# Workshop roof, metal & wood truss, sheets or tiles?



## mindthatwhatouch (10 Jan 2015)

Whilst the blockwork is slowly progressing I am thinking about roof Structure and covering. 

With the overhang 8 metre ridge, double pitched, 4.5m span.

To keep the internal roof height I am considering metal purlins from gable wall to a truss which is supported by a I beam 2/3rds along the building, wooden rafters on top of these purlins. Other 1/3rd will be common rafter and tie beam with ceiling for storage. Good idea or am I missing something. (Wouldn't be the first time  )


Decra plus, metrotile or another supplier for metal tiles. (£1200 ish) More work involved with battening and fixing, greater material cost, looks less industrial than box profile sheeting.

http://www.decra.co.uk/Products/Decra_Plus_lightweight_roofing_tiles.aspx

http://www.metrotile.co.uk/profiles/metrotile-bond/

Or save all that battening out and faffing and get box profile metal sheets. (£500 ish) Need an extra pair of hands to fit, but I can sort that. Worried about condensation, if its fixed to a covered OSB/PLY board will this cure it? Also a little concerned about rusting in the future?
http://www.southernsheeting.co.uk/catlist.php?cat=58

Planning inside to outside:
Rafters,
Insulation,
OSB or PLY,
Felt or Breathable membrane (this will catch any drips/condensation)

Did look at eco slates but they are another jump in price

Any thoughts or experiences please,


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## Charlie Woody (10 Jan 2015)

I have a carport / storage shed with box profile steel roof. The supplier put a membrane on the underside, but still get condensation in shed & carport. I think if I was doing it again I consider the insulated (100 mm?) box profile.


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## blackrodd (10 Jan 2015)

I would have thought that a simple truss roof, @ 600 cntrs, with osb screwed on top (for rigidity in really windy weather) felted and then the final covering, of you're choice.
The Trusses, Very strong, easily and cheaply sourced and fast erection by 2 good peeps!
You will need a fairly hefty "A" frame to take the span and weight of steel purlins, which I asume to be z section.that you have mentioned, have you made any weight calculations at all? 
Most "A" frames centre at around 2 metres supporting a 100mm x75mm purlin and 4"x "2 rafters
If roof storage is an issue, cut a roof on and have a double collar(the ceiling &floor is fixed to) with each end fully on the wall plates each side
HTH Rodders


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## mindthatwhatouch (11 Jan 2015)

Hi Rodders,
Thanks for that, I have attached a quick sketch which hopefully explains it better. Front section for storage with ceiling, rear 2/3rd section open.

Planning to stick a metal I beam across the front set of block piers so thought I could use this to support the metal purlins, can't see a way of using timber due to length of purlin but I have no idea how much weight a metal purlin will take. Do span tables exist for metal?

All ideas comments welcome.


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## kinsella (11 Jan 2015)

Its not clear why you need a clear ceiling? it must be cheaper and easier to use trusses, either made or frabricated but obviously you don't get a clear ceiling height. If you keep the ridge below 4m and your not in a conservation area, not a listed building then your building should be covered by permitted development under planning and you can use the full 4m. 
Assuming you haven't used your 20% of garden etc allowance. Check out Permitted Development
The proposal looks expensive and complicated but its not clear what your constraints are for going this way. You could even do a cambered truss to get the bottom beam higher to get clearance if that is a driver. (do internet search on cambered truss). Using trusses would keep it all in timber.
Regarding tables, you'd have to get Structural Engineering calculations done or go with a full roofing system from one supplier and they would do all the necessary calculations for you.


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## mindthatwhatouch (11 Jan 2015)

Hi,
I just would like the extra height in part of the workshop, My planning permission is for a ridge height of 3metres.
Appreciate this makes it more complicated and expensive just feel it may feel very low ceilinged (is that even a word?) with a standard truss through out. 

Have looked/considered scissor or raised tie, not investigated the cambered, will do so.

Found some span tables.
A 140Z14 has a weight of 3kg per metre, with a span of 5.5m at 1m centres max allowable loading of 1.12 KN/sq metre.
???????

I suppose option number 27 is shift the I beam to the rear piers and just have the rear 1/3rd open, wooden purlin and rafter or Wooden beam under the ridge supported on a king post ?
Will have to look at some span tables


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## monkeybiter (11 Jan 2015)

FWIW Your dimensions are similar to my WS, I have used a steel I beam along the ridge supported at either end, with roof timbers from the wall tops meeting above the steel where they are nailed together in pairs. The roof is tiled and will take my weight roaming about on top of that.

