# Design me a workshop roof



## bodgermatic (22 Jan 2010)

I'm out of my depth here people, and I know there's the knowledge here to put me straight  Over winter I've been relegated to the house because my workshop is uninsulated, and basically uninsulatable. It's a concrete prefab garage, with a low mono pitch roof (front to back) made from corrugated sheets (clear plastic and bitumen), fixed onto steel purlins. The roof is also a little bit low for my tastes, me being 6'2". 

I'd like to replace the roof with a duo pitched one, with a higher ridge, but I've no idea how to go about designing one or making sure that the building can bear the weight. The image below shows the prefab walls, the wooden doors I built at the front, and the metal purlins which are bolted across and give some rigidity to the overall structure. 







Ideally I'd like to get rid of the purlins, because they are only just above my head height, but I'm concerned that they're essential to keeping the walls up! I've sort of imagined that I could build a wooden framework inside the prefab panels which would actually bear the weight of the roof, but this would need to be quite thin to avoid losing too much space from the inside.

Cheers folks!


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## wizer (22 Jan 2010)

What about this:






Any good?


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## eggflan (22 Jan 2010)

Well i had the same problem , i still do with insulating the walls .

My workshop is the same design as yours , i put up with the cold roof for too long and a couple of years ago i re roofed it by builing a flat roof onto a frame on top of the original roof i laid general loft insulation in between and this has worked a treat . (this wont help your hieght problem though .

I also think the roof is too low but there is no way im taking the steel struts out as they hold the sides up as far as im aware , i will be watching this thread with great interest to see what folk recomend ..


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## MikeG. (23 Jan 2010)

Firstly, those bolted on horizontal members aren't purlins! Purlins run parallel with the ridge. 
Secondly, is that picture a true representation of what is there when the roof is removed? Is there really nothing across the top of the window openings? If so, I will need dimensions.

Thirdly, and most importantly..........this is easy. This is a simple roof to design, and to build, and well within the capabilities of any competent DIYer ........with only one critical exception. The exception is that the roof plate (a piece of timber that sits on top of the long walls) must be strapped down to each one of the individual wall panels, and that means drilling holes, quite a lot of them, into the PC concrete. You might need to hire some kit for this.

So, yes, the existing metal ties can be removed (but only once the new roof is up). The new ceiling ties can be higher up, to give you greater headroom. 

Your next step is to answer any questions I have asked, and to put dimensions on the drawing......the key one is the span........and finally to tell me what angle you want the new roof to be, and what sort of covering you are planning (tiles, felt, corrugated sheet etc).

Mike


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## Deejay (23 Jan 2010)

BM

Once Mike has specified the timber, you might find this site useful.

http://www.blocklayer.com/Roof/Gable.aspx

Cheers

Dave


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## MikeG. (23 Jan 2010)

I would do the gables in studwork personally. I would also look to be cladding the whole thing in feather-edge boarding, and that would carry on up the gables......but even if you don't overclad, boarded gables would be fine.

Mike


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## bodgermatic (23 Jan 2010)

Wizer - that design interests me, although I think I'd need to inspect it up close to fully appreciate it. Send me one over 

Mike - I was hoping I could get your interest in this one, and if our discussion can help eggflan, then that's a two-for-one deal 

Below is an updated diagram with dimensions:






My first model was inaccurate, I didn't show the bits of the metalwork which extend above the walls. These provide the slope of the roof from front to back, and the corrugated panels are fixed directly to these. So they're doing double duty as ties and purlins, the ridge of the roof is immediately above the doors.

There really is nothing across the top of the windows.

I filled in the two windows towards the front, the one at the rear and the one on the far wall with framed wooden panels, just so I could hide certain bits out of view - these are bolted in and will probably be providing a bit more strength than the window units do. The remaining two windows are fairly rotten and will need replacing.

I don't have a preference for pitch, except that I'd like to get enough space to store the set of ladders up there, and I had thought of putting a door in the gable end to enable me to get them in and out without the risk of clouting the things in the workshop.

I'm quite happy with a felt roof, to keep weight and costs down. Ideally I'd like to include some roof windows to let some light in, but I guess it might be easier to do that in the gable ends.

Cheers.


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## MikeG. (23 Jan 2010)

Right, I'll look at this later, or tomorrow. I'm a bit busy today. Are you planning on a ceiling at joist level, or do you want to see the underside of your ladders?

Mike


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## bodgermatic (23 Jan 2010)

I thought a half ceiling on the joists  So cover the half that the ladders are stored over, but allow access from the inside for wood storage on the other half. Does that work?


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## MikeG. (23 Jan 2010)

Yep, easily.and I have a nice little design feature for that that I am planning for my own workshop.

Mike


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## bodgermatic (31 Jan 2010)

I've had a bash at a design, but it's very much 'best uneducated guess' stuff. So what I'd like to know is whether it's overspec'd, underspec'd or just plain wrong 











Rafters are 2"x3", cross ties are 2"x2". Wall plate is 2"x4". Distance between rafters is 60cm. Can I get away with even smaller timber dimensions. Can I space them further apart?

Do I need the gussets at the top? I thought having them would enable me to assemble all the trusses in advance, and speed construction.

This will put the ridge height at 2.8m, and the garage is built on the boundary so I guess I'll need to give planning a call in the morning. I tried keeping it below 2.5m, but that doesn't give me enough room to store the ladders above the tie. I guess if I'm going to have to get planning permission anyway, I might go a bit higher and get more space.

Roof will be boarded with OSB and then felted.

Cheers!


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## MikeG. (31 Jan 2010)

Sorry, I forgot all about this! I havent got a chance to look at it now.......but I can tell you without looking that 3x2 won't do it for the rafters.

I'll try and find time tomorrow.

Mike


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## MikeG. (3 Feb 2010)

Sorry for being so tardy over this.

With raised ties such as is proposed here, the usual rule of thumb is to raise them a max. of one third of the length of the rafter, so in your example this will get you about an extra 9 inches of headroom. They can be raised higher, but the timber needs to be beefed up quite a lot, and it would require an engineer's calculation rather than the use of my tables.

You have shown a roof at approx 25 degrees. Using this figure, your rafters should be 100x38 (C16 timber) at a max. of 600 centres. Your joists should be 150x38 (C16), or 125x50 (C16) bolted to the rafters on the centreline's intersection point.

This all assumes a ply and felt roof, and should not be used if tiles are proposed.

Those dimensions are the structural dimensions. However, if you are proposing to insulate between the rafters, I suggest you use 150x38 rafters, and use 100mm Kingspan/ Celotex boards between, with a 50mm air-gap over. This gap should be fully ventilated at the eaves, and fitted with an insect proof mesh.

-

A neat little idea if you are planning to access the loft storage from inside the workshop, is to leave out one of the joists. This will leave you a 1200 gap to angle stuff through. To do this, double up the rafters on the trusses either side of the truss where you will omit the joist, and fix a sturdy structural ridge between the pairs of doubled rafters. This ridge should be 200x50. The rest of the ridge can be 125 x 38 if you use 100x38 rafters, or 175x38 if you use 150x38 rafters.

The biggest area of concern with this roof is at the top of the walls. Those individual panels will each need restraining to the plate (which must be in one continuous length), which will mean screwing galv. MS straps to each one, screwed to the plate. Furthermore, the windows have no lintel over them. You say that most of them are infilled with studwork. This studwork should either have a stud underneath the rafter foot, or it should have a doubled-up top plate. In the latter circumstance, that would mean that there were 3 pieces of 100x50 timber one on top of the other (the continuous plate, then the 2 pieces on the studs). 

Where you have a window you have a much bigger problem. My suggestion here, as you say that you intend replacing the windows anyway, is to take the windows out and build in a sub-frame from 4x2, with a pair of 4x2 ("standing up", rather than laid face down) nailed together across the head to form a lintel. The lintel should come hard up underneath the roof plate.

Finally, studwork at the gables.

Hope this helps

Mike


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## bodgermatic (3 Feb 2010)

Mike Garnham":6jjn1hi1 said:


> Sorry for being so tardy over this.



No need Mike! I'm very grateful for your assistance. Having read over it a couple of times I think I catch your drift, I'll update the model with your suggestions.

I phoned the planning office and got a 'you don't need permission, but drop us an email with a sketch so that you have it documented in case of query later'

Cheers,

Chris.


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## stoatyboy (3 Feb 2010)

My word - I was going to post a question that started "i've got a garage made from prefabricated concrete panels ....."

My roof leaks - so I guess you know the rest

what a wonderful forum - hope you don't mind me cribbing your answers?

Cheers


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## MikeG. (3 Feb 2010)

stoatyboy":1dl0gkie said:


> hope you don't mind me cribbing your answers?



Only if it is exactly the same sized building. Otherwise, post on here and I'll sort it for you.

Mike


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## dh7892 (3 Feb 2010)

Mike, 

As usual, you are providing a very helpful service by going to the effort of writing all of these detailed replies. 

Where do you get/work out the roof calculations from? I haven't been able to find much on the web about what's acceptable in terms of roof structure. 

There are a few "calculators" out there but are there a set of "rules" or something that we could look at?


