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We just bought a California bungalow in Melbourne dating from about 1930s with subsequent extension out back and second floor. The second floor gives me easy access to the roof space above the old part of the house and I would like to use it for storage (ideally somewhere to store my tools rather than a shed in the small yard). How do I estimate the load the existing joists can support with a yellow tongue floor and, if it is borderline, can I stiffen the ceiling by attaching new timber lengths to run parallel to the existing joists? The ceiling joists appear to be original hardwood 90 x 35 at 450 centres and the widest span is 3.7m (over a living room), narrowest is 1.5m (over passage). I can't see any damage to the joists and there are few nails / screws in them. The ceilings are lath and plaster with so any new parallel timber would need to be less than 90mm deep to avoid tapping out the plaster dried on top of the laths (and risking damage to the ceiling below). As part of a check and rewire, I had an electrician tidy up the cabling and J-boxes so there are no services in the area. Gut feel is it should be OK over the passage but will need stiffening over the rooms but don't want to rely on that alone.
Thanks in advance
Mick

Woodsolutions Answer +

A few assumptions would need to be made to assess the load carrying capacity of the ceiling joists, and the assumptions would need to be on the conservative side since we don't know the type or grade of the timber. A lath and plaster ceiling is already quite heavy - possibly as much as 0.5 kN/m2 (50 kg/m2). A simpler approach would be to ignore the ceiling joists and run new timber parallel to the joists, supported clear of the ceiling, and projecting above the joists so that the storage floor is completely independent of the ceiling structure. Then you would not have to worry about any deflection that might occur when the storage area is loaded up with tools, possibly damaging the old ceiling, given that the joists are already quite heavily loaded. 

Answered on 24-09-2020
Please note that our answer is based on the best advice available at the time. If the National Construction Code, Australian Standards or local requirements have been subsequently amended, our answer may no longer be correct in all details. For more information, please read our disclaimer.

Question

AS 1684.4 (2010) Table A1, Page 108.
It has a few alternative sections, i was wondering is it OK to substitute in REVERSE on this table.
For example - I have a span (from one of the other tables) that ends up requiring 2/120 x 35. Can I then use Table A1 and substitute this for 2/90 x 45.
I understand it's meant to be used the other way, if I need 2/90 x 45 I can substitute this for 2/120 x 35. but can i use it in reverse?3

Woodsolutions Answer +

A section size made up of 2/120 x 35 (effectively 120 x 70) doesn't have the same structural properties as 2/90 x 45 (90 x 90). It's all related to the moment of inertia. The moment of inertia is a measure of the efficiency of a shape to resist bending caused by loading. A beam tends to change its shape when loaded. The moment of inertia (I) is a measure of a shape's resistance to change. It's calculated by the formula I = 1/12 x bd3, which shows that the depth (or height) of the cross-section is more significant than the breadth or thickness. In the case of your two sizes, 120 x 70 has an I value of 10.80 x 106 mm4, but 90 x 90 has an I value of only 5.47 x 106 mm4. You will find this explained in more detail on the net here  https://emedia.rmit.edu.au/dlsweb/Toolbox/buildright/content/bcgbc4010a/03_properties/02_section_properties/page_006.htm

Alternative sizes

Answered on 23-09-2020
Please note that our answer is based on the best advice available at the time. If the National Construction Code, Australian Standards or local requirements have been subsequently amended, our answer may no longer be correct in all details. For more information, please read our disclaimer.

Question

We're building a 2 story house. The span tables for N2 stud walls only go up to 4.8m, whereas the atrium walls will extend to approximately 6.1m (including top and bottom plates). The walls form a corner in the north east of the building and the north wall has a 3.6m x 1.2m vertical window spanning both floors.

Ideas we've considered are:

1. Make the walls full height (approx. 6m walls) - should the stud sizes be upped (eg. 90x45 or 2/90x35 or 2/90x45)?
2. Extended the trimmer joist (and separate ground/first floors) on the wall without the window to the north east corner, and make the wall with the window full height - should the stud sizes be upped (eg. 90x45 or 2/90x35 or 2/90x45)?
3. Extend trimmer joists for both walls to the north east corner, but leave a section of the wall frame (both ground and first floor) absent, then retrospectively cut the section of joist in A and slot in a full height wall frame consisting of window lintel / jambs / trimmer / jack studs / etc. between 2 sets of 6m king studs.

How do we build these walls and ensure they are compliant?

Woodsolutions Answer +

We feel your design is beyond the scope of AS 1684 Residential timber-framed construction. It's correct that the tables for studs only extend to a height of 4.8m, but that assumes the studs will have no roof load. Studs up to 4.8m are included to cater for internal walls in houses with cathedral ceilings. There is an overriding height limit of 3.0m on external walls in the AS 1684 span tables to ensure that assumptions regarding racking and overturning forces due to wind load, and uplift at the ends of bracing walls, are not compromised, although a guide document exists to extend external wall heights from 3.0m to 3.6m. The corner of the house that is 6.1m high will definitely need special design, either using multiple studs nail-laminated together or engineered products such as LVL's. We suggest you contact a structural engineer to prepare a design for you.

Wall studs

Answered on 09-09-2020
Please note that our answer is based on the best advice available at the time. If the National Construction Code, Australian Standards or local requirements have been subsequently amended, our answer may no longer be correct in all details. For more information, please read our disclaimer.

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