Double up on raker for dormer?

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Hi.

Builder has said that rather than use two 8x2 better to use 8x4.

Is this correct as what I have seen from most drawings are double up.

I'm sure it would be fine but why is it recommended to double up rather than use full size if available?

Thanks
 
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Costs.
He will change his mind when he sees how much 8x4 cost.
If he finds them available, usually they need to be ordered.
 
I've just done mine by bolting three 7x2 together. Easier to man-handle into place, and means the entire roof plane is made of the same bulk order of timber.
 
great. If it works out cheaper and is easier to handle then it's best to get that. Cheers.
 
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Structurally, two 8x2s screwed together are about 10% stronger in bending than an equivalent grade 8x4; not many joiners seem aware of that.
 
OP,
maybe i dont understand what you are proposing but two things -
one: why are you calling it a "rake" when IME in the UK its typically known as a barge board or sometimes a verge board?
two: why are you aiming for such a heavy section when a simple two by rafter ladder will bring you out to where ever you want to be - where you can then fix a one by of fascia quality material to the 2x?
 
I'd not heard 'rake' before either, but had interpreted it as the doubled-up rafters that dormer cheeks are typically built on, rather than a gable ladder.
 
We always triple up the dormer cheek bearer, as two of the timbers will be used up by the dormer framing, so the third will catch the tile battens etc.

This is exactly how I'm doing mine . Do you have any photos of one of your flatties mid construction? I'm just about to start my corner post studding and would like to see how you intersect yours with the front dormer roof beam.
 
This is exactly how I'm doing mine . Do you have any photos of one of your flatties mid construction? I'm just about to start my corner post studding and would like to see how you intersect yours with the front dormer roof beam.
Not sure...struggling to picture your scenario...? Piccie? Diagram?
 
Not sure...struggling to picture your scenario...? Piccie? Diagram?

Diagram below. The corner posts are made up of 3 off 100mm x 50mm, each a slightly different length to support various elements of the roof such as the doubled up window header.

To get a sense of scale, the dormer has a total width of 1500mm, clear span between tripled up rafters=1300mm.

full
 
Diagram below. The corner posts are made up of 3 off 100mm x 50mm, each a slightly different length to support various elements of the roof such as the doubled up window header.
Would your roof joists not be better going in the other direction?
 
Would your roof joists not be better going in the other direction?

Do you mean from a cross ventilation point of view (cold roof). It hadn't occurred to me and I was going to run the ventilation up in to the loft with a few holes through the trimmers, but yes you're right. That'll save me five awkward mitre cuts too.
 
Thank you for the responses. Quite common to call them rakers around my side.

I now have some more concerns :(

I was going to lower ceiling height of my first floor by 100mm so that I can get around 2300mm height in dormer. Now I'm wondering if I can get away with it by reducing some of the sizes of framing.

1. Can I reduce the size of the dormer roof joists to 7x2s? Or are 8x2 necessary? won't have much load other than snow.

2. For the new floor can use 7x2 (close to max span) or 7x3 (£125 dearer) instead of 8x2 to gain another inch space.

3. Head post is 250x100. Seems a bit big? My friend used 200x100 on their recent conversion on same house.

Is UB of 233x133x30kg too big? a UB of 203x102x23kg was used on same house.

Would anyone know if the following is over engineered for a dormer?


400mm C/C


Screen Shot 2019-10-12 at 22.05.25.png


Screen Shot 2019-10-12 at 22.47.42.png
 
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