New Workshop

[Waggie]

Hi, I'm brand new to this forum, I am planing to build a new workshop for my retirement, this will be 4m X 5m and 2.5m high at the apex, i intend to construct the frame from 4x2 timber but i want a clear unobstructed roof span, I have spoken to a local steel fabrication company who say if i send them a basic line drawing they will fabricate my apex supports. So just a little help please.
Question 1: How can i calculate the roof angle (I tried an online tool and it came back with 14ó seems a little shallow to me)
Question 2: Should i construct straight of the concrete pad of put a wooden base down and build of that.

 

[9fingers]

Welcome to the woodhaven.

You might find your steel fabricators would prefer a dimensioned diagram rather than quoting an apex angle. But if you post the dimensions, I can calculate the the angle for you.

HTH
Bob

 

[Waggie]

Thanks Bob, The roof will be 4m wide rising to 2.5m at the apex, hope that's enough information.

 

[Woodman]

I’m new here too and currently on with building a 2 story structure, ground level for me, upper level for the wife’s art studio.

Can’t help with the steel question, my architect and structural engineer did all that as mine needed planning permission and building regs.

I have a concrete slab floor which will be dry as it has a proper damp proof membrane fitted. But I wasn’t sure about the floor as I will have the usual heavy woodworking machines in there. So I’m going to cover the concrete with 18mm flooring chipboard and paint it. This will help stop the concrete drawing on the feet when stood all day. I may well add rubber matting in the walkways once I know the final layout.

 

[9fingers]

Thanks Bob, The roof will be 4m wide rising to 2.5m at the apex, hope that's enough information.

To work out the angle of the roof, I'll need to know how much higher the apex is than the height of the walls.

Bob

 

[Waggie]

Hi Bob, the walls will be 1.8m high and the apex will be 2.3m high so the height difference from wall to apex is 0.5m. Hope that makes sence. As the building isn't under construction yet, I can alter dimensions a small amount.

 

[Mike G]

....... I am planing to build a new workshop for my retirement, this will be 4m X 5m and 2.5m high at the apex, i intend to construct the frame from 4x2 timber but i want a clear unobstructed roof span, I have spoken to a local steel fabrication company who say if i send them a basic line drawing they will fabricate my apex supports. So just a little help please.
Question 1: How can i calculate the roof angle (I tried an online tool and it came back with 14ó seems a little shallow to me)
Question 2: Should i construct straight of the concrete pad of put a wooden base down and build of that.

Please see here for information on how to build a timber framed workshop properly. In no circumstances should you build the timber frame directly onto a concrete base.

You need a structural ridge beam, not apex supports. I don't even know what is meant by that term. With a single tie this can be achieved in timber alone. Without a tie you are going to need either a glulam beam or a timber I beam for the ridge......or a homemade timber & ply box beam. Otherwise you can use steel if you really want, but raising a heavy steel beam to ridge height is not easy, and it does produce problems with cold bridging.

To calculate the angle of the pitch it is necessary to know the height of the top of the ridge above the wall plate (in addition to the span, which is across the outside of the plates).

 

[Mike G]

Hi Bob, the walls will be 1.8m high and the apex will be 2.3m.......

You just said it would be 2.5m.

 

[9fingers]

Assuming 0.5m difference from wall plate to underside of the rafter at ridge height and 4m width then the pitch of the roof will be

Arctan (height difference / half width)

=arctan 0.5/2
=arctan 0.25

In plain english =14 degrees which is a very very low pitch and will constrain you to certain types of roofing materials eg I don't know of any tiled/slated solutions that will tolerate such a low pitch. EPDM rubber for example would be fine and very long lasting.

Also I don't know how tall you are but walls at 1.8m high could have you banging your head on a regular basis.

HTH

Bob

 

[Mike G]

That doesn't allow for the thickness of the rafters, Bob. The underside of the rafter sits on the plate, and the top of the rater goes to the ridge. This will need 145 thick rafters, dictated by the depth of insulation rather than the structural loadings, and so depending on the design of the birdsmouth the actual pitch is closer to 9 degrees. At that low pitch only sheet materials are suitable for the roofing, such as EPDM, Onduline-type, pressed metal etc.

 

[9fingers]

That doesn't allow for the thickness of the rafters, Bob.

Well it sort of does Mike as I said wall plate to underside of rafter at the ridge for the purposes of determining the angle.

But point well made Mike, the little details of rafter height, the ridge board itself and any ridge capping all adds up and the term "ridge height" needs defining as inside or outside.

