Slab and frame design for a granny flat in Blackwall

The brief

Our client in Blackwall, on the Central Coast, planned a new secondary dwelling, or granny flat, on their block. They needed structural drawings for the concrete slab and the timber frame, to be read with the architectural plans and to go in for approval.

Why it needed a structural engineer

A granny flat is a new dwelling, so its slab, beams, posts and frame have to be sized to carry the building's loads into the ground. The drawings' general notes call for the work to follow NCC 2022 Volume Two and for the slab sub-base to be prepared to AS 2870-2011. Under NCC 2022, the ABCB Housing Provisions say in clause 4.2.2 that the foundations where footings and slabs are to be located must be classified in accordance with AS 2870. Class M covers moderately reactive clay or silt sites, which can experience moderate ground movement from moisture changes. For Class M-D sites, where the expected movement is deep, footing and slab design is beyond the scope of the Housing Provisions and AS 2870 applies.

Approval matters too. In NSW, a secondary dwelling may be built with development consent or as complying development if it meets the requirements of the Housing SEPP. The general notes on these drawings say they are subject to approval by the relevant local authority or council before construction, and that no work starts until written authorisation to proceed has been issued.

Structural plans for new builds, extensions and alterations are part of our structural design and consultation service.

What we designed

We prepared a ten-sheet structural drawing set: a cover sheet, general notes, a site layout plan, a foundation layout plan, section details, a roof framing plan, a wall bracing plan, two sheets of typical connection details, and a wall framing and lintel schedule.

• Ground. The foundation design is based on a presumed Class M site. The contractor should confirm this classification before excavation, and the design will be revised if actual site conditions differ. The general notes also ask the client to engage a geotechnical consultant to verify the foundation soil conditions before construction starts.
• Founding. All footings are founded at least 200 mm into the underlying natural orange, grey or brown silty clay with an allowable bearing pressure of at least 150 kPa, or into confirmed bedrock, unless noted otherwise. Ground conditions may vary significantly over short distances, so founding depths are to be adjusted to suit. If groundwater is found at the design depth of the beams or footings, the contractor must contact the engineer.
• Slab. The floor is a 100 mm slab on ground with SL82 mesh at 20 mm top cover, on a continuous vapour-proof and damp-proof membrane over 50 mm of sand bedding. The top of the slab sits 150 to 400 mm above natural ground level, varying with the site, over compacted fill. Three N12 trimmer bars, each 1500 mm long, reinforce each re-entrant corner. The sub-base is prepared to AS 2870-2011, and the concrete is specified at 32 MPa to AS 3600-2018 unless noted otherwise.
• Beams. EB1 edge beams run around the outside of the slab, and IB1 internal beams run across it. IB2 beams sit where one corner area of the slab steps down 70 mm. Each beam is 300 mm wide and reaches 300 mm below natural ground level, reinforced with L11 trench mesh, N12 bars and N10 Z-bars at 300 mm centres.
• Pier and posts. At the outer corner of the step-down area, column C1 is three 90 x 90 mm hardwood posts on a 590 x 230 mm brick pier (BP1). Each post is bolted with two M12 bolts at 60 mm centres to a 10 mm steel plate on a 12 mm base plate, and eight M12 chemical bolts with 150 mm embedment hold the base plate to the pier. Under the pier, the slab thickens to 300 mm with a 450 mm wide base, N12 bars at 200 mm centres and four N12 dowel bars into the pier.
• Roof. The roof trusses and their bracing follow the truss manufacturer's specifications, at 900 mm centres. Three runs of RB1 roof beam, each two 240 x 45 mm MGP10 members, are shown on the roof plan, and two of them meet at column C1.
• Walls and bracing. The framing schedule sets stud sizes and spacings by wall height, for example 90 x 35 mm MGP10 studs at 450 mm centres for walls up to 3000 mm high. Double studs (DS) of two 90 x 45 mm MGP12 are fixed to the bottom plate where the plan marks them. The top plate is two 45 x 90 mm MGP10, the bottom plate on the slab is 45 x 90 mm MGP10, and noggings are at no more than 1350 mm centres. The wall bracing plan places type B structural plywood bracing panels, each at least 900 mm long, with M12 threaded rods either side of each bracing panel and each opening. The site wind classification is N1.
• Openings. The lintel schedule sizes the window and door lintels and their jamb studs by clear span, up to 3000 mm.
• Drainage. The ground next to the building is to be graded to fall 50 mm over the first metre. Where that isn't practical, ag drains go beside the footing where the ground falls towards the building.

Mark	Element	From the drawings
EB1	Edge beam	300 mm wide, around the outside of the slab
IB1	Internal beam	300 mm wide, across the slab
IB2	Internal beam	300 mm wide, where the slab steps down 70 mm
BP1	Brick pier	590 x 230 mm
C1	Column	90 x 90 mm hardwood posts, three on the pier
DS	Double stud	Two 90 x 45 mm MGP12, fixed on the bottom plate
RB1	Roof beam	Two 240 x 45 mm MGP10
RT	Roof truss	Trusses and bracing to the manufacturer's specifications, at 900 mm centres

The timber notes call for structural timber to comply with AS 1720 and AS 1684, with timber fixing and bracing to AS 1684. Masonry is to AS 3700-2018 and steelwork to AS 4100-2020, and all workmanship and materials are to follow NCC 2022 Volume Two and local council requirements.

The result

Our client received a ten-sheet structural drawing set, issued for approval, with the slab, beams, pier, posts, roof beams, bracing and connections all detailed. Under its notes, no work starts until written authorisation to proceed has been issued, and the contractor should confirm the presumed Class M classification before excavation. The notes also call for the work to be inspected and approved during construction, with each inspection confirmed 72 hours ahead. If required, the contractor certifies at completion that the as-built works match the approved drawings.

Planning a granny flat on the Central Coast?

Our structural design and consultation service prepares residential structural drawings, including footings, slabs and framing. Send us your architectural plans and any soil or geotechnical report, and tell us whether you're going through a development application or complying development. We'll tell you what the drawings would involve and give you a no-obligation quote. Once building starts, our inspection reports and certifications service covers slab and frame inspections. You can request a quote online or call 02 4326 9026.