Build gable extensions and A-frame dormer floors

Updated on 11 October 2026. Building extension remains an experimental dormer mode. These updates improve its model geometry; member sizing and structural connections still need project-specific design.

Flush Wall siding, battens and rafter ends

With an ordinary gable dormer set to Flush Wall, simplified front siding meets the main-wall siding even when soffits lower the main wall’s finish. Front battens align with the wall below, with a 10 mm gap measured from the actual batten ends, including those above the sheathing joint.

Host rafters moved aside around the opening, and rafters inside the dormer’s side-wall thickness, stop behind its front face to prevent protruding tails. Regular, Closed Eave and building-extension placement keep their existing behavior.

Regenerate the building to apply these corrections. Existing saved geometry stays as authored; retain a project copy if you have made manual edits.

Rafter blocking and beam-roof openings

Gable dormers and building extensions have 36 mm rafter blocking on both sidewall top plates, matching the main gable roof. The blocks follow the slope and stop at the main-roof junction, including on raised extensions. They support Edit object.

On gable roofs with Beam structure, extension openings retain the ridge-side rafter pieces and clear disconnected lower remnants. Regenerate an affected design to replace older floating rafter geometry.

Closed Eave interiors

On A-frame/attic trusses, Closed Eave wall dormers share Flush Wall’s interior floor and bearing detail while retaining short rafter tails for eave support. With Loft Floor Deck enabled, floor sheets extend below the front bottom plate and studs. With the deck off, the plate rests on the bottom chords.

The retained tails meet the squared horizontal chords without overlap, and front wall battens stop 10 mm above the roof timber. Regenerate existing designs to receive the updated junction and floor. Supported framing and finish boards can then be edited individually.

Two-storey building extensions

A two-storey design can use a full-height building extension from the Dormers tool. It reaches the ground floor, joins the upper roof and extends both timber floor levels using their own joist/deck settings. Main-wall openings on both storeys must stay clear of its footprint.

The front and both sidewalls have top plates at each storey, following the main building’s single/double arrangement. Upper walls have bottom plates on the deck. The ground-floor bottom plate is removed across the connecting passage; the lower top plates supporting the intermediate floor remain.

Ground- and upper-storey opening jambs follow their own storey bearing. October 2026 update: building extensions keep regular opening headers and sills, with no additional load-bearing headers or their stud notches. Side top plates reach the front timber face wherever the front plate was removed; existing corner laps remain where a front plate is present. Regenerate from the saved design to apply this correction. Successive windows and doors cut the remaining studs, battens and panels without filling earlier openings back in. Hide siding and sheathing to inspect all openings after generation.

At the inside corners, the main-wall siding and sheathing stop at the extension sidewall finishes. An additional backing stud supports each trimmed sheathing edge at every storey. These finish/backing corrections also apply to one-storey extensions.

Separator boards use the main building’s Flat or Capped profile, with matching mitres at both inside and outside corners. Raised extensions stop the side boards at the main wall face. Full-height extensions currently use the upper-storey siding finish over their entire wall.

Regenerate a saved design to apply these geometry changes. Opening a saved model alone preserves its existing geometry. Save a project copy before regeneration if it contains manual 3D edits.

Wall sheathing

Dormers and building extensions use the main wall’s sheathing material, thickness and panel dimensions. This includes the front gable, both sidewalls, regular and wall dormers, shed dormers and paired layouts.

If panel width fits the building’s CC spacing, vertical sheet joints align with stud centres. For example, 1200 mm sheets fit CC 600 mm; the first and last sheets may be cut to reach the wall edges. Incompatible widths keep their natural layout: 1300 mm sheets at CC 600 mm are not shortened to 1200 mm.

Sheets are cut to the wall and roof outline, with additional rows when the wall exceeds the main-wall panel height. Raised sidewalls use a continuous layout across the main-wall junction. Window and door openings are cut into the sheets, and the cut list reports the remaining area. Regenerate the building or dormer to apply these changes; existing saved geometry stays as it is until regenerated.

Simplified sidewall siding

October 2026 update: Building extension sidewalls use a full simplified siding panel across the wall and raised roof junction, so the siding pattern no longer restarts at a vertical panel seam. Roof cuts and the connection to the main wall stay fitted. Two-storey separator boards still divide the finish into upper and lower wall panels; sheathing underneath keeps its stock sheet sizes.

Regenerate the building from its saved design to apply this correction. Check both sidewalls above the main roof and confirm the horizontal courses continue across the old seam. Existing saved geometry does not update automatically.

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

Extension opening correction: single-storey extensions now cut studs and battens left by earlier windows or doors on the same wall. This fixes header studs remaining across loft windows above a previously placed door. Cuts are also retained when the upper window is placed first, and aligned windows share identical jamb studs. New jambs respect existing headers and sills, including staggered windows. Cut battens keep their shortened lengths, and overlapping copies retain only the missing lengths. Regenerate the building from its saved design to apply the fix to existing geometry.

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 front-wall studs follow the lower stud positions. Upper sidewall studs follow the building’s CC grid from the extension front, aligning with the extension rafters across the main-wall junction. 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.

Each upper sidewall top plate continues from the extension front to the main roof slope, with no plate joint or extra end studs at the main wall. Double plates retain their two levels and front-corner overlap. Sloping junction bottom plates extend down onto the uppermost plate of the lower wall, with bevelled upper ends fitting beneath the upper stack. Stud bottoms follow those fitted plates.

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.

Windows in raised extension walls

Sloped windows and battens: building-extension CC battens now stay continuous through intermediate floors and top plates, with cut ends meeting the sill and header battens. Sloped-window side battens stop near their sill instead of extending down to the loft bearing. Sloped opening cuts also clear sheathing sheets with a stepped wall/gable outline. Hide siding to inspect both upper window openings, then hide sheathing to check the battens. Regenerate the building and openings from the saved design to apply these corrections.

In two-storey extensions, opening jambs and headers stay within the window’s storey. Loft windows above the upper plates omit the extra load-bearing header. Regular dormers and single-storey extension windows also keep only the regular opening header.

Gable front studs continue to the rafter underside with fitted bevels. Gable fronts omit horizontal dormer collar ties and front top plates, except for the two-storey extension plates described above. Sidewall plates, main-roof collar ties and Flush Wall floor supports remain. The Openings preview allows gable front windows higher into the triangle when framing clearance remains.

Existing studs crossing a window are replaced by short cripple studs below its sill and above its header. Inspect stacked and side-by-side windows with finishes hidden, and regenerate affected older models to receive the updated framing.

Boundary rafters after generation

Both boundary trusses beside an extension now appear after the Visual Build Generator finishes. Previously, a newly copied truss could retain a hidden state from the build animation, making its rafter appear missing on one opening edge. The opposite edge could look correct because it reused an existing truss.

Included in the 21 September 2026 update. Regenerate affected buildings to apply the correction; existing saved models are not automatically repaired. Check both opening edges after generation and after moving the build timeline backward and forward.

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 and Closed Eave front plate and loft floor

A Flush Wall or Closed Eave 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. Closed Eave retains its short support tails outside the wall.

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.

Openings in extension sidewalls

October 2026 update: In Build > Openings, choose Extension left sidewall or Extension right sidewall in the Wall menu. You can also select the sidewall in the miniature navigator or place a preview directly on the exposed sidewall in the Left/Right elevation.

Use the usual window, door and garage-door tools, door styles (including sliding patio doors), presets and dimensions. Distances run from the left edge when viewing the selected wall from outside. Upper-floor openings use the same height guides as the rest of the building. Raised cheeks above the main roof are also part of the sidewall; an opening must fit entirely inside the wall outline. Regular gable and shed dormers retain their front-wall opening targets.

Opening studs, headers and sills follow the sidewall’s timber section, including 48 x 148 mm walls. Sheathing, siding and battens are cut with the opening. Simplified siding keeps its roof-junction outline after windows and doors are added, so it does not extend into the main building as a rectangular panel. Regenerate an existing design to apply the timber correction; opening a saved model preserves its existing geometry. Save changes as a new project copy when retaining manual edits. New saves use .strek V29 and need a compatible CAD reader.

Hip dormers and extensions

The Build generator supports dormers and gable building extensions on Hip roof sides A and B. Choose Build > Dormers, then add a gable dormer, shed dormer or building extension. Keep roof sheathing enabled and leave room between the new roof and the main roof’s diagonal hips and ridge. Move or resize a design if the boundary message appears. The two end slopes are not placement targets. Existing gable and A-frame options retain their behavior.

Hip extensions now trim the main rafters to the opening framing on both roof sides and remove gutter/flashing outlines across the opening. Regenerate an affected design to replace previously generated long rafters or stray lines; opening a saved model retains its existing geometry.

For raised Hip extensions, the current version removes the thin sheathing strip that could remain beneath the main eave. Regenerate the building to apply this correction; sheathing beside the opening remains in place.

Align eaves now matches the Hip roof’s fascia, roofing edge and flashing height, bringing the gutter rims together. The extension stays seated on its wall plates, with its side overhang adjusted to meet the main eaves. Raised extensions keep their chosen height. Regenerate the design to update previously generated eaves.