Roof wobbles!

G'day all,

I've been building a deck for the past year and it got to a stage where it was serviceable - I could sit on it and enjoy a beer - so work stopped. Eventually, my wife convinced me it would be in my interest to get the thing finished so reluctantly, I started building again. My next move was to get some trusses made and, again at my wife's insistence, I got someone to install them for me. I'm very pleased with the result but there is one problem and that is that that you can push on any of the supporting posts and get a bit of sway going on with the roof. I think part of the problem is the trusses that I had made not having a bottom chord lying on the top plates or roof beams - the plans call them roof beams, everything else calls them top plates. The beams are 225 x 70 x 4000. Posts are 100 x 100 all ironbark.

I have attached a photo of the deck and trusses as well as a plan of the trusses. I had the trusses made the way that they are because I wanted openness on the deck and don't really want to lose that. The lower part of the deck is attached to the house and the posts are single posts down to the ground.

Does anyone have any suggestions on what I could do to brace the roof posts to stop the wobble without losing the openness? It was suggested that I run bracing diagonally between the posts in the horizontal plane (tops of the posts) but I can't see how that would work. It would also spoil the open feel of the deck. I would go back to the guy who drew up the plans but he is a draftsman not an engineer. I want the thing to be structurally sound as well as to look good! Any suggestions would be appreciated.

Thanks
Boris.

Need more pics of the supporting frame really, but from your description and what I can see in that one pic your problem is the framework on which the trusses are sitting not the truss design. You have properly speed-braced in both diagonals on both roof surfaces.

For the trusses themselves you still need to brace end to end (better on that design to run two evenly two centred between the centre strut and the junction of the top chord and the bottom chord) across the top of the bottom chords so there is no sideways movement possible between the trusses. You can use timber for that or speed bracing - IMO timber would look better - you only need 70x19. The pic seems to show triple grips attaching the trusses to the beam and it looks like just one per truss end - if that is so then you should add another one on each side and make sure they are fixed according to spec.

I have to say that the posts look undersize for that structure too - 100x100. Even in good hardwood I would be looking at 140x140 or larger for that height and design. With those high and smallish posts and no angle bracing you will not only have the wobbles, but in a high gusty winds you might find that it will move sufficiently to collapse. You need to brace those tall upright posts ASAP. The best technical solution would be top to bottom both directions at a diagional, but that will look ordinary and impede access too. Bracing below the deck will help the deck stability, but will not fix this problem.

The compromise would be to do 45 degree bracing part way down on each post at all corners and as low down as you can go without the braces being a danger for unwary heads. The appearance you will have to put up with. You could use the larger size patterned bracing brackets (they must have 45 degree support incorporated - triangulation is what braces and resists forces) that can be found in timber, steel and cast aluminium, but the need to be well attached with coach screws or bolts

From your pic I would also be concerned that you have no cross bracing at each end on the posts. There will be considerable outwards pressure under wind loads especially from the truss structure on the beams and I can't see where that will be resisted in the construction so far other than through the posts themselves - and that is not sufficient.

The crude test is pretty simple - a structure like that should offer complete resistance to movement from the force applied by a single person ie: if you try to induce movement by pushing or shoving or rocking back and forth etc then you should be unable to feel any movement. Just as if you walk or do normal activities on your deck you should not feel it move or flex.

You should get a builder engineer to have a look and give you some advice - in any case that is a structure that requires approval in any state so you will need to comply with the BCA and get the plans and work approved.

If you choose not to have it approved then for your own safety and that of family and friends or any visitors at all, you need to get it braced properly. That's not a risk I'd take with a structure of that scale.

Thanks for the comments Bloss.

The plans were aproved for construction with 100x100 posts. The only thing that I changed from the plans was to use trusses. The trusses are attached by tripple grips on either side. I was thinking about adding another 2 on the the other side of the top plate although I didn't think that was going to stiffen things up. It isn't going to hurt though.

I think I will take your advice regarding an engineer. I expect they should know better on what to do.

Boris.