"Top down" design & construction.

What does the term "top down" mean? Well, simply it is a design methodology that refers to designing your layout conceptually and then designing the roof and detailing the building working down to the foundations.

Why do we do this? It's a really good way to understand if your building is going to be buildable structurally (load path transfers), how critical junctions might work and if watertightness of the building envelope can be easier. It is a process in high-rise construction also, where basements are built top down while the tower could be built above. It's a way to expedite construction of a high-rise using a contiguous pile or sheet pile system (with columns and beams around).

Now, the reason I'm raising the subject of top down design to the table is because too often I see homes being designed in a way that treats the roof as an afterthought. And this is especially common amongst those "back of the napkin" or sketchbook type scenarios and what happens is, younger designers especially will match your layout as close as possible but create all sorts of headaches for the builder at the roof and at critical junctions. There are plenty of requirements, in particular around roofing design, that get missed in home design and cause problems later. Worse still, they get missed by the Building Surveyor.

Those issues can be:

• Valley drains not meeting minimum pitch levels.

• Intersecting elements that shouldn't (gable overhangs to falling hips, etc)

• Soaker Flashings inadequately supported, especially pressure flashings.

• Flashing upstands not constructed to complying heights.

• Box gutters with no sump, or non-compliant overflow volumes.

These are a small sample of issues that can occur if you don't take a "top down" approach to designing a home. There are many others, and this is why my advice to people who are scratching up home layouts, or owner building is to get some advice from fully qualified professionals who are registered or licensed in the field, or tradespeople with many years of success behind. It could be the difference between a well-designed, well built home that will last a long time and the Epic Fail, which costs you thousands of dollars over the builds lifetime.

Signing off,

Revell