Building a Shed From Scratch

When we moved house, my wife agreed that as the south end of the patio was shaded by next doors fence and shed I could put a shed on it to use as a workshop as this would keep my "messing" out of the new house. Having measured up and looked at what was available I didn't like the quality of shed I could buy with my budget. This instructable is short of some pictures that might have been useful, and has quite a lot of text, but my approach seems different to other shed builds I've read so I thought I'd publish it any way. I also randomly jump from imperial to metric measurements which will bug the heck out of lots of people but as I was educated in both systems as the UK went metric just after I was born that is how I work

Step 1: Measure & Rough Plan

My design process is normally dynamic. That is I have an idea of what I want and what I've got to hand and make it up as I go along.
This was different I needed to buy everything in.
Having a space about 3m by 2m and standard sheets of 18mm obverse strand board (OSB) being 8'x4' approximately 2.4x1.2m and standard lengths of construction timber being nominally 8' \2.4m long this rather dictated the size of my shed base to be 8'x6'.
Note the approximate I opted for 9mm OSB for the walls as it worked out cheaper than feather edge board. Even though it came from the same builders merchant the 18mm was 8x4/2.44x1.22m the 9mm was only 2.4x1.2m similarly the rough sawn timbers for the base was only 2.4m whilst the finished carcasseing timber I opted to use for the frame was 2.44m a true 8'. Subsequently the final build didn't quite conform to the plan used to buy materials. A roof with a single slope being easier to construct than a pitched roof the height of a standard door and needing a minimum of 6' /1.83m head room gave the rest of the critical dimensions. An additional local requirement that a shed has to be a temporary structure to avoid planning permission being required, meant it had to be designed as flat pack and be able to be dismantled

Step 2: Some Details and Alternatives

Having looked at many shed bases it was apparent that 18mm OSB is pretty standard for flooring. supported on 4x2s\100x50mm at 16"\400mm centers as these are in contact with damp ground they needed to be pressure treated with preservative. had the surface they were going on been just soil not paving slabs I'd have gone for concrete fence posts and not bothered with side rails. As it was 3x2s\75x50mm being about half the price of 4x2s I went with them and gave them an extra coat of oil based timber preservative. OSB comes in several grades 2 is suitable for indoor use only in dry areas, 3 is damp resistant and copes with being wet occasionally, so OSB3 it was to make it last longer I opted to paint it as well. This proved to be a very wise move, either I wasn't supplied with OSB3 or damp resistant means fog, off cuts that got wet swelled up by about a third and became very soft and easily broken, I'd used acrylic wood primer and base coat that was supposedly good for exterior use the front panel got exposed to the rain before I was able to paint it with top coat, the strands became visible and the surface softened, it hardened off again when it dried out but this caused me to switch to oil based paints for the visible sections, and bituminous sealing paint for the ones along side the fence and house wall, the underside of the floor also got two coats of the bitumen.
Some of the cheaper sheds I looked at used little more than 1"1/2×3/4"\30x18mm battens and 1/4"\6mm overlapped boards better quality ones used either tongue & groove boards(t&g) or feather edge board, previous experience suggests a layer of roofing felt between support timbers and cladding boards would be advantageous. t&g was expensive, had I not moved feather edge with roofing felt was viable option coming straight from the saw mill. As it turned out with deliver costs (given the problems I experienced with OSB I should have gone this route but...)
9mm OSB3 worked out considerably cheaper especially as it wouldn't need roofing felt under it a small amount of feather board was still required to do the eaves and gables. I looked at what constructional timber was available and opted for 38x63mm carcasseing timber. pressure treated with preservative was about 2 to 3 times the cost of untreated which while only suitable for indoors I considered inside a shed to be indoors especially with a coat of paint.

