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Workshop roof design

DomP1979

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Name
Dom
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Herefordshire
I'm building a workshop 7200 x 4800 that I intend putting an insulated sheet roof on but I'm trying to understand the roof structure I need to build. I've got a few areas of confusion-

1. I want to be able to have storage in the roof at one end so would need rafters to bolt floor joists to.

2. How does the edge detail work with these panels at the eaves and at the gables as I'd like some overhang at the gables from an aesthetic point. Would that mean the white under sheet is showing on the gables?

3. The one benefit I can see of these sheets is that they remove the need for rafters but on my span of 7.2m that's some hefty purlin.

Any advice much appreciated!

Dom
 
Hi Dom and welcome to the forum. Mike G is the expert on this sort of stuff.

That's a good size workshop you have in mind. What sort of woodworking are you thinking of? Business ? Pleasure? Why not tell us a bit about yourself ?

Whereabouts in Herefordshire as I know it well.
 
I'm building a workshop 7200 x 4800 that I intend putting an insulated sheet roof on but I'm trying to understand the roof structure I need to build. I've got a few areas of confusion-

1. I want to be able to have storage in the roof at one end so would need rafters to bolt floor joists to.

2. How does the edge detail work with these panels at the eaves and at the gables as I'd like some overhang at the gables from an aesthetic point. Would that mean the white under sheet is showing on the gables?

3. The one benefit I can see of these sheets is that they remove the need for rafters but on my span of 7.2m that's some hefty purlin.

Any advice much appreciated!

Dom
Welcome Dom. I'm in the middle of something at the moment, but will come back to this this evening. If for some reason I don't, please remind me as it will drop off the "what's new" list in the forum.
 
Hi Dom and welcome to the forum. Mike G is the expert on this sort of stuff.

That's a good size workshop you have in mind. What sort of woodworking are you thinking of? Business ? Pleasure? Why not tell us a bit about yourself ?

Whereabouts in Herefordshire as I know it well.
Hi Roger, I'm a mile and a half out of Kington
 
Welcome Dom. I'm in the middle of something at the moment, but will come back to this this evening. If for some reason I don't, please remind me as it will drop off the "what's new" list in the forum.
That's great Mike, I've based it largely on your shed plans so a 3 brick plinth on a reinforced concrete pad. I have the framing all up with studs at 410 centres for 1220 width osb internal sheathing and a doubled up wall plate so I can space any rafters where ever needed.

I'll put some cad drawings up in a bit.
 
Insulated sheets vary enormously in their span details. Perhaps if you could link to the product you are planning to use I can check and see what they require. They're very much a commercial rather than residential product normally (big box warehouses, supermarkets etc), so there will be steel involved in the structure, and bits of pressed metal to hide the underside of any overhang at verges or soffits. So, we'll have to cobble together a detail to make them work on a timber-framed workshop. Don't worry, though, it should be pretty straightforward.

You will still need rafters, though, in any circumstance I can think of.

Any sort of image of your ideas will help, as will the manufacturer's details.
 
Ahah.....Hergest Ridge country ! Cue Mike Oldfield :)
Yes indeed, I can see the monkey puzzles planted on top from my garden. Where and why do you know Herefordshire well?
 
Insulated sheets vary enormously in their span details. Perhaps if you could link to the product you are planning to use I can check and see what they require. They're very much a commercial rather than residential product normally (big box warehouses, supermarkets etc), so there will be steel involved in the structure, and bits of pressed metal to hide the underside of any overhang at verges or soffits. So, we'll have to cobble together a detail to make them work on a timber-framed workshop. Don't worry, though, it should be pretty straightforward.

You will still need rafters, though, in any circumstance I can think of.

Any sort of image of your ideas will help, as will the manufacturer's details.
Well I'm not tied to the idea of insulated panels so could probably save money by still using trapezoidal sheet and make my own insulated layer beneath, its just that they require no air gap for ventilation and have a prefinished underside. They do however create their own problems in that I can't see a way of getting a nice barge board and eaves finish as will still need rafters to bolt my roof storage area to.
 
These are the plans my son and I did, they have been updated since these but I've not got the up to date ones on my phone. There is an additional window in the gable end by the doorway and the roof was just a suggestion of how it might work.
 

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Yes indeed, I can see the monkey puzzles planted on top from my garden. Where and why do you know Herefordshire well?
I lived on the Herefordshire/Worcs border for decades and renovated a property at Mansell Lacy....not quite as far West as you are but seemed to spemd a lot of time. It's an empty county but, by God, some of those roads are narrow !
 
