Design

The picture above shows the general modern design I am looking for.  I have made some very rough amendments to the photo to show a design closer to mine.  In my build, there is an existing flat roof shed to the left. Hence I will have a solid wooden door  on the LHS. I will also have a smaller 2.4m sliding patio door, hence I have covered part of the sliding doors in this picture with a copy of the cladding (it will look better in real life!)

The other main deviation will be an overhang over the front of the office to add more interest, and there will be a slight step down to the existing shed roof. The only reason for this, is that time is tight enough.

Dimensions of the new insulated RHS office section will be 3660mm v 2440mm, total height under 2.5m

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And now plaster-boarded and plastered (the only part I've not done myself):

The Base stage 1

The base is to be suspended on concrete slabs.  The frame will be made by 100mm x 47mm timbers.  The outer edge of the frame has 2 timbers together for extra strength (effective 94mm x 100mm when screwed together).  The inside joists suspended by joist hangers to securely hold them above the ground.

The second picture gives a better view of the existing shed, to be incorporated, have a new door cut in the front, and re-clad to blend in seamlessly with the new room.

The Base stage 2

Although most builds don't do this, I was keen to have a better physical separation from the ground, hence I wrapped some standard DPC under the frame, wrapped around the edges and staple gunned to the top.

The Base stage 3

I used Recticel 70mm foil backed insulation for the base.  Cut to measure to fit between the joists.  Cut carefully for a good tight fit.  18mm OSB board fixed over the top.  Three 2440mm x 1220 sheets.

Wall Panels

I am building by myself, hence a big consideration is the weight of materials and being able to construct straight and true without an extra pair of hands to hold whilst I fix.  Hence the best solution is to pre-fab the panels comprising the stud work (100mm by 47mm) to the 18mm OSB panels, with the baseplate already on the bottom.  That way you can affix the upright stud work and OSB external board on the base plate, which will then simply lift up and sit nicely on the OSB floor ready to be screwed down.

Originally I was due to leave an internal door between the garden room and shed.  I've decided to now fill this in and cut a new door
on the front of the shed.... All in good time.

 The LHS made in 2 sections

 The LHS complete

 Turning the corner (see below for 3 stud corner diagram)

 In no time the back and RHS are complete.  The back built in three sections, the RHS in 2 sections.

Covered and ready for the rain.  The first of my two top-plates are fixed to give strength.

Diagram of a 3 Stud Corner.  This gives strength, and also two internal surfaces in the corners for when it comes to plaster-boarding

The next step is to complete the front RHS stud wall panel (1220mm wide), and a smaller 400mm front LHS panel.  I will then be in a position to start fixing the roof rafters.

Total working time so far:  no more than three working days over a few Saturdays.

Addition of the front panel and top plate across the front.  Built in the same way as previous panels.

Top Plate

During construction of the wall panels, I left 58mm below the base plate at the bottom of the OSB to overlap over the base, and 94mm at the top to allow for an overlap over a double 'top plate'  these will cross in the corners to give strength.   The Picture below shows the double top plate.

Roof

The roof construction will be one of a 'warm roof' design.  This meaning that the insulation is above the rafters, which keeps the rafters at room temperature, and minimises the chance of condensation forming and causing damp problems.

There are many designs for a warm roof, but the order from the rafters to the waterproof top are:

- Waterproof membrane (likely rubber EPDM membrane)
- Recticel Plylok 100mm (This incorporates a 6mm ply top, 90mm of insulation, and a foil faced bottom (vapour shield)
- Breathable membrane (optional I believe)
- Rafters

As the insulation sits on top of the rafters, I decided to first build the end rafters which would ride up at the ends to provide an enclosed area to sit the Insulation.  Pic of the pre-built LHS end rafter:

Rafters to be positioned every 600mm.  Currently two end rafters one one additional fixed:

The remaining rafters went up very quickly, with double rafters at the places where the 1.2m wide sheets will meet.  The pics below show the progress:

Now time to fix the roof sheets.  I am using pre-made warm roof sheets (Recticel brand) which comprise of a foil bottom (Vapour barrier), 90mm insulation, and a ply top.  these will be fixed directly onto of the rafters.  Fixing is achieved by eight 6" screws per sheet (3 sheets will cover the internal roof area).  It is also wise to run a bead of sealant on top of the rafters where the sheets will it and meet the adjacent sheet.

I should add that i added another piece of timber along the entire back edge on top of the rafters to give something for the Recticel sheets to sit up against.

Pics below of the bottom of the roof from inside, you can see the foil facing.

The Insulted roof panels are only large enough to cover the area of the roof which covers the inside of the office, not the overhang at the front.  Maybe would have been easier, but unnecessary expense and cutting, so I am covering the overhang with 5mm ply on top, a facia board on the front and soffit underneath.  The pics below show the woodwork ready to take the plywood from atop the ladder.  Once this is covered., and the facia board fitted, we can get on to fitting the EPDM rubber membrane over the whole roof.

Pic of the two new timbers fixed lengthways at the front and back of the overhang.  Note pre-drilled with 6" screws part screwed in, so I can lift it up, and finish the screwing easily.

And from the ground:

As shown elsewhere, the ply top was fitted to the over hang, as well as the cedar soffit and facia boards.  The roof deck was then ready for the EPDM top:

I'll not go into any detail on the fitting of the EPDM, as this can be found easily on any of the supplier websites.  Pic of the top - one of the front corners:

EPDM laid out before being glued:

Picture of the front before the trims were fixed:

Insulating the walls

The walls also need insulating, and I have chosen 70mm Rectacel foil backed & faced insulation panels, to sit between the OSB external sheets, and eventually the plasterboard interior.  Although the roof hasn't been finished, it is now waterproof with the sheets and temporary cover, so I am able to make a start on cutting and fitting the insulation.  They need to be cut to a good tight fit.