Shed to office conversion - interior framing and ceiling

Done something similar on a bigger sized shed with 4.2m high walls.

The spanning of rafters like you describe will be fine so long as they are sized correctly.

How high are the current shed walls?

Assume the water we see is the result of you hosing it out and not dampness?
Is the slab infilled so it virtually seals the tin sides to the concrete? or is the tin sitting on top of the slab or partway down the side of the slab?
Do you know if the slab had a plastic membrane before the pour?....possibly easy to find out if you dig a small hole on the outside of the shed
....just something to consider for possible future dampness issues, may need to consider base plate options (others here will know better than I)

The walls could easily be 90x35mm @ 450 centres as you planed but nothing wrong with 90x45. Just staple the siso foil to the back side of the frame before standing the frames up.

The ceiling rafters and walls aren't really holding a lot of weight (10mm plaster board) which simply needs the correct size for the span and the 10mm plaster board. One thing to consider is will you use the crawl space as a small storage area keeping in mind it will be as hot as hell!!! If so, factor what sort of weight and place some 12mm ply or yellow tongue flooring in strategic areas....even a 400m strip down the length in case you ever have to crawl up in that tight space will make it really easy Suggest to buy a roll or packing tape, after you run the lighting wiring, staple this to the underside approx 450mm apart, stack in your insulation b4 you fix the plaster https://www.ebay.com.au/itm/37158443...79f073bc69c5cb

If you stick R2 in walls and R4 or R6 in the ceiling, you will be amazed at just how cool it is in a "shed" as the walls wont hold the heat with this type of construction (will flow up into the ceiling). If you can rig up you ceiling so the heat escapes easily, its likely to be cooler than you house!!!....that been my experience however with mine its 4.2m high walls with a mezzanine so slightly different.

If you're after an additional 100/200mm usable width, just set back your stud walls exposing the single side post and butt your plaster up each side.


There are 2 other construction methods to consider that will be a lot cheaper but different finish result and not as well insulated in the roof.
1. Roof: Just fix 6mm or 9mm ply to the roof. Use cement joiners to butt the joins to each other. Ive not done this method, but others here have and appears to look quite good.
2. Walls: Screw yellow tongue flooring directly to the wall tophats with 5 to 10mm packers on the bottom to keep it off the slab. Could seal the bottom edge for better dampness protection. Ive seen 3 sheds done this way and the finish looks really great, just paint your preferred color. You will still get R2 batts into the walls.

Brilliant, thank you very much for your response Bart. I'll go through your post and respond to each of your questions.

Originally Posted by Bart1080

How high are the current shed walls?

The walls are ~2.4m. This should result in a flat ceiling height of ~2.2m once the joist and plasterboard thickness has been taken into account, which is totally fine for my application.

Originally Posted by Bart1080

Assume the water we see is the result of you hosing it out and not dampness?

Correct, I took this photo after we'd just moved in and I spent the afternoon removing all the previous owner's tool boards, shelves, etc. and then gave it a thorough pressure clean.

Originally Posted by Bart1080

Is the slab infilled so it virtually seals the tin sides to the concrete? or is the tin sitting on top of the slab or partway down the side of the slab?
Do you know if the slab had a plastic membrane before the pour?

It appears that when the shed was originally built, the slab was not infilled. However, it looks like the previous owner infilled the left hand side and rear of the shed at a later date. The right hand side was still open when I moved in, but I've since filled that right hand channel in as well.

As for the slab itself, I've seen one section of the slab on the outside of the shed where some plastic membrane is sticking out, so it's fairly safe to assume that it has been adequately protected.

Originally Posted by Bart1080

The ceiling rafters and walls aren't really holding a lot of weight (10mm plaster board) which simply needs the correct size for the span and the 10mm plaster board. One thing to consider is will you use the crawl space as a small storage area keeping in mind it will be as hot as hell!!! If so, factor what sort of weight and place some 12mm ply or yellow tongue flooring in strategic areas....even a 400m strip down the length in case you ever have to crawl up in that tight space will make it really easy

I don't plan on using the triangular section above the ceiling for storage; that may change in the lead up to commencing the work, but I've got adequate storage elsewhere in the house/property so would be happy to just leave that area empty.