Step 3: Detailed Plans

To be realistic the back of the envelope would have done OK. But I felt the need for more details in some areas notably the roof construction sections of that got draw out full scale. (whilst this helped with design it turned out to be mostly unused)
The final plan was thus:-
Base.
8'x6' made of pressure treated 3x2s. a single beam at the rear with 7 cross beams 5'8" (6'-4" for the thickness of the front and rear beams) @ 16"/400mm centers the front beam being made up of the off cuts, each joint screwed together with a pair of 1/4x4" wood screws. Decked with a sheet and a half of 18mm OSB3.
Rear wall.
Same as base substitute carcasseing timber for 3x2s with two solid end rails not one made from off cuts, clad with 18mm OSB3, I contemplated using 9mm but as it saved wasting half a sheet of 18mm and 3 sheets got a healthy bulk discount 18mm it was.(hindsight again that half sheet would have made a work top or shelving half a sheet of 9mm would have made cupboard or window shutters)
Sides 6' rear to 6'8" at the front top the sloping top beams also being the end rafters, with a further 4 uprights again at 16" centers starting from the rear this makes the front bay the short one. cladding cut from 3 sheets of 9mm OSB3, all 3 are first cut down to 6'x4'. one of these is split length ways to form 2 panels 6'x2'. 3 planks of 4" feather board are used to clad the top tapering section.
Front wall. 8'x6'8"
With an eye to economic use of sheet materials having 3 bits of 9mm OSB3 2'x4' left over from the sides and the sizes being part dictated by the door size 78"x30". which is situated to the right of the center line, the18" to the right of the door being wide enough to store a bicycle. and a 6'x4' working area to the left of the door. cladding for the door and the area to its right being made from 1 sheet of 9mm OSB3. the 4' square base of the left-hand side being clad with 2 of the 2'x4' off cuts, the standard 16" centers for the uprights gives a window 32"wide and 30" tall. The remaining 3'x2' panel is used to clad top of the 16" wide left-hand bay. an additional 2 planks of feather board are used to fill the gap at the end of the rafters to the peak of the corrugated roofing sheets.
Roof.
An additional 5 rafters made from the carcasseing timber screwed to the top beams front and rear (this changed during construction as did some other minor details all of which will be documented) support battens for the roofing sheet in line with each of the side vertical beams. Sheeting 3 panels of corrugated bituminous sheeting.
This was the plan materials were ordered accordingly

Step 4: Materials

Despite the fact that timber basically still comes in imperial units. fractions are a pain to type and as the UK has been metric since 1970 so all dimensions are metric.( see foot note to introduction)
Obverse strand board (OSB) class 3 damp proof, 2.4m x 1.2m
18mm thick 3 off
9mm thick 4 off.
Timber
40 off kiln dried carcasseing 38mm x63mm @2.4m
7 0ff pressure treated 50mm x 75mm @2.4m
8 off pressure treated roofing battens 25mm x 38mm @2.4m
10 off pressure treated feather board 100mmx11mm @2.4m
Due to design changes during construction the following were added
1 off pressure treated feather board 150mm x11mm @ 2.4m (rear soffit board)
4 off pressure treated fence posts 75mm x 75mm @2.4m (corner posts)
4 off planned untreated 119mmx18mm @2.4m (door and window lining)
Materials for window sills and construction of a window frame are detailed in the relevant text.

Step 5: Construction

I used screws through out for all joints, nails would have been fine but screws are easier to rectify errors or change design. I used 2 per joint and drilled clearance holes in the outer timber for ease of fixing and alignment. All panels got modified.
The base:
Just a dimensional change due to actual sizes of materials it ended up 2.4m x1.8m The frame was laid out and screwed together using 6mm x 100mm wood screws, a full panel of 18mm OSB was laid on it aligned with the front and left-hand edges squareness was confirmed and it was screwed down using 4x50 chipboard screws approximately 1 every 300mm along each beam. A second board of 18mm OSB was sawn(I used a circular saw with a length of carcasseing timber clamped to the panel as a guide it would have been possibility a hand saw) to fill the remaining section of the floor due to the change in dimension this was slightly under half the width of the panel. Once this was secured in place the floor was stood on its rear edge and the overhang on the right hand side cut off with a hand saw flush with the edge of the right hand floor beam.
Once laid down again the floor was used as the construction base for the walls.
Rear wall:
To allow for the shed to be bolted together and be disassembled the corner post were made from pressure treated 75mm square fence posts, they were cut down to 75x 68 mm the same thickness as the carcasseing timber using the circular saw its maximum cut is only about 40mm so the post were cut from both sides and planed smooth. This would have been easier with a table saw and could have been done with a bow saw/panel saw I think but I'm glad I didn't have to try that.
The height from the top of the top rail to the bottom of the bottom rail was reduced to 1.8m, the corner post and intermediate rails were all cut to length the corners were fitted first then the center post the other intermediate posts were fitted at 400mm/16" centers from the central post making the end 2 bays slightly short. Cladding was again 18mm OSB (in hind sight I should have gone with 9mm the back wall is as a result very heavy) the lower board was the full sheet and was again aligned with the left-hand edge but dropped 20mm bellow the bottom rail this means the end fibers of the floor are protected from water run off the other 'half of the third board of 18mm OSB was used for the top panel the over hang to the right was again trimmed flush.
Front wall:
This was constructed as planed, with the substitution of cut down fence posts as corner post, and not clad until temporarily erected, the cladding was in 9mm OSB

