafter or truss top-chord dimensions. Edit it for this dormer, or choose Use main roof size to restore the defaults. Set dimensions before creating the opening. Changing the controls does not resize an already-created extension.

The purple extension preview follows the rafter roof planes. It is a framing reference: finishes and overhangs project beyond it. Building extensions automatically use a full-height opening frame.

Raise the extension walls

Gable building extensions have a Height above main wall (mm) control, starting at 0 mm. Increasing it keeps the original lower wall framing and adds a separate upper wall section above the main building’s top plates. The front and both side walls rise together, while width, roof pitch and extension length remain independent.

Upper studs follow the lower stud positions. At the main roof they are cut to the slope, and the wall finishes fit the roof junction and the main wall surfaces. Roof sheathing is retained where it supports the upper wall, without leftover sheet tails or unsupported truss connector plates inside the opening.

The preview’s H measurement shows the added wall height. Roof rise is the triangular gable height. An indicator shows whether the eaves align or how far above the main eaves they are. Align with main eaves resets the added height to zero. Raised fascia, gutters and soffits follow the extension roof and end at the main roof; at zero, the original connected eaves return.

Use zero or at least the minimum height shown by the tool so there is room for upper studs and top plates. Placement is also blocked if the extension roof would pass beyond the main ridge. Reduce height, width or pitch to fit. Regular dormers and Flush Wall dormers without Building extension keep their existing Face Height controls.

Set the height before creating the opening. To update an existing extension, return to a saved model from before its opening was cut, or regenerate the building, then create the extension again.

Plates, birdsmouths and aligned eaves

Extension walls inherit the main wall’s top-plate elevation and single or double plate arrangement. Double plates overlap at corners. Studs finish below the plate stack, and rafters receive birdsmouth seats on the upper plates.

At zero added height, side fascia, sheathing, simplified roofing, flashing and gutters meet the main eaves. The flashing lip points outward and the extension gutters connect at the junction. The resolved side overhang can differ from the entered value; Gable-end overhang controls the projection beyond the front wall.

Hide finishes to check the seats and shortened rafter tails behind the fascia, then show the roof layers again. Inspect both Roof Side A and Roof Side B. Regular gable dormers also receive the corrected birdsmouths and tail cuts. Flush Wall removes leftover truss eave pieces inside its opening; Closed Eave retains its intended eave structure.

Gable dormer gutters and flashing

New regular gable dormers, Closed Eave wall dormers and Flush Wall dormers now inherit gutters and hooks when Flashing and Gutters is enabled on the main building. Building extensions use the same inherited mounting. No extra dormer setting is needed.

The eave flashing drops slightly outward into the gutter and has a folded lip at the free gable end. Its drop is fitted to the actual gutter height, including raised extensions. Ordinary dormer gutters finish with a closed end before the main roofing; aligned building extensions retain their connected gutter corners. Guttered dormers also receive flashing when the roof has no battens.

Included in the 18 September 2026 update. Existing modelled dormers keep their saved geometry. Generate a fresh building or reopen a baseline from before the opening was cut, then create the dormer again to receive the new detail.

Framing where the roofs meet

Gable rafters continue on the building’s center spacing into the roof junction and become progressively shorter as the main roof rises. The old extra end pair is removed. Counter battens follow the current rafters through this transition.

Building-extension valley framing connects to the inner triangular opening support. Host top chords meet that support, and roof sheathing extends beneath the valley jack rafters with allowance for its thickness. Sheathing and roofing are trimmed at the interior eave, and panel cuts avoid visible internal triangulation lines.

Leftover blocking from trimmed trusses is removed inside the opening. Host rafters intersecting extension walls are seated on their plates. Trimmed bottom chords in projecting extensions have square ends and a perpendicular cap across the opening. That cap is a modelled connection, not a substitute for a supporting beam or wall designed for the project.

A-frame ceiling connections

For A-frame hosts with gable dormers, disconnected main-roof collar ties receive a plumb header across the triangular opening. It uses the main opening header’s section. Dormer rafter collar ties align vertically with the host collar ties so the modelled ceiling supports continue through the dormer.

These collar-tie additions are specific to A-frame hosts and gable dormers.

Flush Wall front plate and loft floor

A Flush Wall gable front wall over the main A-frame bottom chords receives square chord ends, a continuous bottom plate and blocking between the chords, including the edge bays beside retained trusses. This arrangement excludes projecting building extensions.

With Loft Floor Deck enabled in Build, the existing loft deck continues into the opening. New panels match its material and thickness, stagger their joints on actual bottom-chord centerlines and notch around retained posts, top chords and side-wall plates. Slight setbacks at retained boundary chord ends do not cancel the floor.

The sheets run beneath the front plate to the inside face of the wall sheathing. The plate and stud bottoms rise by the deck thickness while the stud tops stay fixed. The stack is bottom chords and blocking, floor sheathing, bottom plate, then studs. Without a loft deck, the plate sits directly on the chords.

Update and verify an existing project

Save a baseline before making openings. To receive these generation changes, return to the model before creating the dormer or generate a fresh building, then create the dormer again. Deleting a dormer alone does not restore the openings already cut in its host wall and roof.

Inspect the junction from inside and outside, confirm the floor and ceiling connections, and save/reopen a test project before continuing with detailed work.