I have thoughts about expanding a storage shed next summer by about 10 square meters, and I realized that I don't know how to start. Obviously, I will need building permits and other hassles, but that will be a question for later; right now, I just want to get an idea about the job.
The storage shed currently has a flat roof, and the plan is to continue having that even after the expansion. The addition will be about 3.5x3 meters, and I'm wondering what size timber I will need, etc.? I've framed walls before, both interior and exterior, but I've never had to consider load-bearing, as that was resolved from the start. Do I put a sturdy post in each corner, or how do I begin? Since no roof trusses are needed, what do I do instead—do I need laminated beams, or will regular joists suffice given that it's so small? Currently, there's a concrete slab that the shed stands on; should I just pour new concrete next to it, or what's the simplest way to do it? It's a grotesque slab today, probably about 12-15 cm above the ground.
Grateful for tips.
The storage shed currently has a flat roof, and the plan is to continue having that even after the expansion. The addition will be about 3.5x3 meters, and I'm wondering what size timber I will need, etc.? I've framed walls before, both interior and exterior, but I've never had to consider load-bearing, as that was resolved from the start. Do I put a sturdy post in each corner, or how do I begin? Since no roof trusses are needed, what do I do instead—do I need laminated beams, or will regular joists suffice given that it's so small? Currently, there's a concrete slab that the shed stands on; should I just pour new concrete next to it, or what's the simplest way to do it? It's a grotesque slab today, probably about 12-15 cm above the ground.
Grateful for tips.
"...building permits and other mög" suggests a Skåne resident 
Have you considered using the Friggebod rule? As of the new year, it has been expanded to 15 sqm. Completely without a building permit. As long as the building is freestanding, not closer to the property boundary than 4.5 m, etc.
But if you want to build together and spend money on the building permit, 10 sqm is hardly such a large construction that it’s something to get worked up about requiring detailed structural calculations. You can very well cast a concrete slab in a way that you find best for the area with a perimeter strip and crack reinforcement in the slab (centrically) with welded reinforcement mesh and a couple of longitudinal bars in the lower part of the perimeter beam. Insulating with cellular plastic under the slab won’t hurt either. Just make sure the subbase is well compacted.
You can easily frame the structure with 45x95 cc 600 and place a wall plate of the same dimension on top and a sill beneath with a layer of foundation felt in between against the concrete slab. Window and door openings may require a header with a short beam of the same dimension (if you have larger openings than 60 cm in width and therefore have to cut off a post/wall stud) standing on edge over the opening between the posts against the inside wall. Or you can place a beam of the same dimension under the wall plate around it. Also on the inside.
Thus, you have the support ready for the roof truss, which can consist of 45x120 cc 600 or 45x? cc 1200, where the question mark depends on what insulation thickness you want in the roof and double the distance between the roof trusses.
The rest is a ‘nail-and-slam project’ at a low amateur level.
_______________
The Builder
Have you considered using the Friggebod rule? As of the new year, it has been expanded to 15 sqm. Completely without a building permit. As long as the building is freestanding, not closer to the property boundary than 4.5 m, etc.
But if you want to build together and spend money on the building permit, 10 sqm is hardly such a large construction that it’s something to get worked up about requiring detailed structural calculations. You can very well cast a concrete slab in a way that you find best for the area with a perimeter strip and crack reinforcement in the slab (centrically) with welded reinforcement mesh and a couple of longitudinal bars in the lower part of the perimeter beam. Insulating with cellular plastic under the slab won’t hurt either. Just make sure the subbase is well compacted.
You can easily frame the structure with 45x95 cc 600 and place a wall plate of the same dimension on top and a sill beneath with a layer of foundation felt in between against the concrete slab. Window and door openings may require a header with a short beam of the same dimension (if you have larger openings than 60 cm in width and therefore have to cut off a post/wall stud) standing on edge over the opening between the posts against the inside wall. Or you can place a beam of the same dimension under the wall plate around it. Also on the inside.
Thus, you have the support ready for the roof truss, which can consist of 45x120 cc 600 or 45x? cc 1200, where the question mark depends on what insulation thickness you want in the roof and double the distance between the roof trusses.
The rest is a ‘nail-and-slam project’ at a low amateur level.
_______________
The Builder
Thanks for the good input, it was about what I hoped for. A Friggebod is unfortunately not an option. A private plot of 630 sqm doesn't leave many spots with 4.5 meters to the property line... An extension is the best option.
