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Structural roof design help...


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Can anyone recommend a SE to help with an efficient roof design?  We are nearly at plate level and discovering all sorts of issues with the existing roof design and structural supporting of it.  SE and architect are impossible to get hold of and don't really want to know.  I feel like someone looking at it afresh (quickly) may be the best answer and tbh ultimately save money even though I'd be paying again.

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Photos and drawings...

 

Architect should definitely have an idea of what the roof looks like. SE may or may not have it in their original scope - often with builds it's a trussed rafter which sits with the manufacturer to design (SE only to review/comment).

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To provide some context - here are the plans...

 

I have specified 45 degrees on the side, 37 degrees front to back

Trimmers picking up the ridge of the 4 dormers

Two steels across the centre picking up the valleys on the large dormers/gables

Posts off of two spine walls picking up the main ridge

Flitch plates in the hips

Screenshot 2022-06-09 at 20.37.56.png

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So challenges.

 

Different pitches make the roof extremely difficult, especially given the style of the house really requires valley and bonnet tiles

The beams indicated roof plan indicated doesn't work as the large gables eaves height is below that of the main roof so the valleys don't end on load bearing walls, they end in mid air

The plate level is about 2.1m but ceiling height is 2.5m so we will have skeilings at the edge, so the trimmers effectively need to be cranked beams?

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Questions/solutions we (me and builder have)

 

We've amended roof pitch to 40 degrees all round

Cranked trimmers, can they pick up the hips and therefore remove the need for flitch plates

Can we support the large valleys on cantilevered steels, they will end about 250mm off the inside of a supporting wall

 

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Firstly I’m surprised your design team have let you get so far without a roof plan.

 

Secondly I think you you’ll need a roof frame - probably steel. Steel hips and a ridge and purlins to break the rafter span and pick up the ridges of the dormer windows.

 

You might also need a couple of steel portal rafter frames, a steel trimmer and steel ridge beam to help trim out the large return to the front and back.

 

You will also need steel ridge beams for the single storey gables.

 

You’ll also probably need something to anchor the steel to the walls.

 

In hindsight the building would have benefitted from a steel portal frame with infill steels.

 

Or as previously mentioned - trussed rafters. This will be a bespoke design and quite a complicated roof to design. Difficult but not impossible.

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On 09/06/2022 at 20:34, matthyde83 said:

Can anyone recommend a SE to help with an efficient roof design?  We are nearly at plate level and discovering all sorts of issues with the existing roof design and structural supporting of it.  SE and architect are impossible to get hold of and don't really want to know.  I feel like someone looking at it afresh (quickly) may be the best answer and tbh ultimately save money even though I'd be paying again.

That is a cracking and interesting job you have there, especially for an SE. Have had a brouse through the posts and the drawings and it looks like there is a lot to get your teeth into SE wise.

 

The SE drawing you posted looks like a preliminary sizing drawing to give the Architect a feeling for the structural envelope. Maybe there is a lot more detailed information?

 

On a job like this, to review and value engineer, if there is only preliminary sizing information available, I would probably look to model in 3D the ground floor primary walls on the two floors to see how things all line up and the vertical load paths and just as importantly check to see that the building stability system proposed looks adequate. Nothing special, just enough to check the loads paths and get a feel for the structure. This gives you confidence that what is supporting the roof is ok and nothing serious has been missed.

 

This would provide the platform for the roof design and associated steel work in detail. The 3D software is then used to generate the sections, particularly the bits where that are really hard to draw in 2D and the steel fabrication drawings.

 

The great thing about this is that the model is in basic colour so you take screenshots and put them on the construction drawings, if the joiners have a "picture" to look at life is easy for them. You can turn the model into a file that can be spun round using a laptop and say using Navis Works (free viewer) on site to help the joiners visualise where all the bolts go, the awkward timber cuts and so on.

 

Question..