Originally Posted by Bart1080

If you can rig up you ceiling so the heat escapes easily, its likely to be cooler than you house!!!

I plan on installing a whirlybird to aid in ventilating the roof cavity.

Originally Posted by Bart1080

If you're after an additional 100/200mm usable width, just set back your stud walls exposing the single side post and butt your plaster up each side.

This was my initial plan, which I've moved away from because I'm not sure if the additional 100/200mm width would really make much difference in my application, and I'd prefer the extra air gap and a continuous plasterboard wall. I might change my mind on this, I'm still playing with the layout of my office equipment etc.

Originally Posted by Bart1080

There are 2 other construction methods to consider that will be a lot cheaper but different finish result and not as well insulated in the roof.
1. Roof: Just fix 6mm or 9mm ply to the roof. Use cement joiners to butt the joins to each other. Ive not done this method, but others here have and appears to look quite good.
2. Walls: Screw yellow tongue flooring directly to the wall tophats with 5 to 10mm packers on the bottom to keep it off the slab. Could seal the bottom edge for better dampness protection. Ive seen 3 sheds done this way and the finish looks really great, just paint your preferred color. You will still get R2 batts into the walls.

Thanks for the additional options, I'll certainly give them a look. My main priority is definitely the insulation of the shed, followed closely by the finish quality, so I think the plasterboard option is still going to be the best, but I'm happy to explore other options as well.

I've just come across the hySPAN 'Span Guide for Residential Framing' document again (I've been through it a number of times previously), and realised that it does actually provide a span table for ceiling joists 'supporting ceiling lining only' (Table 11 on page 15 for those following along, I've included a screenshot as well).



Given that my span is ~4900mm at a 450mm spacing, I think I'd opt for the 140x45mm joist which gives a little bit of headroom (5200mm maximum span).

From that screenshot, I do have a couple of questions:

1. "Ceiling joists not fixed to rafters require blocking at supports to prevent rollover" - I'm guessing blocking is not required if the joist is secured to the 2x45x90 top plate with some sort of steel bracket (which both secures the joist and prevents rollover)? If so, what sort of bracket should I be using for the joist-top plate connection (e.g. z-style bracket, hurricane tie, etc.)?
2. Is a single, central overbatten sufficient for my application, or should I be considering noggins as well?

Originally Posted by jars121

The walls are ~2.4m. This should result in a flat ceiling height of ~2.2m once the joist and plasterboard thickness has been taken into account, which is totally fine for my application.
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With walls only 2.4 resulting in a final ceiling height 2.2 as opposed to something like Davids finished design with a roomy "cathedral" ceiling, you might be kicking yourself later on. Allows for more flexible uses that you havnt thought of yet and maybe more appeal for buyers when you sell at some point. Could still achieve good insulation by running tophat battens across the existing ones that would allow for R3 batts.

Looks an interesting project.

Originally Posted by Bart1080

With walls only 2.4 resulting in a final ceiling height 2.2 as opposed to something like Davids finished design with a roomy "cathedral" ceiling, you might be kicking yourself later on. Allows for more flexible uses that you havnt thought of yet and maybe more appeal for buyers when you sell at some point. Could still achieve good insulation by running tophat battens across the existing ones that would allow for R3 batts.

Looks an interesting project.

I've now come around to this very idea Bart! I'll be opting for a pitched/cathedral interior ceiling after all, mounting rafters/battens to the existing tophats, with the central c-beam and apex brace poking through.

One question I have, for which I've been researching and haven't been able to come up with a definitive answer: should I install sarking/anticon (the type where it's secured between the colorbond panel and the tophat) in addition to the insulation batts, for the purpose of controlling/reducing condensation? I would need to remove each of the roof panels, install the sarking/anticon, then re-install the roof panel, so I understand it's a considerable job, but I'm also mindful that the shed/office will be housing some fairly sensitive equipment, so I'd rather put the effort in if there's going to be a demonstrable reduction in moisture within the shed.

