Green Oak Gazebo Design

[restorationcouple]

Hi All,

We are just about to start a timber frame project, a gazebo measuring approx 2.4x4.8, it will be a flat roof design with a green/sedum roof.

Therefore on the face of it a simple design/build, (hopefully). I originally planned on 6 150x150mm posts which would be set at the corners and centred, i.e all 2.4m apart. Looking at the design it would look much nicer if I could remove the centre post to open it up a bit on the long side/front and give more space for tables and chairs. As it is being built against a wall the other long side can be left.

My question to anyone in the know is, what sort of span can I expect from 150mmx150mm oak or 200x150mm construction grade oak? The plan was all 150mm oak however I could increase the horizontal timbers to 200m if I have to. The clear span would be 4.5m, however with the wind braces this is probably nearer 3.6m.

Is this viable? Many design by oak garage type joinerys seem to have similar spans but without a spam table it is hard to find any info.

 

[r896neo]

Your sedum roof is going to add a significant load and worth considering double checking this.

There are some structural engineers and other guys in the building section who will be much better suited to answering this for you. I'll ask the mods to move it. Again. Sorry mods

 

[restorationcouple]

Yes once saturated they can be quite heavy but the growing medium is fairly lightweight. The recommended load figure I have read is 120kg/m2 inc live loading if that helps at all?

I went to view the oak yesterday and I think after seeing the 200x150 beams they look overkill and also not very manageable to move around and erect. Therefore the plan stands at 150mmx150mm posts and beams with 75mm thick braces on each corner. If measuring the clear span between the braces rather than the posts then the distance is only 3.34m.

The build up on top will be 150mm treated joists cut on top to reduce to over the length 100mm, i.e 50mm drop over the 2.4m to create roof pitch. Then ply and EPDM with the plan to add sedum to the top of that, either at the time of building or the following months.

Sketchup design below to give an idea. The thicker oak at the top is just a fascia to trim the end of the joists.

Any input very welcome.

 

[CambridgeBrummie]

Quick calculation shows this is going to be deflexion controlled. A 200 x 150mm (D35 grade) is fine in bending but the beam would deflect by about 35mm, spanning the 4.8m length. The knee braces help but really just add some frame action to the connection in the corner. You could go with a flitched timber beam (ie a steel plate sandwiched between two timber beams).

The proposed rafters I would keep at the 150mm depth throughout and put firrings on top to create the fall.

 

[restorationcouple]

Thank you for this. Is it still possible with a 150x150mm oak or would 200mm be the minimum?

By the looks of a 5m 200x100 green oak beam I am going to struggle working alone in the construction phase! Otherwise I guess I am back with the centre post halfway along the span. Trying to keep it all pegged traditional joints so prefer not to go the for the steel plate route.

Makes sense to keep rafters at 150mm with firrings so I will allow for that.

 

[restorationcouple]

If looking to construct in 100% oak, could 100x100mm oak be used to substitute the 150mm softwood rafters? Really struggling to find any form of comparison between using hardwood v softwood.

 

[CambridgeBrummie]

It would help to know what timber grade your oak man can supply.
Sometimes these will be stated as Structural Grades, eg D24, D30, D35 or they may be stated using THA, THB, TH1 or TH2. THA Grade is equivalent to D40 but usually quite expensive. THB is roughly equivalent to D30 and is probably the one to go for, when working with smaller sections.

100 x 100 D30 rafters at 400 centres, spanning 2.4m will support the proposed roof load (taken as 150kg/sq.m all in and including imposed loading allowance of 75kg/sq.m). 100 deep x 75 wide also just about works.