Well, the verge detail would seem pretty straightforward, by extending the purlins out past the gables, and cladding underneath with ply or similar, and the ends with timber (orthodox barge boards). If you have enough rafters then the purlins will only need to be small (say 50x50), and dealing with that at the eaves is then pretty straightforward too.

Don't give up on the insulated panels just yet. I think that having say 5 pairs of rafters at about 1.8m centres or 6 pairs at 1.4m centres should enable small enough purlins, as well as enough ties as to enable some loft storage. You are going to either need ties or a structural ridge beam.......or a somewhat complex mixture of the two.
 
Thanks Mike, as I've got the studwork up while I had helping hands available I'd now really like to get on with the roof to keep the weather out, I just can't visualise or settle on a method.
 
My experience of working on insulated roofing sheets is that they normally interlock at the edges and have steel U shaped purlings to fix to and are supported by steel framework.

Rafters are not the norm as the sheets are normally self supporting between U shaped steel purlings but thats not to say you can not use rafters just they are not normally required with this type of roofing material.

If the span for the steel purlings becomes to wide a steel truss would be used when needed. As Mike has said this type of roofing material is normally used on large stores or warehouses that have steel frames.

You might consider using a wooden truss at the centre point of the 7m stretch and then using steel U shaped purlings at the recommended spacings for the roofing sheets.

As has been suggested take a look at the manufacturers information regarding support for the sheeting and contact the technical team for clarification on methods of construction. A lot will depend on the manufacturers technical guidance as to how you place your supports.

As you have a timber framed building you could overhang your sheets at the verges by say 45mm and then just fix a 45mm x 95mm piece of timber under the verge to fix a barge board to. A 90 deg metal angle strip is normally screwed on at the verges though to cover the edge of the exposed sheeting.

The screws used normally have a hex head that has a rubber or similar washer under the screw head and are driven through the insulated sheeting into the steel purling below, once the sheets are fixed down a plastic cap is used to cover the exposed hex head of the screw.

Mark

Some details:

https://www.cladco.co.uk/media/pdf/Installation_Guide_Insulated_Panel_Roof_1.pdf
 
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I'll let those who know what they're about say whether this is a good idea, but there's another approach to it here - the insulated panels span from a ridge beam straight to the wall plate, and have the insulation stripped back by 150mm at the bottom so that just the top steel box section overhangs the cladding. The eaves detail can then be handled ignoring the thickness of the insulation, as if it were just plain steel roofing sheets.

As far as I can tell most home workshop builds would get away with this approach as the first product I looked up specifies a maximum purlin spacing of 2500mm - but at 4800 wide you might be pushing it if you want any sort of pitch to the roof.
 
The problem with that, Stephen, is finding a 7.2m ridgebeam at domestic scale. If this was a steel framed building than that would be a doddle, but not in timber. And don't forget, our OP wants some joists (tie beams) to use for storage, so this building doesn't need a structural ridge.
 
So would creating 4 trusses and using joist hangers to hang purlins between them to save lifting the whole roof height yet further work? I can then use a light weight ridge so that I can add rafters at my storage end to bolt joists to. A bit of a hybrid roof.
 
This is my frame, it's not done in this photo so ignore some bits.
 

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So would creating 4 trusses and using joist hangers to hang purlins between them to save lifting the whole roof height yet further work? I can then use a light weight ridge so that I can add rafters at my storage end to bolt joists to. A bit of a hybrid roof.
I don't think so, Dom. The problem with lightweight rooves (such as these insulated panels), is that the greatest forces on them can be upwards, rather than downwards. Wind uplift is the issue, and there wouldn't really be very much holding the roof down onto the shed in a gale if you put purlins on hangers between rafters. You're getting out of the territory of the architect here and into the territory of the structural engineer.
 
Back to the drawing board I suppose for now, the chap in the YouTube video linked above just had a huge ridge beam and an oak timber frame which I suppose give the anchoring weight for the sheet. Alternatively I revert to ridge board, rafters and insulate with an air gap and membrane. Noggins between the rafters for the sheet steel
 
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Back to the drawing board I suppose for now, the chap in the YouTube video linked above just had a huge ridge beam and an oak timber frame which I suppose give the anchoring weight for the sheet. Alternatively I revert to ridge board, rafters and insulate with an air gap and membrane. Noggins between the rafters for the sheet steel

Dom, take a look at:
https://www.cladco.co.uk/media/pdf/Installation_Guide_Insulated_Panel_Roof_1.pdf

I put the link above after I edited my reply.

