Design me a workshop roof

[bodgermatic]

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!

 

[wizer]

What about this:

Any good?

 

[eggflan]

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 ..

 

[MikeG.]

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

 

[Deejay]

BM

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

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

Cheers

Dave

 

[MikeG.]

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

 

[bodgermatic]

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.

 

[MikeG.]

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

 

[bodgermatic]

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?

 

[MikeG.]

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

Mike

 

[bodgermatic]

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!

 

[MikeG.]

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

 

[MikeG.]

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

 

[bodgermatic]

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.

 

[stoatyboy]

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

 

[MikeG.]

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

 

[dh7892]

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?

 

[MikeG.]

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

 

[jack55]

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.

 

[bodgermatic]

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.