I wanted loads of headroom without spidery storage above.


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## RobinBHM (11 Jan 2015)

3.0 metres is a very low ridge height, I assume if its a condition of planning consent then that height will be from ground level to top of ridge.

Perhaps you could consider a structural load bearing ridge, as Mike suggested. Either in glulam beam, steel beam or flitch, could be pretty big for that span though. In glulam beam Id be thinking of a 115 x 595 section! Thing is though one beam and everything can rest on it with a clear span. 

Maybe consider a flat or mono pitch for max headroom. 4.5m could be done in timber, or put steels at 1/3 and 2/3 and 
run rafters other way.

For a wood working shop I would try hard to get maximum height


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## mindthatwhatouch (12 Jan 2015)

Mike,
Can I ask you what Metal beam and timber sizes you used.


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## blackrodd (12 Jan 2015)

Attic trusses, which are nearly clear spsan, inside, would be good if storage will likely be an issue, which, at a 3 metre ridge height, it may well be, guessing at a 2,2m, or 2,3m finished floor, to wall plate height.
Not a lit more expensive than standard trusses and nothing else needed, just the locating clips, on the plate.
Obviously not designed to store loads of concrete blocks, type of weight
On that span, would probably mean rafter section of around 125mm instead of the usual 100mm.
Attic trusses would be a simple calculation for the maker to decide given you're details, span etc.
There's lots of manufacturer's about, so shouldn't be too expensive.
HTH Regards Rodders


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## monkeybiter (13 Jan 2015)

mindthatwhatouch":zb3wex6v said:


> Mike,
> Can I ask you what Metal beam and timber sizes you used.



I'm on nights at the moment, I'll try and have a look later today [to remind myself :roll: ] as I built it a few years ago.


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## mindthatwhatouch (13 Jan 2015)

Cheers Mike,
No rush as its a way off yet.

What about a king post truss 1/3rd and 2/3rd? Purlins then rafters?

How would one size the tie beam or can you sit the king post truss on top of a metal I beam.......

Thinking along the lines of 7x2 for the ridge, 4x4 purlin and king post, 4 x 2 rafters.

Seems like a lot of timber for a lightweight roof.

More pondering. :roll:


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## blackrodd (13 Jan 2015)

mindthatwhatouch":13jjsi6t said:


> Cheers Mike,
> No rush as its a way off yet.
> 
> What about a king post truss 1/3rd and 2/3rd? Purlins then rafters?
> ...



If its storage you were after, Wouldn't A queens post would be more suitable.
Google it and have a butchers.
The ridge is usually a 7"x 1", or a 7"x 1 1/4", 'as you say, 'cos of the weight. Regards Rodders


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## mindthatwhatouch (20 Jan 2015)

Right slowly forming a plan.

King or queen post truss at 1/3rd and 2/3rd positions. Purlins onto principle rafters with common rafters from ridge to wall plate.
From 2/3rd truss to one gable wall will be joisted to give me some storage, I was considering for simplicity 6"x2" common rafters with tie beam, but do we think its not a good idea to mix & match like that.
Rest of roof, the other 2/3rds open to give me the headroom.
Will also ask the planners if I can increase ridge height.

Tie beam 8" x 4", 4500mm long (must try to stop mixing metric and imperial!) King post 150 x 75, Common rafter 75 x 50mm, Ridge 175 x 25/38mm (as per suggested above, or 175 x 50 as I have some in stock) Purlins 100 x 100. 


Q1, Any comments on the sizes.

Q2, Placing purlin, does it just go halfway between ridge and wall plate?

Q3, will the King post work if I sit it on top of metal I beam, can't see a problem, although the King post may now not be in tension?

Q4, Another query re: Lintels and things. I have an L shaped metal lintel for over the pedestrian door. The pier to the side of this door will have a beam sitting on it so the upstand of the L will interfere with the placing of beam. Using a concrete lintel as per the below diagram, with engineering bricks and or padstones. Is this acceptable? I know its not a good idea to put one beam above another but cant see a way round it?






