What sort of joints can I use?

[mailee]

I am in the process of designing this gazebo type structure to go over a hot tub.

Can anyone help me as to what type of joints to use on the roof beams? As you will see the top square section is there to house a piece of clear Polycarbonate so must be kept clear of any obstructions from the roof beams. I am thinking along the lines of a mitred top square with birds mouth type of joints on the roof beams although I am not too sure of the strength in this instance. All the roof beams will be 2 by 4 and the uprights will be 4" square concreted into the ground. Anyone any ideas, please? :?:

 

[SketchUp Guru]

I think birdsmouths would be fine for a structure of this sort. What will you use for the rest of the roof? Shingles? Shakes?

 

[mailee]

Hi Dave and thanks again. On the surrounding roof it will be felt shingles. Not a lot of weight but enough to test the joints. I have been playing around with mitres and birds mouth type of joints and thought of using a half lapfor the mitre and roof beam along with the birds mouth as the weight would then press inwards onto the squareand lock the joint tighter. Just a thought but seems feasable. It will entail some pretty tricky cutting but I like the challenge. :roll:

 

[Shultzy]

Is this feasible - put the roof beams around the outside of the uprights and put the beams for the polycarbonate around the inside of the uprights.

 

[mailee]

?? Sorry Shultzy you have lost me there. Can you clarify what you mean? Thanks. :?:

 

[Shultzy]

After re-reading your post I've realised that I have misunderstood where the polycarbonate was meant to be. I, for some inexplicable reason (age :lol: ), thought it was going to rest on the beams at the top of the posts and not in the top square(what a muppet).

I would, as you say, mitre the beams at the top; but I would overhang the horizontal beams and cut halving joints.

I think 4x2 might be overkill as there is not much weight to support if you are using shingles. A hip type roof construction might be better.

As shown here
http://www.cwc.ca/products/trusses/hip_system.php

Calculations are here

http://www.blocklayer.com/Roof/RoofEng.aspx

 

[mailee]

Thanks for the info Shultzy. I am going to use the mitred top corners with half laps and birds mouth on the frame tops. I will use the hip roof construction method as it looks like this will be the strongest. Many thanks for the replies guys.

 

[Anonymous]

Just one thought ocurred to me, it might be worth considering putting in dragon ties at the corners to stiffen the roof structure. As theres no ridge board, theres no triangulation to give any stiffness, it might twist and heave a bit in a gale (its effectively a large umbrella type sturcture and needs some rigidity built in). I agree standard birdsmouth's would be fine, plumb, seat, plumb. I'd also consider putting angled brace's at the junction's of the posts and wall plate's, again to resist compression and tension force's caused by gale's. Also your wall plate looks a bit small in cross section, like it will eventually sag unless itts deeper and wider, or braced, or both, and possibly have more common rafter's as well or the roof will sag as well

 

[mailee]

Yes i agree it does need deeper wall plates Mr Spanton. After seeing it to scale I thought they were a little thin too. I do intend to add more rafters the picture was just to show the sort of design and ask about the joints. I am puzzled by the term 'Dragon joints' as I have not heard of them before, could you explain? There is also going to be some lateral bracing and triangular plates to the uprights of course. Many thanks for the comments and suggestions.

 

[Anonymous]

Hi Mailee

Dragon tie's are used generally on hipped roof's especially if they have a low pitch. They are diagonal brace's going across the corner's from one wall plate, to another adjacent to it at 90 degrees (similar idea to having small braces sinside a joyned chair frame to stiffen it). The hip rafter is joined to it also, this prevents the natural tendency of the structure to want to flatten out under its own weight and puts less strain on the actual corner preventing the wall plates from bulging apart. The roof load's (sometimes tons of weight) are consequently spread over a greater number of joints and surface area. Here is a picture that shows the basic idea

http://www.traditionaloakcarpentry.co.u ... enham.htm#

On this example the dragon tie is morticed into the plate's, but they can also be dovetail half lapped from above. Will yours be assembled with spikes or spax type screws or will it be jointed?

Cheers Mr S

 

[mailee]

Thanks very much for the information on the Dragon ties. I will use those I think as that will provide plenty of strength for the corner joints. I was intending to joint them as this would be stronger. Those would certainly spread the load fine.