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Doubling up rafters


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Starting to strip back roof to build hip dormer on a catslide roof.

1, Do I put the new rafters on the inside or outside of the rafters that the cheeks will sit on? 

2, The new rafters are 6m long ( so I'm having to join two pieces to make 6m,) so...at what interval do I bolt the new to the old? 

Edited by Dee
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Bcon suggested I join them at 4 and 2. Don't have time to order engineered stuff. Joining them with overlap joint but how far? 

So put the new rafters on the outside of the old and cheeks on the new?! 

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No, you will have to build the dormer cheeks on the same line as the spars otherwise you will have a step on the inside. The stoothing for the cheeks will sit on both spars anyhow because of the width, even if only using 50 x 75 which would be the absolute minimum.

Edited by RichS
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The original rafters are 50 x 125 and the new ones are the same so the cheeks (made from 50x100 studs) will rest on those... What should the cheeks be built of.? 

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1 hour ago, Dee said:

Bcon suggested I join them at 4 and 2. Don't have time to order engineered stuff. Joining them with overlap joint but how far? 

So put the new rafters on the outside of the old and cheeks on the new?! 

 

So my rafters are doubled up under the dormer cheeks and it's M12 bolt @600mm spacing up the middle of the rafter with square plate washers either side and a spiked washer between the rafters. 

 

That makes them 95x175 so they are going nowhere ..!!

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57 minutes ago, Dee said:

Just re read your post.... I get it, sorry but I can be soooooo thick sometimes 

 

Ha ha, not a problem.

Just as an aside, I don't know what you're cladding the dormer cheeks with (lead or tile hung or other) but if you simply nail another piece of timber to your outside spar you will have something to nail the main roof tile lath into where it butts up against the dormer. Yes it will carry without but it's a better job with.

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3 hours ago, Dee said:

What size should the ridge timber be? 

I have 50 x 250mm 

 

I'm assuming you mean the ridge board as opposed to a ridge beam.

If it's only a ridge board 50 x 250 is way overkill, with 50 x 125 rafters I would use more like 25 x 200.

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I'm not sure of the difference between the two. The timber in question is at the apex of a new Hip dormer that I'm building, where the trusses come off. 

It will be about 2.5 long. 

I've had delivered 50x250 but I could cut it down on the 250 dimension.  Do you think weight will be an issue if I use it as is? 

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A ridge beam is a structural element to the roof that carries and spreads the loading on the rafters.

Am I correct in thinking that you are doing a "cut roof" and not using trusses or have I got the wrong end of the stick (again) :)

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A ridge board isn't load / weight bearing. 

A ridge beam is. 

If your making a 'cut' dormer then your looking at triangles, with the bottom horizontal timber being your ceiling rafter.

If that's the case, and they're not vaulted, then you don't need a ridge beam ;)

That help ?

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As above, with a cut roof and 50 x 125 rafters I would use a 25 x 200 ridge board.

As Nick explained better than me the ridge in this situation is not structural. The roof gets it's strength from the triangulation of rafters / ceiling joists etc all being nailed together. This stops any spread loading on the wall plates (even though it's only a dormer).

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22 hours ago, Dee said:

Oh Lord, haven't got that far yet.... Still got to figure how to build the damn thing.... Never done a hip roof before 

?

 

Dee, have you mentioned that your a brave self-building lass yet ;)

Avatar please ! 

And are you really doing the dormers ? I know you can drive an excavator so I won't be one bit surprised TBH. ;)

 

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Can't delete the above name! 

Anyhoo @NickfromWales ....What's an avatar? 

No I haven't as I thought that was fairly obvious?

Yes, I really am building a dormer window.... (With my husband, too complicated on my own!) He's the brains of the family  and I've taken 2 week's AL to tackle this

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Part vaulted? Er, if by that you mean there will be rafter ties up in the apex rather than at ceiling height.... The former is my preference but do I have the option? 

Part stripped the the tiles off tonight so we're committed,!!!!! 

Edited by Dee
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1 hour ago, RichS said:

. The roof gets it's strength from the triangulation of rafters / ceiling joists etc all being nailed together. This stops any spread loading on the wall plates (even though it's only a dormer).

not if there aren't any ceiling joists.

@Dee i presume one of you knows how to work out the true lengths, plumb and seat cuts? this, https://prezi.com/qnjmw9dbfnic/traditional-cut-roof-geometry/ may help, we don't use saddle boards, just the crown rafter which is the same as a common, the compound cuts of the jacks/cripples are fun.

 

simon

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