Originally Posted by Moondog55

I'd simply use rigid insulation board directly.
I used 10mm Foilboard because that's what we had. If I was doing it from scratch and had no building wrap beneath the steel sheeting I would pay the extra money and use 25mm and install some effective roof mounted venting at the gable.
Does the concrete floor have a vapour barrier under all of it?
If not and the equipment is both sensitive and delicate you may need to address that issue at the same time, so it pays to double check that before going too far.

Thanks moondog, that's actually very helpful. Following reading your comment, I gave Foilboard a call and explained the situation; their advice is pretty much exactly what you've said. They recommended the 25mm board, and recommended foregoing sarking (which would mean I wouldn't have to remove roof and wall panels which would be a huge bonus).

Given the air gap and 25mm Foilboard, we calculated an R-value of 2.5-3.0 for the ceiling, which would be great. However, we didn't discuss the R-value potential of using the 25mm foilboard for the walls, so I assume it's going to be similar. Is this sufficient insulation on the walls, or should I be looking at using the Foilboard in conjunction with batts? I raised the idea of using batts in conjunction with the Foilboard for the ceiling and was told not to bother, which I'd be fine with, but then when looking at stockists of the 25mm foilboard, I came across this statement (on pricewiseinsulation.com.au): "We recommend installing Foilboard Insulation in combination with insulation batts".

Originally Posted by jars121

One question I have, for which I've been researching and haven't been able to come up with a definitive answer: should I install sarking/anticon (the type where it's secured between the colorbond panel and the tophat) in addition to the insulation batts, for the purpose of controlling/reducing condensation? I would need to remove each of the roof panels, install the sarking/anticon, then re-install the roof panel, so I understand it's a considerable job, but I'm also mindful that the shed/office will be housing some fairly sensitive equipment, so I'd rather put the effort in if there's going to be a demonstrable reduction in moisture within the shed.

Depends....If I personally had the opportunity I'd prefer for a shed roof installing a blanket (insulation that is foil lined on one side) with the foil facing down. The blanket presses tight into the roof profile eliminating any exposed tin and air underneath and therefore any chance of developing condensation to drip down. Generally there is a wire mesh layed first to support the blanket and the tin screwed on top.

A lot of work for a good result but maybe the next best option is batts pressed in underneath fitting snug between the top hats. You could try the blue packing tape up over the tophats and back underneath the bats to hold in place as you screw the ceiling lining in place. I've never used foil boards in this application, would be concerned if condensation would be created - others that have used it between the ceiling lining and tine may be able to provide feedback.

Wouldn't bother with foil boards in the walls. If framing, then siso foil on the inside of the frame before standing the walls and batts. If flooring screwed straight to the walls, the just batts. Had excellent results with both methods.

Originally Posted by Bart1080

Depends....If I personally had the opportunity I'd prefer for a shed roof installing a blanket (insulation that is foil lined on one side) with the foil facing down. The blanket presses tight into the roof profile eliminating any exposed tin and air underneath and therefore any chance of developing condensation to drip down. Generally there is a wire mesh layed first to support the blanket and the tin screwed on top.

A lot of work for a good result but maybe the next best option is batts pressed in underneath fitting snug between the top hats. You could try the blue packing tape up over the tophats and back underneath the bats to hold in place as you screw the ceiling lining in place. I've never used foil boards in this application, would be concerned if condensation would be created - others that have used it between the ceiling lining and tine may be able to provide feedback.

Wouldn't bother with foil boards in the walls. If framing, then siso foil on the inside of the frame before standing the walls and batts. If flooring screwed straight to the walls, the just batts. Had excellent results with both methods.

Thanks again Bart, much appreciated. I'm leaning towards siso foil and batts in the walls instead of the foilboard as you've said; do you have a particular recommendation as to the siso foil?

Originally Posted by jars121

Thanks again Bart, much appreciated. I'm leaning towards siso foil and batts in the walls instead of the foilboard as you've said; do you have a particular recommendation as to the siso foil?

Not really, just what ever the hardware store had. Didnt research it ....but my thinking is how many ways can you make siso??? and just grabbed a role.