Step 6: Joints

The joints between cladding panels were with one exception all butt joints, due to several of the panels being cut with a hand saw they were not all very tight. I applied a bead of window frame sealant to each prior to fixing the panels to the frame work, I decided to tape each joint to improve their weather resistance. The technique is similar to that used to cover seams and stitching on fabric covered aircraft.
I found an old pillow case in the rag bag and unpicked the seams this gave me a strip of fabric about 1.5 to 2m long and about 0.4 m wide. Using pinking shears I cut it into 50mm(2") wide strips ( pinking shears cut a saw-tooth pattern it is resistant to fraying and works around curves better than a straight cut ordinary scissors and a straight edge would do)
I'd primed my sheets of osb on one side before I started cutting them, the joints need taping after the first coat of primer is dry this means it isn't trying to penetrate two unprimed surfaces at the same time.
Apply a line of primer/ undercoat along the joint it needs to be a little wider than the tape. Smooth the tape along the joint the joint the paint should work its way through the weave of the fabric,. Paint over the top of the tape.
Run the ends round the corners of the panels, ends at cut outs can be trimmed flush once the paint has dried with a sharp blade. The tapes don't have to be long enough to do each joint in a single run so long as the ends over lap.
The tapes can be painted over with further coats of undercoat and top coat without any further special attention and gives a smooth joint resistant to weather

Step 7: Construction Part 2

To build the sidewalls and roof the front and rear walls were temporarily erected screwed down to the base with only 3 coach screws not one per bay as final construction two of the roof battens were used as temporary spreaders level with the top inside of the rear wall. The lower side panels of 9mm OSB were now fitted again covering the edge of the floor, a couple of nails were knocked in just bellow the floor board for it to stand on then after making sure the front and rear walls were square it was fixed in place temporarily with just 4 screws one in each corner.
A change in design to make the roof a detachable panel meant an additional beam was fixed to the top beams of the front and rear walls again just with 3 screws for now positioned so they can be removed with the roof beams in place.
The Roof structure:
I'd done a lot of drawing calculations and measurements to work out angles and lengths for the roof beams, this was mostly a waste of time.
I positioned a beam (untreated carcasseing timber 7 had been selected and painted with oil based preservative due to them being most likely to get wet) over the end wall so it was just over hanging the rear wall drilled it and screwed it down with one 6x100 wood screw at each end. I then held a straight edge to the wall and across the beam and marked the continued line of the vertical wall on the beam front and rear. The beam was then removed the line was continued top and bottom of the beam with a carpenters square theses were then connected on the other side of the beam with a steel rule giving the lines for an angled cut. The rear was cut first then repositioned to check it was right, the front position was again checked whilst it was in place then cut in the same manner, the remaining 6 roof beams were then cut to this template drilled and screwed in place each inline with a rear wall upright. A batten was then screwed in place on the front and rear edge, the rear one was later planed to the same angle as the rafters an additional batten was screwed diagonally across the underside to hold the roof assembly square when removed the rest of the battens were fitted just before covering.
Side walls:
These deviated greatly from the plan.
They ended up with just 4 uprights at approximately 600mm/2' Centers. I cut a floor rail that was a good fit between the front and rear walls. Using an off cut for support I held a beam against the front wall and end rafter and marked the top and bottom of the rafters position. Again using the carpenters square and a steel rule I carried these marks around the beam and cut it to length then 're-checked the lower position. Note I started at the front, if you mess up you get a second chance to use the beam further back. Using an off-cut of the rafter I marked the thickness of the beam that needed removing to accommodate the rafter then cut down the lower diagonal squarely to this line before cutting down the end grain to remove it. I cut the remaining 3 uprights in a similar manner, and did the same for the other side. Make sure the intermediate uprights are squarely vertical and mark both sides of them on the floor beam drill the holes in the floor beam for the screws and fix the uprights, note if you don't have a drill long enough to go through the corner posts and the front/rear uprights of the side walls in one go if you fix the front and rear uprights then stand the frame in position and drill through from the inside into the corner posts the holes for the fixing bolts then complete them with the frame removed this makes life a lot easier. Once the central uprights have been fixed in position refit the frame add your fixing bolts I used 8mm coach bolts, secure the 9mm OSB cladding to the uprights and floor beam I used 4x30mm chipboard screws at about 150mm spacing. Then fit the upper section of cladding a bead of frame sealant along the joint doesn't hurt but isn't essential (see later section on joints) then I filled the tapered eaves section with 100mm feather board.

