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Gambrel Roof Design Options

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    Posted: 06 Mar 2010 at 1:50am
I've spent a few hours over the last week trying to finalise the Gambrel Roof and Storage loft Design for my new workshop.

The American book I've been consulting (One of Taunton's 'Build it like a Pro' series ) shows the outer part of the roof at an angle of 54 degrees with the shallower centre section at 27 degrees.

Unfortunately I've found that without reducing the wall heights i cannot achieve these angles at the span I want.

My first and, in some ways, aesthetically preferred design is as below:



Unfortunately this gives a very shallow centre section pitch of 19 degrees. Using bitumen tiles I should be all right so my roofer Son in Law tells me as they can even be laid flat. I'm a bit concerned that a repeat of snow like we had this winter,(or worse) might put too much load on the structure. What do people think.


Reducing the eves height but not the walls below the max permitted 2.5 m gives two other possible designs which I have combined in the following illustration:

I'm very committed to the gambrel roof idea as it maximises the storage space. Since the maximum possible volume for the loft would be a half cylinder the left hand design represents the nearer approximation to this.

Any advice / thoughts very welcome.

Ray

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Turnpike View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Turnpike Quote  Post ReplyReply Direct Link To This Post Posted: 06 Mar 2010 at 1:41pm
8 lookers but no advice?    I need help here people!
 
 
Ray
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pfeil it View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pfeil it Quote  Post ReplyReply Direct Link To This Post Posted: 06 Mar 2010 at 1:52pm
Hi ray
you have cracked on if your ready for the roof to go on well done mate.
mine,s all but finished just a few bits n bob,s to do
regards ian

Edited by pfeil it - 06 Mar 2010 at 1:54pm
just another pfeil chisel wont break bank
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Turnpike Quote  Post ReplyReply Direct Link To This Post Posted: 06 Mar 2010 at 2:17pm
No.... I'm not as far on as it seems Ian. Unhappy With this design of roof you have to know where the angle changes to drop a supporting timber down to the ground at that point. The only thing in so far is the base plate of the walls!
The quicker I receive some advice, the quicker I can 'crack on' though! Big%20smile
Ray
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stanleyman140 Quote  Post ReplyReply Direct Link To This Post Posted: 06 Mar 2010 at 8:26pm
hi ray,i'm not a roofer but from work i've done in the past i would definitely not lay felt tiles flat or at a very shallow angle,for the reason you pointed out heavy laying snow and the effect of capillery attraction.why are you using felt tiles and not a 3 layer system of felt?
you dont state what timber section sizes you are going to use?
how are you intending to joint? if you sandwhich the timbers and bolt using toothed connectors i don't think you will have a problem with your design.
if you allow the sloping sides rafters to run through to the wall plate you could tie the flloor joist by bolting through thus creating a structure under tension which will add to the strength. hope this helps.
andy
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Turnpike Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar 2010 at 12:32pm
Originally posted by stanleyman140 stanleyman140 wrote:

Hi Ray, I'm not a roofer but from work I've done in the past I would definitely not lay felt tiles flat or at a very shallow angle,for the reason you pointed out heavy laying snow and the effect of capillary attraction.why are you using felt tiles and not a 3 layer system of felt?


I was planning on the tiles purely for the cosmetic appearance but as you say with the shallow pitch perhaps this is not such a good idea. I'll discuss with my roofer Son in Law later today.

Originally posted by stanleyman140 stanleyman140 wrote:


You don't state what timber section sizes you are going to use?


Nominal rough sawn 100x50mm (which come out at 95x45)

Originally posted by stanleyman140 stanleyman140 wrote:

How are you intending to joint? if you sandwich the timbers and bolt using toothed connectors I don't think you will have a problem with your design.


The joints will be gusseted with good quality 13mm ply on both sides and bolted through. By 'toothed connectors', did you mean 'T' Nuts?

Originally posted by stanleyman140 stanleyman140 wrote:

If you allow the sloping sides rafters to run through to the wall plate you could tie the floor joist by bolting through thus creating a structure under tension which will add to the strength. Hope this helps.
andy


If I understand you correctly, I'm planning to make the front half of the shop, where the loft will be, with a bottom cord of probably 150x50 mm nominal forming a braced truss. The back half will not have a cord bracing the full width but under the central section will be open to facilitate moving around longer timbers when the workshop is in use. Two timbers, in line with the change of roof pitch, will brace the shortened cords with verticals going up to the roof forming a ridgid triangle structure. (Hope you can follow that... if not I'll have to do some more graphics tonight!)

Your comments certainly have helped and thank you!

Ray 

PS Which pitch do you think I can get away with for the central roof? 19, 22 or do I have to go the full 27 which wastes loft space?

R


Edited by Turnpike - 07 Mar 2010 at 12:37pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Turnpike Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar 2010 at 9:16pm
Discussed the options between bitumen tile and triple felting with my son in law. I'm definitely going with the triple felt.

Ray
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stanleyman140 Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar 2010 at 10:11pm
hi ray.i think that 22 degrees is the best option.it seems to suit your requirements best .
andy
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Turnpike Quote  Post ReplyReply Direct Link To This Post Posted: 09 Mar 2010 at 9:06pm
Thanks Andy... that's the conclusion I've reached as well. So full steam ahead to finalise the design and get building! Big%20smile
 
Ray
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