[Closed] Any structural engineers in da house?

I'd agree with you. My only thought is that ply may support one rafter if it has a tendency to slip spreading the load onto neighbouring beams.

I'm a student civil engineer so only partially qualified to answer......but probably also more qualified to answer than many advice givers on forums.

The roof rafters needs boarded in to create what is in engineering terms a diaphragm. I'm presuming all tension forces normally dealt with by the omitted joists are now transmitted to within the gable ends - the forces are still there. For this to happen effectively the roof itself needs to be more rigid.

The idea is also (as I understand it) to remove any forces to the gable end as this is to be a glazed panel, so in theory, the roof is "self supporting" but I just cant see how it is supposed to work. Even the architect cant expalin how it works, and just told me "it does and thats all you need to know" !!!

Potentially the plywood deck is acting compositely with the rafters as a diaphragm/deep beam and causing the resulting beam to span between gables?

what is the detail at the ends for connecting back to the gable walls?

The thing about diaphragms is that they have to be continuous. Just sheeting it with 12mm ply might not be enough if the joints slip or open. Our standard spec for roof diaphragms is "2 layers of 9/12mm exterior grade plywood, joints staggered in each layer and lapped between layers. tack bottom layer to supporting structure, glue top layer in place then nail through both layers into the supporting structure 3.75 diam nails at 100crs, all sheet edges to be timber backed."

There is a longer version!

The second difficulty is that there is a spreading force in the roof that is contained by the diaphragm. This has to be resisted somewhere and it's usually through fixing the diaphragm panels to each other via the gable. If your gable is insubstantial then you may have a problem.

The third aspect is that you can't jack-in a diaphragm but it has to sag a bit to generate enough bending moment to work. If this sag is noticeable then the ridge may dip in the middle. If it does then you are stuffed because you can't just wind up a cable or tie rod to pull it in/push it up again.

Tricky things diaphragms.

Just fit a ridge beam instead of a ridge board. Rafters cant move down as they are supported at the top and bottom and therefore cant move out (spread). The diaphram would need to be able to transfer the force to the gables so would need to be fixed somehow which i doubt a typical builder would do adequately for the forces involved. Obviously dont just do this - get a structural engineer to produce the required calculations.

What he says. A ridge beam usually wins. It does depend on the angle of the roof though. If it's a really steep pitch your diaphragm has better chance, but I probably wouldn't go for it on less than 50-60 degrees.

The spreading thing happened on my mates roof while the roofers were loading up with tiles, took the top 6 courses away with it, luckily the scaffolding held firm.

got any pics?

the ply wont help downward pressure (well it may give a little extra depth to the rafer by acting compositely but that will depend on fixings) the rafters will likely be tied at the base and this creates a rigid traingle to deal with the roof loads. the ply helps robustbess of the structure to prevent lateral movement. i`m guessing that there are ties at the lower corners too providing vertcal wwall and crawl space behind?

lt may be the gable triangle has been designed to support a ridge beam?

there are loads of ways of doing this sort of roof detail. without seeing the calcs and scheme its difficult to say what your enigineeer has done.

Sillyoldhector - Member
. Even the architect cant expalin how it works, and just told me "it does and thats all you need to know" !!!

When someone say this it usually means they don't understand it themselves.