How to cantilever a flush beam stair landing?

[Avago]

Hi, thanks for any advice you can give me regarding my project.

I require a lower landing to accommodate some stairs that are to be fitted to an external deck.

This landing, I feel, needs to be a flush beamed so that minimal disturbance of the natural slope of the land and proximity to a Gum tree. 

I would prefer to place posts (single span) as far away from the tree as possible. However, I am limited to the actual position of the landing itself, thus I am considering an allowable overhang on the long edge closest to the tree.

The landing frame is likely to be 1400 x 915, and I am considering using 90 x 45 H3 Treated Pine for framing and joists.

If I placed a suitable size bearer under the frame on the long edge closest to the tree, do I need:

1) A third post on the long edge with no overhang due to the span of 1400mm?

2) Noggins between the joists on top of the bearer?

3) Additional support for the stair stringer placed on the overhang? The foot of the stringer is 200 mm long. I was considering placing the first joist from the short edge ~ 200mm, then a further two joists equidistant to the opposite edge.

[JacobZ]

Hi @Avago,

Due to the complexity of the situation, where you are combining a cantilever, as well as having the stairs land on that cantilever, it would be best to speak with a builder or engineer to confirm that your plans are practical and safe.

Having said this, you might like to check out the advice I've given in What spans and cantilever for deck for an idea of what the rules are around cantilevering joists.

If you are using 90x45 as your joists, then they have a maximum single span of 1400mm. This means they can have a maximum cantilever that is 25% of 1400mm, which is 350mm. This is provided that the backspan is at least double the cantilever, so the joists must span at least 700mm to allow the maximum allowable overhang of 350mm. 

In saying this, we generally recommend that cantilevers do not exceed about 300mm without sign-off from a suitably licensed person, such as a builder or engineer.

If you were to position your posts 610mm apart, you could reasonably cantilever your joist out 300mm, as long as all joists were connected to the outside bearer with Joist Straps.

You could use a 90x45 for your bearers, but as the maximum allowable span is 900mm, you would need a third post on each bearer. I think a better option is to use 140x45 for your bearers as they can span 1400mm. 

Yes, I would suggest that you attach noggings between the joists relatively close to the top of the bearers, as long as they do not interfere with the joist straps.

Any additional reinforcing you could reasonably fit in to support the stringer, such as doubling up the joists or adding extra noggins, would certainly be valuable.

After finalising the plans, again, I would suggest you speak with a builder, engineer or other suitably qualified person to confirm that they are both practical and safe.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob