newbi here, with outhouse question

[mr. chips]

hi all my names Gary and i`m an alco---- sorry wrong place my names gary and i`m new here , i have been a chippy since i left school,

i am building a workshop of an octagonal design, the centre being the workshop the two ends open as smallish patio areas . The structure is on a 4 course b/wk and concrete oversite . I originally wanted to do this structure using all oak beams and featheredge. this was to dear ,way to dear, so i have settled for 6x6 treated posts and 6x3 timbers..

the idea is to have the 6x3 running around the the b/wk with the posts coming off theses at the corner points and 6x3 on top posts which will take the hipped roof and tiles..

Is the 6x3 laying flat ways going to be insufficient to take the roof load, the longest run without an up right will be 1.7 meters, I will put in corner braces..is this going to be ok.. i want flat as opposed to on edge as it will look right and aid in construction, or am i looking at this wrong?

also the angles are causing me a problem with the 6x3 plates can i halth lap them at the angles , and still tenon the 6x6 posts through it?

sorry if this is not making sense its driving me insane at the moment... :?

thanks for reading

 

[engineer one]

not sure about all the tech stuff, but there are a number of chinese type joints which involve 3 pieces being joined together with a through tenon.

basically i seem to remember the upright has the tenon, whilst the two other beams have the mortices but are each half lapped horizontally.

hope that makes sense. :?

paul :wink:

 

[JFC]

I suggest you look into timber framed construction , as a chippy you will know what you are looking for . 6x3 seems rather large as most old timber framed buildings in the UK are 4x2 with A frames and have lasted hundreds of years . Also why go for a tiled roof ? Shingles would cut down the weight and maybe ease your mind .

 

[mr. chips]

thanks for your replies
the reason i`m going for 6x3 with 6x6 posts is so i get an oak building look, and a nice meaty construction. the tiled roof is a must as it will match surrounding buildings ( i have to confess this is my girlfriends house , and she is a graphic designer.... need i say more :wink: ) i was quite happy with a 4x2 shed covered in featheredge and SQUARE :lol: why why why did i let her talk me into this...
oh well things are sent to try us...... oh and did i mention the workshop will have velux windows in .. :?

 

[jasonB]

Hi Garry, I think we have spoken on this subject elsewhere.

Half lapping the sole and head plates will be fine. Use the tennons on the uprights to "peg" the lap joints. I would keep the posts all aligned at 90degres to the structure rather than trying to make them fit the eight corners, will just make the corner brace joints interesting :wink:

Can you elaborate on the roof structure as you say its a hipped roof but an octagonal structure, do you mean 8 hips or will the roof be square and overhang at the corners?

Jason

 

[Andy Pullen]

From experience if i was going to do this anything other than four sided i would have an upright with rails tenoned into it. The upright would need to be cut to the correct angle of each corner. Tenoning into a corner post would mean the rail could be sat on edge for beam strength and also form a fascia for guttering etc to fix to if necessary. I have done it for porches aswell with the lap joints you mention but the timbers all tended to be square, ie, all 5x5's and you can bore and drive a large dowel thru your laps into the uprights with this method. Regards Andy.

 

[mr. chips]

Hi jason
yes we have spoken before in screwfix about this subject.

firstly i was unsure about using a tenon through the bottom plate to attach post, the top plate i can wedge through to fix in place, but unsure of fixing bottom, as structure will be in place..

the roof was ideally going to be hipped in four places.. the construction has a centre square of 3.4 x3.4 approx the two shaped end pieces are 1.5 wide, sought of like an elongated 50 pence piece.

i was going to keep common rafters to edge of square section then hip off the two furthest points- the pitch has to be 30 degree ( for the type of tile im using ) and this keeps structure to under 4.0 mtrs .

would using eight hips still look ok bearing in mind elongated shape, or am i correct going with the four?

so with this in mind can i use my 6x3 the way i hope which is flat ways on top of posts? like i said the furthest lengh they will be without upright support will be 1.7 mtrs

thanks for your time, sorry its long winded and probably confusing :wink:

 

[mr. chips]

From experience if i was going to do this anything other than four sided i would have an upright with rails tenoned into it. The upright would need to be cut to the correct angle of each corner. Tenoning into a corner post would mean the rail could be sat on edge for beam strength and also form a fascia for guttering etc to fix to if necessary. I have done it for porches aswell with the lap joints you mention but the timbers all tended to be square, ie, all 5x5's and you can bore and drive a large dowel thru your laps into the uprights with this method. Regards Andy.

hi Andy
think I got what you said.. so your idea would be to keep 6x3 on edge tennon into corner posts and dowel through..which as you said would give me ideal timber for guttering ect..
what about bottom plate this will be sat with the 6" on b/wk would you still tennon through posts this way? and i asume i would have to use two tennons on bottom 6x3 to be able to dowel through the 6x6 post into tenon. is this correct?

thanks andy

also to jason i like the idea of keeping posts 90 as this will look good but bracing may be - as you said- interesting...