As always thank you in advance for the input, comments and suggestions welcomed, sorry for all the numpty questions, but I like 'thinking out loud'
Now just need the frost to go away and I can crack on with the blocklaying. Managed a whole 12 blocks today and moved a ton of Ballast off the road.


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## blackrodd (20 Jan 2015)

These are mostly questions that only the Building inspector can answer. with you're on site conditions as a consideration.as a final say so. 
I presume that you are building under "The 24 hour notice" choice and not by previous approved and stamped plans?
For the roof, There are many variations in timber quality and load bearing abilities, stress graded, etc.
The "A" frame calculations will have to include the stresses given by it's own weight, as well as it's intended loading etc.
3 courses of common bricks on top of a standard (not pre stress) concrete lintol will, I believe, triple the strength, but this may not be relavent because of other unidentified factors that you may, or may not be aware of.

Regards Rodders


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## Ed Bray (21 Jan 2015)

I covered my Garage last weekend in Plastisol sheets, 5.8 mtrs by 1 mtr (4 sheets). Apart from some help from my wife to get the sheets up onto the garage I did all the work myself. I also fitted plastisol bargeboards around the 2 sides and the back, I will do the front over the next couple of weeks weather dependant. I am not the fittest and have bad knees but I managed all this pretty much on my own but would have been much easier with 2 people.

About 18 years ago I had used OSB for the roof onto which I bitumen sealed the roof and thenused 3 layers of felt to cover it. It had perished in a few places and I decided just to put the full length sheets directly on top.

I bought:

4x 5.8x1 mtr Slate Grey Plastisol Sheets
7x 3 mtr Slate Grey Bargeboards
200 Wood Fixings with Caps
100 Stitching Fixings with Caps
Delivery to Plymouth and this was all less than £500

I can recommend Hornsey Steel http://www.roofing-sheets.co.uk/ for the quality of their products, service and price. (I have nothing to do with them other than as a very satisfied customer and would use them again without hesitation.

If you do decide to go with box section profiles you will need at least one of these 5/16 Nut Driver: http://www.amazon.co.uk/Laser-3134-Nut- ... F16+driver

Good luck with your project.


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## monkeybiter (21 Jan 2015)

mindthatwhatouch":8toeyq8c said:


> Mike,
> Can I ask you what Metal beam and timber sizes you used.



Sorry for the delay:-

FWIW my I beam is 5.5M long and 130mm width by 200mm height. The size was calculated for me by an engineer to allow plenty of spare weight capacity. At one end the beam sits on a block pillar, at the other it passes through a wall and sits on a vertical steel. At this point I extended the ridge by another eight feet with a pair of timbers which fit in snugly either side of the I beam central web [like a sandwich].
The roof timbers are too small really, but I like the method [i.e. wall top direct to ridge, no additional support].


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## dombarber (29 Jan 2015)

If using metal sheet roof you will need to insulate the underside or you will have an absolute nightmare when it comes to condensation.


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## mindthatwhatouch (18 Mar 2015)

Thanks for the info Mike.

I have decided on the Decra or Metrotile type tile. Doesn't seem to be much to choose between them.

Any advise on venting the roof space? What dictates whether I need to do this? 
There will be a breathable membrane below the metal tiles, insulation with a chipboard or ply covering on the interior.


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## mindthatwhatouch (27 Mar 2015)

Roofing will be Decra or Metrotile Tiles.
So I have fitted a wooden beam (3" x 9") across the gable end (opening opposite the main garage door). Upright to support the ridge.
Metal beam to go over the garage door and also on the first set of piers at the 1/3rd position.
These three will support a 9" x 3" structural ridge.
Rafters will be 4x2.

Do you think I will need a truss at the 2/3rd position as well???


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## mindthatwhatouch (1 Apr 2015)

AAaaaaahh.

Can anyone give me a definitive answer on ventilating the Decra product.

Breathable membrane (possibly with a breathable board under this) Insulation between the rafters and lined with ply or OSB as a VCL. Happy to stick a VCL plastic in but don't think it needs it. Will be sticking a little heat into the workshop but it will be occasional.

Problem is that there are a couple of ridge vent options as well as over the eaves type available, Do I need both either or none?

Have spoken to Decra technical but the lady was friendly but not very helpful.


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## mindthatwhatouch (22 May 2015)

Decisions made, no longer updating this post, see main thread.

yep-it-s-another-workshop-design-questions-t84326-45.html


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