I'd suggest Waggie draws some detail sections though the ridge and wall plate areas just to illustrate for himself (and us!) how his design is going to work. for things like headroom befroe getting too detailed with angles etc.

Bob

 

[Waggie]

Hi, looking at the comments I can see i'm going to get myself into a whole world of problems, when i spoke to my local council about this they set the parameters i.e 20sqmtrs @ max 2.5mtrs high, so i simply took this as a 4mx5m building @ 2.5mtrs high. they also stated that the max height was to include any roof coverings. I know i can purchase this sort of building but prices are out of my reach (well i just don't want to spend that much) and of course there's the internal height problem. i'm 5'10" so i naively thought that with the walls at 1.8m (5'10") and the roof rising off that there wouldn't be a height issue. I can jiggle about with the wall height and use 1.9m or 2mtrs which is about 6'3" to 6'6" but i have to keep the ridge below 2.5Mtrs and there seems to be the problem, not enough slope on the roof. so would you recommend I go for full planning permission? to gain height.

 

[StevieB]

Do you specifically need a dual pitched roof? If you went for a 'pent' roof that would alleviate your problems somewhat.

The council have just given you the maximum size allowed without planning. 20m2 could be 6x3m for example with a bit left over, making a pent roof even more feasible as you only have a 3m span. Even if you do not want to buy, looking at sheds, workshops and garages online gives a pretty good idea of what is available in terms of style and shape, and many will have internal photos as well - all useful for extra ideas. Summer houses/log cabins are also good sources of ideas for shape:

dunsterhouse.co.uk

gardenbuildingsdirect.co.uk

passmores.co.uk

 

[9fingers]

Hi, looking at the comments I can see i'm going to get myself into a whole world of problems, when i spoke to my local council about this they set the parameters i.e 20sqmtrs @ max 2.5mtrs high, so i simply took this as a 4mx5m building @ 2.5mtrs high. they also stated that the max height was to include any roof coverings. I know i can purchase this sort of building but prices are out of my reach (well i just don't want to spend that much) and of course there's the internal height problem. i'm 5'10" so i naively thought that with the walls at 1.8m (5'10") and the roof rising off that there wouldn't be a height issue. I can jiggle about with the wall height and use 1.9m or 2mtrs which is about 6'3" to 6'6" but i have to keep the ridge below 2.5Mtrs and there seems to be the problem, not enough slope on the roof. so would you recommend I go for full planning permission? to gain height.

Did the council say if the 20sqm was the external footprint at ground level, the maximum overall Length x Breadth eg including gutters, the internal floor area or something else.

Provided the parameters they gave you (and the definitions used ) are acceptable to you then in your shoes, I'd not put yourself through the agro and expense of a planning application.

As earlier I suggest you draw out detail at wall plate level, using Mike's 145mm rafters, with birds mouth , the soffit and facia boards, roof deck and covering with the position of the gutter.

Do a similar section for the ridge, roof deck and covering and ridge capping. Put the top of the capping section 2.5 m above ground level, and your wall plate section with the wall plate at your 1.8m internal clearance offset to the side by a couple of metres.

Join up the rafter ends and you have the nominal design of the roof and the angles you originally asked about.

If you can do these sections in some sort of computer drawing package so much the better as you can move things about to try things out.

hth

Bob

 

[RogerS]

Where did the 20sq m come from ? Unless you're living in certain special areas such as a conservation area, I thought that the floor area didn't come into consideration as far as permitted development rights for outbuildings was concerned. Building control DO have an interest if it's over 30sq m internal area.

I'll defer to Mike G if he says I've got it all wrong

 

[9fingers]

Equally unsure but pdr area limit might come from the total of previous permissions or in the back of my mind is a limit of not having buildings covering more that 50% of the plot.

Bob

 

[Mike G]

Permitted development rights allow you to cover up to 50% of the land area around the original house (it would take a paragraph to describe what the original house was). Therefore if you have been told that you can have a 20 sq m building under PDR we can assume that the entirety of the land around your house amounts to around 40 square metres (with the caveat of other outbuildings and post-1948 extensions being subtracted from the total). If that isn't the case then let us know.

The 2.5 metres height restriction applies if any part of the proposed building is within 2m of a boundary. If it isn't, then a duo-pitched roof can be up to 4m high, eaves 2.5m.

 

[Mike G]

...... i'm 5'10" so i naively thought that with the walls at 1.8m (5'10") and the roof rising off that there wouldn't be a height issue........

There won't be. You'll be fine with that.