Building a Workshop, Roofing advice and general hints!!!

5 Aug 2010
Reaction score
United Kingdom

I'm in the early stages of building my new workshop/office/garage in the back garden, I'm just finishing the concrete base which measures 6m deep and 7m wide. I intend to build it in a way so it can be flat packet easily to transport in the future as plans to build something more permanent will be drawn up, maybe in a couple of years. The frame will be made from 3x2” with shiplap cladding to the outside and 6mm Ply board inside with insulation in-between.

I had intended to have a pitched roof but for simplicity I would prefer to have a flat sloping roof, I'm just not 100% sure I could make it strong enough over such a large span?? I don’t mind buying some 7m long 3x7" joists or similar if they will do the job, especially in winter with 6” snow on the top!!

I have some rubbish hand drawn diagrams of roughly what I would like to achieve and can scan them in when I am at home! Sorry if I am being vague but it’s all in my head and hard to write down!

Any help is appreciated!


Sponsored Links
Osb [11mm] would be cheaper and sturdier than thin ply, not sure what you mean by flat pack, are you intending to move it or just want to dismantle easily?Internal wall would help reduce the roof span allowing support to mid-span.
for get it you need an internal load bearing wall/piers or double the frame to support probably 9x3 timbers :(

although the loading will be less than a floor a 40 percent increase in length requires around double the timber strength
so if you look at the 4.4 m loading s that should give you the rough size you need at 6m at half floor loading

if your looking at 7m span its 5m loadings

this is only approximate give or take

400mm 450mm 600mm
Size of joist
38 x 97 mm 1.72 m 1.56 m 1.21 m
38 x 122 mm 2.37 m 2.22 m 1.76 m
38 x 140 mm 2.72 m 2.59 m 2.17 m
38 x 147 mm 2.85 m 2.71 m 2.33 m
38 x 170 mm 3.28 m 3.1 m 2.69 m
38 x 184 mm 3.53 m 3.33 m 2.9 m
38 x 195 mm 3.72 m 3.52 m 3.06 m
38 x 220 mm 4.16 m 3.93 m 3.42 m
38 x 235 mm 4.43 m 4.18 m 3.64 m

47 x 97 mm 1.92 m 1.82 m 1.46 m
47 x 122 mm 2.55 m 2.45 m 2.09 m
47 x 147 mm 3.06 m 2.95 m 2.61 m
47 x 170 mm 3.53 m 3.4 m 2.99 m
47 x 195 mm 4.04 m 3.89 m 3.39 m
47 x 220 mm 4.55 m 4.35 m 3.79 m

50 x 97 mm 1.98 m 1.87 m 1.54 m
50 x 122 mm 2.6 m 2.5 m 2.19 m
50 x 147 mm 3.13 m 3.01 m 2.69 m
50 x 170 mm 3.61 m 3.47 m 3.08 m
50 x 195 mm 4.13 m 3.97 m 3.5 m
50 x 220 mm 4.64 m 4.47 m 3.91 m

63 x 97 mm 2.19 m 2.08 m 1.82 m
63 x 122 mm 2.81 m 2.7 m 2.45 m
63 x 147 mm 3.37 m 3.24 m 2.95 m
63 x 170 mm 3.89 m 3.74 m 3.4 m
63 x 195 mm 4.44 m 4.28 m 3.9 m
63 x 220 mm 4.91 m 4.77 m 4.37 m

75 x 122 mm 2.97 m 2.86 m 2.6 m
75 x 147 mm 3.56 m 3.43 m 3.13 m
75 x 170 mm 4.11 m 3.96 m 3.61 m
75 x 195 mm 4.68 m 4.52 m 4.13 m
75 x 220 mm 5.11 m 4.97 m 4.64 m
Thanks for the advice, I’ve had a good long think about it and I think a pitched roof will work best for us, the timbers for a flat roof would be more expensive than the truss I would imagine!!

I'm thinking a simple King post truss design should do the trick, not sure what wood I need to construct them from or how to go about it!!

The final dimensions are 6m x 6.8m and I’d only like a 450mm high apex (above the 2.2m building height!!) as not to arouse any neighbourly complaints. Are there any calculators I can use online to work out what I need?


Sponsored Links
I ended up doing the trusses like this:



6x2 and 3x2 with 21mm ply plates. They wont bend with 3 people swinging from them!!

Almost finished article:


Not bad to say I’ve no joinery experience, lol
is there any joint apart from the ply plate? what sort of nails did you use?
4" 12g screws were used throughout, no nails. The ply is then held on with 2" 10g screws.

the ply was not really needed as they were strong enough but i thought i might aswel as it only cost an extra £40 to do it.
Just remember to peel the tape off your Rehau door, before it weathers itself to the profile, spoil a good job otherwise ;)
great job well done
thanks for letting us know how it went :D
my only concern is with a few tons off snow on the roof the point loading is going to be high on the walls
Fingers crossed it's OK, I put gussets on every corner of the wall panels to stop them wanting to turn into parallelograms and each truss rests on the vertical 3x2 timbers so as long as the walls dont want to push away from each other I wont need to worry. Is there anything you would consider to prevent it?
The inside is lined with 9mm ply, behind that is 90mm insulation and then breathable roofing membrane stapled behind. i've no pictures of the walls in progress.


you can see the gussets in that pic.


The walls have the same membrane and insulation as the roof.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links