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## MikeG. (3 Feb 2010)

Well. it's a bit of a minefield. You really need a bit of training or experience in structure, which is why it is best to ask me, or some other professional, rather than just have a go yourself with some tables off the internet. This job here is a good example, with the roof being very straightforward, but its relationship with the walls being more awkward due to their panelled nature and lack of lintels. This is why I will generally ask for photos before I get too involved with advice.

Mike


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## jack55 (3 Feb 2010)

Hi all. If you don't mind me jumping in on this thread. 

I have just had the letters from planning and building warrant saying I don't need either, so i'm planning to pull down my old shed and build a new one along the lines of this drawing. 

The roof has been the hardest bit and it may still not be right? We had three foot of snow sitting on the shed and house roofs this winter, and so it has to be able to shed snow. 

Before I start gathering materials any comments are more than welcome.


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## bodgermatic (3 Feb 2010)

Mike,

I've redone the trusses with the specifications you gave, specifically: rafters and joists are now both 150x38. I've raised the pitch to ~27 degrees to pull the ceiling up a bit higher now the joists are thicker. I reckon there won't be much more space up there than the ladders will take up, so I'll probably scratch the idea of accessing the space from inside. Instead I'll build opening doors into one of the gables. 

I've gone for 620mm spacing between trusses - can I get away with that? Bearing in mind that I've gone with the larger rafters to allow for insulation as you suggested. I did try 600mm, but the centre two trusses were very close together, so I removed one and made all the distances the same - which came out to 620mm.













> which will mean screwing galv. MS straps to each one, screwed to the plate



I'm not sure I follow this - what are MS straps? If I remember correctly, the panels each have a through hole top centre - would it do to bolt the wall plate down to every panel through this hole?

I understand that the wall plate needs to be continuous, so that's 2x5m lengths and 2x3m lengths - Is there a recommended joint at the corners? Half Lap?

So assuming that the theory's sorted - now some stoopid practise questions: how do you actually fix the joists to the rafters? Does anyone have a link to the order in which to assemble a pitched roof?

Cheers!

Chris.


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## MikeG. (4 Feb 2010)

bodgermatic":3bj3egxl said:


> I've gone for 620mm spacing between trusses - can I get away with that?



Yes, structurally. However, practically, it wont work for your boards. The OSB or ply will be 1220x2440, (or possibly 1200x2400) so going to 620 spacings will produce a lot of waste as you cut it back to the previous truss. Obviously the joints between boards must occur on a rafter. You will find it cheaper to make up an extra truss.




bodgermatic":3bj3egxl said:


> what are MS straps? If I remember correctly, the panels each have a through hole top centre - would it do to bolt the wall plate down to every panel through this hole?



MS= Mild Steel. These are the normal galvanised straps bought from any builder's merchant. If you can bolt the plate down to the top of every panel, so much the better, but I can't picture it.



bodgermatic":3bj3egxl said:


> I understand that the wall plate needs to be continuous, so that's 2x5m lengths and 2x3m lengths - Is there a recommended joint at the corners? Half Lap?



Half-lap is perfect.



bodgermatic":3bj3egxl said:


> how do you actually fix the joists to the rafters?



They bolt onto the face of the rafters.........not as you have drawn them. Correctly sized (ie 10mm for a 10mm bolt) holes on the centre-lines (ie where the centreline of the joist intersects with the centreline of the rafter). Plate washer front & back. Don't bother with dog-toothed connectors.

As for building the roof (you'll need a scaffold tower).........first, fix your plates in position. Second, cut a piece of batten the exact length of the span across the plates. Use this to set out your first pair of rafters on the ground (don't forget to cut a short section of ridgeboard so that you have the proper gap at the apex......mark and cut.....and the best one of this pair becomes your pattern. Mark it clearly. Cut out all your rafters and joists.

What happens next depends on whether you are working alone, or your own preference. My usual technique is to clamp a vertical timber into place in the middle of each end wall, sticking up to above the ridge position. Then, clamp your ridgeboard to this.......having first marked out your rafter positions, properly with a square, both sides. Now, offer up your first pair of rafters at either end of the building. Nail them into position at the plate, regardless of how they meet the ridgeboard. Then, go and adjust the ridgeboard to suit the rafters......releasing the clamp, and tapping rafters and ridgeboard until everything sits right...then nail.

Once the 2 ends are in place, I would do a temporary diagonal brace on the underside of the timber.....ridge to plate, then just infil all the rafters.

Finally, having got the rafters into position, fix the end two joists in place, ping a string line between them at the junction of the rafter and the joists, nail a pair of battens into place along this series of lines, then you can sit your joists on this batten at each rafter position to ensure that they are all horizontal. (Eye up your joists so that any curve you sags *down* in the middle of the roof.........some will find that controversial).

