Forces in a gambrel roof

Symmetrical loading won't impose partucularly large forces.

But the gussets can take it.

In essence consider whether the gusseted rafter would break at the gusset or elsewhere under load.

Once the answer is 'elsewhere'. its strong enough :-)

You could tie the top section for extra strength.

And put in verticals in other places. The ply skin adds a huge amount of strength in all other directions

However in this case I would pay a structural engineer to sign off any plans

Then you can sue if it falls down

That is certainly possible.

Not possible here as they would become columns to the floor.

Yes.

Possibly metal gussets would be an option - very very strong...

I either have a bookmark (or an open tab hiding amongst the others) for sketchup plugin that did calculations for various roof/truss types, I'll see if I can find it ...

So your 'roof' would actually for part of the shape of the structure, rather than being a roof over an existing/separate floor/roof, as per the example in your link?

I ask because I assumed the 'tie' would have been provided by the ceiling of the existing structure and with only 2.5m high to the top ridge I'm assuming you would have to put ties across at maybe the first joint (as TNP suggests) to be able to store stuff in the roof in any case?

Basically I's imagine that unless the gussets were 100% strong enough to stop the base of the roof splaying under *any* (worst case) load, then you would probably need ties somewhere?

Interesting project idea / project though, I could do with more space in my garage / workshop roofspace but with the walls already 8' high there isn't much room for a gambrel roof (and stay under the permitted development limits). ;-(

Cheers, T i m

Cool - that would be very interesting Andy :)

Description say it does gambrel trusses, not tried it

In the wikihow, the top of the roof was way above the top of the walls. So, how will that work if you are going to be limited to 2.5m total height? Will you have dwarf walls, with almost the whole structure being roof? The height of the roof part is a function of the width of the building and the angles used, but it looks about half to two-thirds of the width being spanned.

So, if your workshop is 5x3m, your roof height is around 1.5-2m, leaving you not much vertical wall at all, and much of the interior with reduced headroom.

Won't it look a bit like a Nissen Hut? :)

Personally I would glue the gussets and fit them with 6 mm coach bolts. Then they are going to be stronger than wood.

Yes - very typical 4x2" framed shed sort of thing :)

I think ties at the top level will work well - will be around 2m off the floor.

It would be ties at the base of the roof that would be hard - although a tension wire hooked up to the ridge and down the other side could be an option (sure I've seen that in a real large roof somewhere once).

That's what I'm wondering. Or buttresses outside (that would look a bit heath robinson!)

I'm building on a slope - the PD tech notes say measure from the highest local ground to the highest point on the building.

So I might be able to buy a foot or so legitimately. It was my plane at least to dig it into the ground a bit so the floor structure was lost downwards and the entry was dead level with the lower part of the hill.

Super - thanks very much :)

Yes. Walls would be reduced to suit - and the roof would not be that extreme. Depending on the rood covering options, the top slope might be anywhere from 10 deg to 22.5 deg.

And the sides 45 deg to 80 deg.

Theer are some nice self bonding plastic "slates" thate are certified down to 10 degrees pitch but only up to 45 degrees.

There are some more normal synthetic slates that I think are OK down to

I hope not :)

I reckon this is a non starter for 2.5m ridge height. Note that the wiki article shows a flat deck on which the beams bear. this is critical to the strength as it will be in tension to support the roof weight. Even if you replaced this with horizontal joists they would end up around chest height. If you want to stay within PDR then there is little option but to have a very very low pitch roof which rules out most tiling systems and you possibly will have to go with EPDM rubber which is practical but not pretty. Bob

You can use tiles at a very low pitch if you fit asbestos or similar first. Joints must overlap.

NT

I suggest spending a lot of time producing visualisations of the end product, to make sure you are completely happy before you build it. With the lower sections at 80 degrees and the top sections at 10 degrees, it may just look like a flat roofed structure put together by someone without a spirit level. :)

Its probably stronger than a straight rafter design in some respects since you are approximating a more "arch like" design.

It will make the internal space a bit more usable. (try and avoid having any horizontal timbers at eves level that are less than 8' from the finished floor. Being able to get full sheets of ply through on end really helps. (and having "high" areas where you can poke longer stuff up is even better).

I'm starting to think you're right Bob.

It's either that or sink the structure into the ground a bit to gain some extra height - but there's a limit as anything other than a gentle ramp down isn't a great idea for a workshop.

I've got an awkward corner plot with the house set back to boot - that gives me two front elevations that I cannot build beyond (PDR) and forces anything else to be near a boundary.

I am glad the old 5m from house rule was scrapped!

I also have a shared sewer I need to stay clear of (I'm going to sink footings about 1.5m away and at the same depth or slightly lower and have about 50cm of overhang on the base (cantilever).

The old church builders wanted to avoid beams halfway up the height of the nave. Besides that, they were building in stone, which is not good in tension. Their solution was buttresses, and a similar idea would work with your shed. :) (Not seriously.)

Basically, Tim, what you are gradually working out from first principles is why most sheds have a roof with a very low pitch, and covered in roofing felt. It's simple, it maximises internal space, and it's easy to build.

I helped put up an apex roof type wooden 10 x 6 shed the other day that was modular (5 x 6 x 2) and the middle truss was, useless. If you put it in place and put any weight on it you could feel it sagging / collapsing. It wasn't even the same profile as the two ends of the building and they stated in the instructions that the roof wouldn't carry the weight of a person.

We rebuilt the truss (properly, with a tie either side and at a lower level) and daughter sat on the roof putting in the felt tacks and it didn't move at all. ;-)

Yes, a pair X ties like that can work well. Isn't triangulation clever. ;-)

Well, they could be made to look ok, many Churches have them as features after all. ;-)

So worst case for you then. ;-(

Yes, I think keeping something as low as possible (subject to drainage and holding back any cut-away land etc) all helps it to become less visible. ;-)

Cheers, T i m

The construction method is similar to my ply on oak framed boat, which uses 2 gusset plates at each joint epoxy glued and copper riveted through.

What surprises me is why the longitudinal ridge piece (I don't know names of building components) is not one or more long pieces of wood notched into the frames at the ridge. The graphics show short lengths fitted between each frame with no indication of what holds them in place.