Flat roof construction for detached garage.

[Russdl]

The plan is to build a garage sometime next year. Irregular shape to fit the tapering plot and low height to comply with Permitted Development. 

 

I want it about 9m deep. About 5m wide at the back (narrow end) and about 7.5m wide at the front. The first question that I have is how do I construct the flat-ish roof?

 

Can I do it all in C24 as per the image below? Or does the red C24 in the image have to be a steel or something else? TIA.

 

[markc]

At 6.5m a 9x3 will struggle to hold itself up and definitely not the rest of the roof plus a big snow load (flat roof and presumably insulated)

[joe90]

I think an SE would need to give advise on this, yes snow loading is a real thing (till we get some decent global warming 🤣).

[Russdl]

49 minutes ago, markc said:

At 6.5m a 9x3 will struggle to hold itself up

 

The span tables I've seen says it will, but...

 

50 minutes ago, markc said:

definitely not the rest of the roof plus a big snow load

 

...I guess span tables don't consider hanging all the other roof joist of it?

 

52 minutes ago, markc said:

(flat roof and presumably insulated)

 

Yep. Plan is a flat warm roof.

 

15 minutes ago, joe90 said:

I think an SE would need to give advise on this

 

That will definitely happen, I want to understand it all myself though, plus see what this wonderful community can throw up by way of alternatives.

[Temp]

Even running the rafters the other direction would give  a 7.5m span. Think that's much bigger than the standard Trada span tables cover.

[Conor]

I think you'll need to swap the central 7x2 with a steel, something like a 203x102. 

[Russdl]

So, if the red joist becomes an RSJ am I on the right track?

 

Or could I use an LVL I joist?

 

Or a pultruded GRP 'I' beam?

 

[joe90]

4 minutes ago, Russdl said:

I want to understand it all myself though,

As I would, get the SE to talk you through the figures. If I was younger I would not mind studying to be an SE.

[Russdl]

3 minutes ago, Temp said:

Even running the rafters the other direction would give  a 7.5m span. Think that's much bigger than the standard Trada span tables cover.

 

Yep, i don't think that's an option (but willing to be educated otherwise if anyone knows better?)

[joe90]

Or a glulam 🤷‍♂️ (I like them, you can get nails and screws in them).

Edited October 24, 2022 by joe90

[joe90]

2 minutes ago, Russdl said:

if the red joist becomes an RSJ

Which you can get an engine hoist on 😱

[Russdl]

3 minutes ago, Conor said:

I think you'll need to swap the central 7x2 with a steel, something like a 203x102. 

 

I'm trying to steer clear of the steel, difficult access etc, but it may come to it.

[Russdl]

1 minute ago, joe90 said:

Or a glulam

 

I looked at them, for a previous iteration of my fermenting plan, maybe I need to look again.

[Russdl]

I like glulam for the reasons @joe90 mentioned but I think for what I’ll need it would be big and heavy. 
 

I’ve just been informed that a GRP ‘I’ beam at 6.5m would be “difficult and expensive” (the standard size is 6m). 
 

I guess I’m looking at an RSJ, just don’t want one. 

[Temp]

If we assume a steel across the middle the rafters end up about 4.5m span. The old Trada table I have suggest rafters would be something like 47 x 220 C16 on 400mm spacing. 

[joe90]

9 minutes ago, Russdl said:

I like glulam for the reasons @joe90 mentioned but I think for what I’ll need it would be big and heavy.

I guess it depends on what the SE decides, large glulams can be beautiful (IMO). Not sure about being heavier than a steel 🤔. I put 8 of them in a loft conversion once and just the two of us, no sweat.

[Mr Punter]

You could consider a flitch beam, using 10mm steel plate sandwiched between two timbers, bolted every 400mm.  A fair bit lighter than an equivalent steel beam or column section and you can fit joist hangers easily.

[Russdl]

1 hour ago, Mr Punter said:

You could consider a flitch beam

 

Can you enlighten me? Is that exactly as you describe, sounds good to me if so.

 

2 hours ago, Temp said:

If we assume a steel across the middle the rafters end up about 4.5m span

 

My drawing shows slightly off centre so that the joists (rafters) at the narrow end of the garage wouldn't be so deep, but actually its only 1" so a bit of a pointless exercise from me.

 

 

[Russdl]

Google enlightened me about flitch beams.

 

Would I be right in thinking I would have to attach timber to an RSJ anyway if I used one, to hang the rest of the roof off - or is there another way? If that would be required, then the flitch beam has moved to the top of the pile.

 

 

 

 

[joe90]

15 minutes ago, Russdl said:

Would I be right in thinking I would have to attach timber to an RSJ anyway

Not necessarily, you could fit the timbers into the web. Saves having joist hangers.

Edited October 24, 2022 by joe90

[markc]

No specific need to attach timber into a UB or UC steel, depending on steel section size and joists being fitted it can be easier to fit into the web and sit on bottom flange.

Flitch beams look great if on show and are easy to put together but need to be deeper than a UB or UC for given span and deflection

[Russdl]

1 hour ago, markc said:

UB or UC steel

 

Good old Google, saves me asking too many questions!