Roof Span, Load - Gable

[Nate B]

Hi,

Roof is covering small 11x13 outbuilding. Slope is 4:12. Using 2x8 ridge board and 2x6 rafters.

No problems with the rafter trig - got that covered. No problem with the span of the 2x6 rafters either. A mere 11 ft / 2 = 5.5 feet (roughly) - all to code. Overkill, but I want an R-19 roof.

My question is this: I've looked at a bunch of codes, and I can't seem to find anything that covers the span of the *ridgeboard*, especially as it applies to the roof slope. I'm certain the ridgeboard can not span as far on a 4:12 as it could on a 6:12, for example.

I'm probably overkill like the rest of the roof, but I'd like to know for

100% certain. Snow and 100 mph wind load codes apply at this location.

I know a steeper roof would be more bomber, but I want to match the house.

Any references?

Thanks,

- Nate

[Harry K]

I don't think there is a code for it. The 'span' in a properly built roof is only from one rafter to the next. Collar ties (or joists) keep the walls from spreading and the rafters support the ridge board.

Harry K

[Paul Franklin]

I'm not a structural engineer, but...

What Harry says is true unless the OP is planning a structural ridge, meaning no joists or collar ties. In that case, I believe the ridge supports one-half the total load of the roof, and the walls support the other half. Posts under the ends of the ridge carry the load to the foundation.

A steep pitch increases the total weight of the roof system (because of longer rafters and more sheathing). The snow load stays the same, so the ridge carries more load (in the case of a structural ridge).

HTH,

Paul

[Nate B]

(roughly) - all

So many books show a gable roof with a ridge beam, that isn't really a ridge beam kind of roof. I guess this is one of those things where there isn't really a cookie cutter standard design one can follow.

I'm probably way overbuilding the thing, but I've decided to add in some 2x6 ceiling joists, providing a flat ceiling about 9 inches above the top of the framed wall. It will look like a capital "A", with the same 4:12 pitch. I was shooting for a cathedral roof, but that's not to be at this stage. This will be a best guess at an engineered truss design, I suppose.

- Nate

[Nate B]

"Paul Franklin"

Okay - did a quick redesign. This is what I have going on now:

Still using a ridge board. Probably should just build these things on the ground and skip the ridge board, but the former sounds like a little bit less work. We'll see.

Thanks,

- Nate

[Paul Franklin]

Ok, now you've got collar ties, and you don't need a structural ridge. The collar ties counteract the force that would otherwise try to push the walls out due to the roof load. Using collar ties instead of joists will give you more headroom.

You are right that now you don't need a ridge board; you could build them as trusses if you like.

Just remember that the collar ties will be in tension, so overlap the ties and the rafters and nail them off good.

It will be fine, it's just not that big a roof!

Good luck and have fun,

Paul

[Nate B]

"Paul Franklin"

Probably squirt some PL100 in there, too.

Like playing with Tonka toys. Fun stuff.

- Nate