Step 8: Flat Pack It

This isn't necessary if ironically, as I was you are building it where it is going to stay, but if you are moving elsewhere, or as I had to show it was a temporary structure it had to come apart.
Each of the panels was just managed on my own an assistant or two would reduce the risk of injury.

Put a diagonal brace across the door. Remove the screws securing the rafter support beams to the tops of the front and rear wall, remove the screws in each corner of the lower sections of the side walls lift the edge of the roof clear of the sidewall uprights an push the wall outwards and stack. Remove the battens acting as spreaders lift and tilt the rafters lower the roof and lean on the back wall for now. remove the 3 coach screws securing the front wall and stack the front wall stack the roof, remove the 3 coach screws securing the rear wall and lay it on the base.

Step 9: Final Assembly

I took the opportunity whilst the shed was flat packed to give the underside a further coat of bituminous paint and to fully paint the two walls that would be nearest the house and fence and so inaccessible, also with bituminous paint.
I devised a castor and screw jacking system to make the shed mobile enough to permit access regrettably whilst this worked to move it to its final position they have failed and won't lift the shed enough to permit further movement. During the final assembly I painted all the seams with the same bituminous paint and assembled them whilst it was still wet, all the seams are therefore effectively sealed with tar.
I started by raising the back wall and refitting the 3 coach screws but did not tighten them fully.
I then put the side walls in place fitted the coach bolts both sides heads on the outside threads and nuts inside, the front frame was then fitted only top and bottom bolts and only finger tight note at this stage it is still just a frame. With the walls confirmed as square the sides were screwed down with 4 coach screws per side one per bay, the rear wall also received an additional 4 coach screws making that also 1 per bay and they were all fully tightened down. The front frame work was again removed, and the roof lifted into position but not fixed. The front frame work was then refitted the coach bolts are done up but not fully tightened as they need to come out to fit the cladding, the frame is bolted down with coach screws 2 in each bay either side of the door and one in each of the rest, the diagonal door brace can now be removed.
Secure the roof I used 6mm coach bolts 1 per bay at the rear, drill through both beams and drop the bolts through from the top putting nuts and washers on from below. This is only possible in the outer two bays at the front, above the door and window the beam needs counter boring deep enough with a flat spade bit to take the head under flush then drilling through, the bolts also need cutting to size so they do not come above the level of the battens.
(This could also be done with the front fixing bolts, but I choose to do it differently)
To fit the front cladding I again used nails as temporary supports removed the lower left fixing bolts and held one of the 2'x4' panels in position, marked through with a pencil and drilled the board, which was then secured in the same manner as the sides and the fixing bolts fitted and done up tight. The space under the window was filled with second panel in the same manner the remaining panel was cut to size and fitted to the left of the window. A strip was cut from the remaining 8x4 sheet of 9mm OSB and used to clad to the right of the door this time all 3 fixing bolts had to come out at once but with both frames secured at the bottom and to the roof nothing moved.

Step 10: Roofing

I used corrugated bituminous roofing sheet.
To fill in the ends of the rafter I used feather boards, at the rear this was a single 150mm board sat square on top of the rear 18mm OSB screwed to the rafters and the edge batten then trimmed and planed flush with the battens,this also acted as the mount for the gutter brackets. At the front as ridge boards I used 2 lengths of 100mm feather board similarly attached to the rafters but left to stand above the ridges of the roofing sheet. The battens were spaced according to the manufacturer installation guide as were fixings the water proof caps come as standard with nails, I opted to replace these with screws.(supplied with screws they cost the same for 20 as 200 with nails 200 screws cost only a few pounds
To save playing guess where the batten is, I worked out where the fixings needed to be from below a and used a gimlet to pierce through the ridges above the battens. I used expanding foam to fill in the ridges at the front by the ridges board with heavy rain and the wind in the right direction this leaked. I then used cloth and more bituminous paint to form a ridge flashing, however I suspect In will have to buy the manufacturer version.