Leave your temporary diagonal brace in place until you have boarded the outside.

Put the kettle on and enjoy your new space.

Mike

_edit_ PS. Be very careful when taking the existing roof off that you don't leave the walls unstable. You might consider temporarily fixing a sturdy piece of wood along the outside face of each wall, towards the top, with a couple of sturdy diagonal props staked to the ground. Get the roof plates fixed in position as soon as possible.


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## MikeG. (4 Feb 2010)

jack55":mv0eeyju said:


> Hi all. If you don't mind me jumping in on this thread.
> 
> 
> The roof has been the hardest bit and it may still not be right? We had three foot of snow sitting on the shed and house roofs this winter, and so it has to be able to shed snow.
> ...



Jack,

I really will save myself some typing one day by doing the definitive "Mike's Guide to Building a Shed"!!

Firstly, the key to a decent long-lasting timber-framed workshop is to build it on a low brick plinth. This is the single most important thing you can do to improve your design.......it protects the sole plate, adding decades to your shed's life. Two courses of bricks is all you need, but 3 can look quite good.

Secondly, a building that size I would build of 4x2s not 3x2s, oh, and don't forget to double up the studs at openings.

Finally, whilst on the walls, if you are lining the inside of the walls with OSB or ply, you don't need a vapour barrier as these materials are so resistant to the passage of water vapour. Put it in if you feel more comfortable though. The next huge detail that elevates this above a temporary store shed is to not fix your cladding directly to the studs, but to counter-batten vertically on each stud with 2x1 battens, and then clad on that. Forming a ventilated void this way virtually guarantees that your walls will stay dry. You should place insect mesh at the top and bottom.....and don't have a horizontal batten at the bottom.

As for your roof......well, you've drawn a pre-made truss roof. If that is what you want the truss manufacturer will tell you exactly what is required. However, I wouldn't bother and would cut my own. Let me know and I'll calculate it for you. Same comment re- air gaps as per Chris's roof......there should be a 50mm clear ventilated void above your insulation running eaves to ridge......again, insect mesh is vital.

Again, as for Chris, the rafters should be spaced to suit your sheet materials. 24" centres should suit, even if there is an odd gap at one end of the roof. You waste a lot of material otherwise.

Mike


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## jack55 (4 Feb 2010)

Thanks for the advice Mike. There is definitely a book here, or maybe a DVD. I did get a book called Building a Shed, but its not a great deal of use. 

I have a separate plan for the foundation. I need to keep the floor about a foot off the ground to keep it out of the snow. Normally I step up into my shed, at the moment its almost a step down. 

I plan on using concrete blocks on top of some drainage material with the joists placed to support the edges and middle of the flooring material (18mm T&G chip board). Going across the floor joists.




The roof joist were going to be positioned on top of the studs which should be around 24" centres, to accommodate the insulation. I wasn't planning on having them manufactured was going to build them on site. 

I take it fitting the internal walls on secondary studs is to keep an airflow around the wall, hence no horizontal board at the bottom? 

Any ideas on the outer cladding? The last time I clad the wifes garden shed the sun caused it to dry out and shrink. That was 18mm ship lap T&G from the local builders merchant.


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## MikeG. (4 Feb 2010)

jack55":1dafo833 said:


> I have a separate plan for the foundation. I need to keep the floor about a foot off the ground to keep it out of the snow.
> 
> The roof joist were going to be positioned on top of the studs which should be around 24" centres, to accommodate the insulation. I wasn't planning on having them manufactured was going to build them on site.
> 
> Any ideas on the outer cladding? The last time I clad the wifes garden shed the sun caused it to dry out and shrink. That was 18mm ship lap T&G from the local builders merchant.




Hmmm......no sign of a masonry plinth! And all that space for the rats to run around in......

Even with a timber suspended floor I would still be going for a brick plinth with the frame built off that, and the floor hanging off it.



> I take it fitting the internal walls on secondary studs is to keep an airflow around the wall, hence no horizontal board at the bottom?



This I don't follow. There are no internal walls. If you are talking about the external walls and the vertical counterbattens, then yes, that is the purpose. Any moisture that makes its way through the wall from inside gets dried off by the airflow behind the boarding. Any water that gets through the boarding from the outside can't migrate across to the main body of the wall, and wil similarly be dried off by airflow. It works brilliantly.

As for your roof.......too tired now. I'll look at it tomorrow. At least, if I remember I'll look at it tomorrow. :wink: 

Cladding? Ex 7" x1-1/4" feather-edge board is a robust yet cheap solution. For a really posh version of it, you could buy a rebated version which sits flat against the battens (or you could rebate it yourself).

Mike


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