The recomondation is usually to use steel purling because the fixings then have the ability to lock into the steel purlin and resist wind uplift. Wood purlings may not offer the same fixing adhesion so wind uplift may be a problem pulling out the fixings (wood purlings and fixings have probably not been tested by the manufacturer so not 100% recommended and that's about it).

The standard form of construction for this type of roof would be to use steel frames spaced along the building with steel purlings in between spaced at the manufacturers recommended spacings for support.

It's really a case of making sure your support members are anchored to the main framwork and the main framework is suitably anchored too the base. All members of the roof should be capable of taking both wing and dead loads.

I would suggest you contact the roof sheet material manufacturer, show them your design and ask their technical department for advice. If I was considering using this type of sheeting that's what I would do. Mark

Roof Ridge.JPG
 
Thanks Mark, eaves height is something I need to be mindful of so I think (and MikeG might agree) doing a standard rafter with ridge board with ties design, then running timber across the rafters to take normal box section sheeting might be my best option. It will be initially cheaper to get a watertight covering so that I can insulate from beneath and board in osb at my leasure.

Thats back tracking on my idea of a quick way to get watertight and an instant finish on the inside but it may make more sense.
 
Thanks Mark, eaves height is something I need to be mindful of so I think (and MikeG might agree) doing a standard rafter with ridge board with ties design, then running timber across the rafters to take normal box section sheeting might be my best option. It will be initially cheaper to get a watertight covering so that I can insulate from beneath and board in osb at my leasure.

Thats back tracking on my idea of a quick way to get watertight and an instant finish on the inside but it may make more sense.
What do you mean by the highlighted, Dom?
 
What do you mean by the highlighted, Dom?
Height? To keep under 2500mm at the point where the side would meet the top surface f the roof covering to satisfy permitted development rules.
 
Thanks Mark, eaves height is something I need to be mindful of so I think (and MikeG might agree) doing a standard rafter with ridge board with ties design, then running timber across the rafters to take normal box section sheeting might be my best option. It will be initially cheaper to get a watertight covering so that I can insulate from beneath and board in osb at my leasure.

Thats back tracking on my idea of a quick way to get watertight and an instant finish on the inside but it may make more sense.
You do not need to use rafters with either insulated or not insulated sheeting...it is not the correct method of construction for this sheeting.

On a low pitched roof the rafters will push outwards at the eaves (they have a tendency to push outward anyway) so if you ususe rafters you will need a form of restraint in the form of collars or ties.
It is better to use trusses when needed in this form of roofing because they form the necessary restraint.

This type of sheeting is supported on purlings (bearers) running along the lenght of the roof.

If the run is too long for the purling to span then intermidiate support is needed so you may consider constructing trusses for intermidiate support and space them along the length of roof where needed then fix the purlings between the trusses.

The trusses can be made of wood but I would advise using steel purlings. As Mike has said the design of the trusses is the domain of a structral engineer.

The trusses need to be anchored (strapped) to the main framwork to stop uplift likewise the purlings need to be secured (strapped) to the trusses to resist uplift and the sheeting also needs to be secured properly to the purlings using the correct type of fixing.

You may be able to obtain truss design guidance from the internetet based on your span, pitch, spacings between trusses and load.

Mark :)
 
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I agree that I don't need rafters for sheet type roofing but as I want to create a storage space in the roof I will need rafters to bolt floor joists to so a hybrid design will be needed.
 
I agree that I don't need rafters for sheet type roofing but as I want to create a storage space in the roof I will need rafters to bolt floor joists to so a hybrid design will be needed.
You do not need rafters to bolt floor joist to.

Floor joist normaly rest on either wall plates at either side of a building or on intermidiate walls. They can be built into walls but also they do sometimes rest on Joist hangers built into a wall etc.

Floor joist are normally supported by the walls either side of a room or building wichever is the case.

The floor joists should be of sufficient section size for the span.

Just so you know, I have been a carpenter and joiner for 48 years.

Regards Mark
;)
 
Height? To keep under 2500mm at the point where the side would meet the top surface f the roof covering to satisfy permitted development rules.
No, I highlighted "normal box-section sheeting".
 
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