Thread: Is it safe?

It's a bit of clever timber engineering! Nicely done too.

The rafters must be full length and are cantilevered over the central hall / room and actually support the beam. The same may be done with the support beam, though there are not any photos to show what is happening with it. The interaction of both cantilevers (beam & rafter) also rely upon the outside wall resisting lateral forces that tend to push the top of the outside walls further outward i.e. increase the span. The design of the window mullions may provide minimal support as well, however they must be designed to permit deflection of the rafters under roof loads.

Somewhat like the kids seesaw game where a heavy person sits at one end whilst a lighter person walks the plank to the high end. This example is like two seesaws with the high ends fixed to each other, that joint in itself also contributes to the resistance of down load forces.

The internal walls support the download (weight) forces, i.e. the seesaw fulcrum, whilst the external walls resist the uplift forces of the tail of the cantilevered rafters (heavy person). The ratio's of the cantilevered rafter over hang to supported over hang is different from LHS to RHS.

Other examples of similar roof loads are "scissor trusses" which rely on lateral support from the supporting walls.


Calculating Cantilevers - Timber Trader News

It's still a popular design concept, this is a home constructed approx 12 years ago in TSVL -

With each 'half-rafter' there is about two-thirds of the length resting on the outside wall and inner walls, which leaves about one-third of the length over the central room (on each half).

The inner walls support most of the weight. The outside walls less so. Though, I take your point about lateral forces.

In this setup, the central beam seems to serve little purpose other than to locate the rafters. In fact, it could probably have been left out of the construction and the roof would be just as strong. Less weight on the walls too.