 

What level of drawing and design information do you have to work with? As this is a new house it could be worked up in a lot of detail as there is much less uncertainty compared with say a renovation in terms of dimensions for example. That would include all the connection details, steel fabrication drawings, timber connections and all the tricky bits.

 

Is it the case that the Architect and SE have included a lot of "contractor designed elements" and left it to you to figure out the construction / steel fabrication details and sequence?

 

Below is a small trussed rafter roof screen shotted out the software with some quirky angles and cuts. Joiners love the visual aspect and can then easily understand the dimensioned 2D drawings and wind bracing system

 

image.png.398b76aeec1be831daa43cfb45a7d0d0.png

 

Keep us posted and posting as very interested, wishing you good luck too!

 

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9 hours ago, Gus Potter said:

That is a cracking and interesting job you have there, especially for an SE. Have had a brouse through the posts and the drawings and it looks like there is a lot to get your teeth into SE wise.

 

The SE drawing you posted looks like a preliminary sizing drawing to give the Architect a feeling for the structural envelope. Maybe there is a lot more detailed information?

 

On a job like this, to review and value engineer, if there is only preliminary sizing information available, I would probably look to model in 3D the ground floor primary walls on the two floors to see how things all line up and the vertical load paths and just as importantly check to see that the building stability system proposed looks adequate. Nothing special, just enough to check the loads paths and get a feel for the structure. This gives you confidence that what is supporting the roof is ok and nothing serious has been missed.

 

This would provide the platform for the roof design and associated steel work in detail. The 3D software is then used to generate the sections, particularly the bits where that are really hard to draw in 2D and the steel fabrication drawings.

 

The great thing about this is that the model is in basic colour so you take screenshots and put them on the construction drawings, if the joiners have a "picture" to look at life is easy for them. You can turn the model into a file that can be spun round using a laptop and say using Navis Works (free viewer) on site to help the joiners visualise where all the bolts go, the awkward timber cuts and so on.

 

Question..

 

What level of drawing and design information do you have to work with? As this is a new house it could be worked up in a lot of detail as there is much less uncertainty compared with say a renovation in terms of dimensions for example. That would include all the connection details, steel fabrication drawings, timber connections and all the tricky bits.

 

Is it the case that the Architect and SE have included a lot of "contractor designed elements" and left it to you to figure out the construction / steel fabrication details and sequence?

 

Below is a small trussed rafter roof screen shotted out the software with some quirky angles and cuts. Joiners love the visual aspect and can then easily understand the dimensioned 2D drawings and wind bracing system

 

image.png.398b76aeec1be831daa43cfb45a7d0d0.png

 

Keep us posted and posting as very interested, wishing you good luck too!

 

 

This looks great Gus and exactly what I need tbh.  Someone interested in the job! We’re slowly working things out but I really need some SE input.  For instance turning the trimmers designed by the SE into cranked beams because of the outside plate height and supporting the valleys on the large gable front and back as being a lower plate height means they need to finish about 300mm inside off the supporting wall.  
 

I’ve sent you a pm…

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Probably easier and less costly to send it over to a truss manufacturer, who will deal with this sort of thing every day.

 

I previously used DWBoston who did structural calcs as part of the design and quote

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Our roof is way less complicated, but the truss manufacturing company produced cad drawings, based on the the original structural engineering ones, and produced a full set of structural calculations etc, these were then sent to my own structural engineer for sign off and approval and added to the structural engineering certificate.

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I don't think there would be a true trussed roof solution to this. They would certainly be able to come up with a solution but it'd be based on a similar system of beam supports. Worth enquiring but they'll need an accurate drawing with levels.

 

There is, I think, enough structural information to build from (albeit messy and I wouldn't be happy issuing that - my entire career has been CAD based). But they have left you with significant practical challenges. You really need more setting out information from the architect, else the carpenter will need to set things out.

 

I'd be tempted to set up a site meeting and knock some heads together.  

 

 

Edited by George
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