Step 11: Paint

I started by giving all the OSB a coat of acrylic primer on one face, whilst this claimed to be exterior grade it had little waterproofing benefit. I subsequently used a lot of bituminous paint on both sides and the rear, whilst this recommends just 2 coats I've used 3.
The front has had 2 coats of oil based under coat and two coats of oil based gloss.
I believe had I used pressure treated feather board for cladding my initial build would have been more expensive not only due to timber prices but I would also have used roofing felt under it or at least polypropylene sheet but a single 5Lt can of fence paint would have done for painting.

Step 12: The Window, and Door Liner

This is an on going project it is at present a fixed 3 pane it is my intention to make it so the top two open, but for now.
I found in a scrap pile/ it's too good for fire wood pile, a 6' length of 120x50mm pressure treated timber, I set the saw fence on the circular saw at 80mm, carcass timber plus cladding plus a bit and 20mm deep, so it would make a door step to that covers the edge of the floor, having cut one slot I then cut another series of slots in the scrap area and removed what was left with a chisel then planed it smooth, this L shaped section was cut to length to fit the window sill and door step, bituminous paint and screwed in place.
The frames were lined top then sides with 100x12mm planking, fixed as the sill/step.
To hold the glass I used a router to cut a 12mm square rebate down the edge of a length of 38x38mm section.
I cut 4 sections the width of the now lined window and removed a 38mm section off each end level with the rebate, on two of them I also removed a 38mm section from the center of each on the rebated side.
I worked out how far into the lining the framework for the front wall was and fixed one of the unnotched sections to the sill with wood glue and screws, I fixed one of the notched sections to the top lining in the same way set the same depth back I cut a further section of 100x12 to the width of the window and fixed the remaining two sections of rebated wood to it the same depth back on opposite faces off the plank.
I then measured the width of my salvaged glass added 5mm and this gave me the length of the lower side bars I then calculated how long my upper side and central bars would need to be I then routed a 12mm square rebate slightly longer than this off the other side of the end of my already routed section giving me a short T section for the central bar.
I then cut my lower side bars and fitted them. The plank with the two glazing bars was then fitted above these here I used 2 pack epoxy and panel pins.
I could now measured and cut my upper bars and fit them the central bar was held in place just with epoxy.
I then cut my salvaged glass to the right size( www.instructables.com/id/Cutting-Glass ), and found some additional pieces for the top panes. I then glazed the window using 'traditional' putty (well it's linseed oil based but doesn't contain white lead anymore) If I can't find an instructable on glazing this way I may get round to writing one.

Step 13: The Door

I thought there was a suitable door laying around at my parents house but it turned out to be rotten so I built one.
First I did a practice mortise and tenon joint see instructable. (https://www.instructables.com/id/Mortise-and-Tenon-Joint/)

I then cut to beams to the height of the lined door frame and 3 cross pieces. Learning from my practice piece I laid them out and labeled them so I kept the same orientation for all marking out and cutting.
The center bar has a full depth joint both ends, the upper and lower the tenon has the upper and lower 1/3 removed respectively this means the height of the door can be trimmed a little without cutting into the mortise.
Having done a dry assembly and tried it for size I set about paneling it for this I used most of the rest of the 9mm OSB . I used a 3/8" router bit to put a slot down the insides of all the beams and both sides of the central beam. Again in hind sight I should have done this first to the cross beams especially the central one, for the sides though doing it after cutting the mortises was correct. Having measured the sizes of the panel whilst assembly I measured the depth of the slot doubled it added this to the dimensions and subtracted 5mm for play. I then cut the panels to size and did another dry fit with the panels, I had to sand the edges smooth to get them to fit nicely. I then applied glue to the slots and mortise cavities and assembled the door again, I fixed the tenons with screws from the inside face ideally I should have used dowels or wedges.
The door was then hung so it opened outwards, a further 100mmx12mm plank was fitted to the opening side to act as weather sealing and a stop