 

[inventor]

Desperate for a sketch here, please!

If the rafters angle up from your beams towards the centre of the building, and aren't tied together by ceiling joists somewhere, then most of the force at the rafter-beam joint is outwards, not downwards. The roof will be trying to burst your octagon apart. Laying the beams flat would then be a stronger construction than upright, but I'd start worrying about the strength of the half-lap joints and tenons.

If the rafters are tied together with joists to make rigid structures independent of the walls, so the roof would sit nicely if you just placed it on the floor, then the force at the rafter-beam joint is all straight down, and the beams would be stronger sitting upright.

As to whether the beam is strong "enough", that depends on how heavy your roof is! How many tiles are there? How many windows? How much does a tile weigh? A window? Then we might have a chance of working it out!

Anyway, best of luck! Sounds fun!

 

[jasonB]

The problem with tennoning the heads into the posts is that by cutting a tennon you are reducing the cross section of the timber and therefore its strength. So if you cut a 100x50 tennon onto the 150x75 you will have less than half the strength.

Regarding the roof I would make it rectangular with a short ridge and 4 hips coming down to the corners that can overhang the octagonal frame.

Don't worry too much about pegging the posts to the plate the weight of the building will keep it down.

Jason

 

[engineer one]

jason, i was thinking about pagodas, and other such asian buildings which from pictures i have seen have these kinds of fixings.

there is even a famous wooden bridge in i think shanghai which relies on these joints and they are not even glued. :?

paul :wink:

 

[mr. chips]

inventor, i understand how my questions could make you want to throw a lump hammer at my head.. so here goes to best of my ability!

the roof tiles work out at 73.8kg per m2... i have a building aprox 6.5 x 3.5
the window has no info on weight, but its a velux 450x500 in size..

cheers

 

[mr. chips]

It did work but a better view is now below.
DaveL with his Mod hat on.
not sure if this has worked,
its the only picture i have so far. hope you now get an idea of what im trying to build
cheers

 

[mr. chips]

[oh wow..... that was supposed to be a link to photo bucket... but you guys have to suffer a huge pic.... sorry but i am new ....

 

[DaveL]

Mr Chips,

Well here is your picture, as a thumbnail

and I rotated it to stop cricked necks!
I have removed the link from your post.

 

[engineer one]

you guys must be careful with those legs on show, your' e not tommy walsh :twisted:

paul :wink:

 

[mr. chips]

thanks for sorting my picture out.. very clever... I have much to learn

many thanks

 

[mr. chips]

you guys must be careful with those legs on show, your' e not tommy walsh :twisted:

paul :wink:

we were trying to impress the sunbathing neighbour who was totally unimpressed

 

[inventor]

6.5*3.5 metres is 22.75 square metres. Add 50% to account for the hips (that's more than enough) and you have 34 square metres of tiles. This doesn't allow for the windows, which will reduce the weight, or the rafters, which will increase the weight. At 74kg/square metre, your roof will weigh about 2.5 tons!

For a regular octagon, each post-and-beam section supports one eighth of the weight, 315 kg. Say the post supports half this, and the rest is sitting in the middle of the beam, a very rough approximation. That's 160 kg pushing down in the middle of each beam, or two hulky guys. Take the roof off and have two hulky guys standing in the middle of the beam. Does it break? Have them jump up and down to represent dynamic load like wind. Does it break now?

Given that model, I think you'd be better off with the beams upright.

BUT still, what pulls the ends of the rafters together , stops the roof squishing down flat? If it's ceiling joists, fine. If you're relying on the posts and beams to pull the rafters together, that's just like an old building with a girding beam round the roof line. Failure of the girding beam joints is a classic structural problem in old buildings. The forces pushing the beams out are higher than the forces pushing them down.

PLUS: thanks for the photo! I see you have a brick foundation. Does that mean that this is meant to be approved by the buildings police? If it does, then show them a sketch of the design. They should tell you if your beams are too weak. I hope!

 

[mr. chips]

thanks for your reply inventor, I totally understand what you said.
i`m going to keep beams upright- i feel a lot happier with this- also i will use beams -like joists- to tie rafters together in the square section which will give better stability to stucture.
Jason mentioned the problem of using a tenon joint for beam to post, i see what he means , it will reduce its strength...have you any ideas ? i would need the beam to finnish flush with post, would a large tenon make such a big difference in its strengh?


and no this building is not being approved by the building police...
thanks for all replies, i have now got a better understanding on how i will approach this project
cheers