I got most of my ideas confirmed, what I'm still a bit unsure about is the roof. I have supplemented and insulated the outer walls, but what do I do with the roof, what are the layers there? I assume it will be felt or possibly sheet metal on a reasonably flat roof.
I got most of my ideas confirmed, what I'm still a bit unsure about is the roof. I have supplemented and insulated the outer walls, but what do I do with the roof, what are the layers there? I assume it will be felt or possibly sheet metal on a reasonably flat roof.
Since you are hesitating between sheet metal and felt as roofing on a 'fairly flat roof' (i.e., with no or low slope), you will face problems with water (which can freeze into ice and lift at the seams with felt roofing if not professionally installed) leading to leaks after a few winters of repeated freezing) and should therefore be avoided. Felt requires a 'completely covering' water roof, usually laid with 7/8" rough sawn lumber, but it can also be done with 15 plywood sheets.
Metal roofs that remain completely watertight at low slopes exist, but then you have to consider a seam-welded roof in stainless steel, installed by professionals with an electric seam welding machine. Alternatively, you need at least a 4-degree slope and double-lock standing seam metal roofing, installed with the help of a regular metalworker. Here too, you should expect a 'completely covering' water roof with 7/8" rsp. or alternatively 15 ply.
The water roof is laid directly on the top of the trusses.
Then you should have a ventilation gap that is at least 1", open towards the eaves. How you will then solve the air outlet at the junction with the other building must be solved on-site. If there is a ventilation gap in that roof too, it's easiest to connect them. The ventilation gap is usually ventilated at the eaves and ridge.
Then you need a wind barrier on the insulation. It can be paper, but in this case, I recommend a 1/8" hard wood fiberboard that is oil-impregnated. You nail it to the inside of the trusses with 1" battens to create the air gap. You may also need to batten between the trusses/rafters if you have 1.2 m centers on the trusses/rafters. In that case, you nail it up to the bottom of the water roof first and then nail the wind barrier board and side battens last, and drive a few roofing nails (with wide heads) through the fiberboard and up into the central batten. (Otherwise, the board will be pushed down over time into the insulation, reducing its insulation value.)
After that, insulation in the chosen thickness. You may need to scab on the trusses/rafters at the bottom to get enough height. Either parallel to them or across them.
Then it’s the vapor barrier's turn. A UV-resistant plastic film.
Afterward, furring strips if you don't have a completely covering roof.
Then paneling on the underside towards the room side.
______________________
The Builder
Metal roofs that remain completely watertight at low slopes exist, but then you have to consider a seam-welded roof in stainless steel, installed by professionals with an electric seam welding machine. Alternatively, you need at least a 4-degree slope and double-lock standing seam metal roofing, installed with the help of a regular metalworker. Here too, you should expect a 'completely covering' water roof with 7/8" rsp. or alternatively 15 ply.
The water roof is laid directly on the top of the trusses.
Then you should have a ventilation gap that is at least 1", open towards the eaves. How you will then solve the air outlet at the junction with the other building must be solved on-site. If there is a ventilation gap in that roof too, it's easiest to connect them. The ventilation gap is usually ventilated at the eaves and ridge.
Then you need a wind barrier on the insulation. It can be paper, but in this case, I recommend a 1/8" hard wood fiberboard that is oil-impregnated. You nail it to the inside of the trusses with 1" battens to create the air gap. You may also need to batten between the trusses/rafters if you have 1.2 m centers on the trusses/rafters. In that case, you nail it up to the bottom of the water roof first and then nail the wind barrier board and side battens last, and drive a few roofing nails (with wide heads) through the fiberboard and up into the central batten. (Otherwise, the board will be pushed down over time into the insulation, reducing its insulation value.)
After that, insulation in the chosen thickness. You may need to scab on the trusses/rafters at the bottom to get enough height. Either parallel to them or across them.
Then it’s the vapor barrier's turn. A UV-resistant plastic film.
Afterward, furring strips if you don't have a completely covering roof.
Then paneling on the underside towards the room side.
______________________
The Builder
Thanks! Perfect info, I will have a few degrees of slope, when I mean "fairly flat" I mean "no 30 degrees here"... The problem is that the existing roof slopes the "wrong way" (lower at the end where I want to extend), so I might consider trying to reverse it. The problem then is that there's an attached carport that I would also have to change the roof on, which suddenly becomes 38 square meters of roof to change instead of 10...