Building a shed from scratch

[Steven Briggs]

Hello all.

This years project looks set to be a large shed at mums place.

I'll be building from scratch, as I want a challenge and to achieve something better, more substantial and different from the usual stuff.

It'll be about 18' x 8', part potting shed, part summer house, with covered veranda deck area. There's a CAD render of the design at

I have a few questions for the collective wisdom of the group on construction methods and materials.

The latest thinking on the base is to mount it on single brick pads (on a large, existing concrete platform), then a 50x100 joist frame, T&G floor, then the shed structure.

Wall & roof framing I'm thinking 25x75 sawn timber studs & rafters,

50x75 sill & head. Inside skinned with 6mm WBP ply, outside with VT&G weatherboard. I'll put about 30mm of Celotex / Kingspan on the inside face of the wall cavity. I'm debating the need to vent the remaining cavity to the outside, I can easily have a 10mm hidden gap around the framing at ground & eaves level, and over the roof ridge board. Is this a good idea? Should I put a vapour barrier in there somewhere? If so, what to use?

Timber treatment is another issue. While the floor joists will be tanalised timber, I don't think my local timber yard has on-site treatment plant, so everything else (weatherboarding, frame, floor etc) will be untreated red or white wood. I could then paint or spray a suitable preservative on, but would it really be worth shopping round and getting the whole lot prevac treated (which adds about 20% to the cost)?

Any other construction or material suggestions welcome.

Finally, this is grand excuse for compressor and nail gun purchase. Probably the Axminster £100 compressor (2.5HP /25L /7cfm FAD) and 50mm brad nailer. Are 18ga brads going to adequate for the cladding?

Cheers all,

[Andy Hall]

Nice design - I think she should be very pleased.

With this, I wouldn't go for more than 450mm centres.

You will need a matrix of brick piers.

Alternatively, what I did was to start with a concrete base and then use pressure treated wooden bearers on it and the joists orthogonally on top of that. It's worth putting some DPC material like Visqueen down to isolate the timbers from the concrete. You can also use packing pieces on top of the bearers in order to level the final floor.

I'd up that to 50x75mm for good strength.

I would go for 12mm or even 18mm there because you can fix things to it. 18mm ply is used in large quantities (or OSB as an alternative) so it is not that expensive. You will get a much sturdier result.

That's fine.

Yes. What I did was to tack nail small spacers to the sides of the vertical studs such that the Celotex front face was level with the front of the studs. Then I tape sealed the Celotex to the studs with metallised tape and ply clad on top of that. This created an air gap behind and ventilation was arranged to the air gap.

Celotex has foil on both sides and you do not need an additional vapour barrier.

It would also be worth insulating the floor and ceiling.

For the floor, build the floor frame and then tack nail spacers in the same way as I described for the walls. Drop in the Celotex and put flooring on top of that. I used 18mm T&G for that.

it is also worth insulating the roof. I used exactly the same technique for that, making sure that there is ventilation on the cold side.

I dealt with this by using Cuprinol Clear Wood Preserver. which is a spirit based product.

When you get your compressor, get a cheap air powered paint sprayer and then spray both sides of the timbers. Three coats are recommended. You can apply any other finish afterwards if you want to.

Use a very good quality roofing felt. The heavy grade is not a lot more than the cheap shed stuff and lasts a lot longer.

Not really. I used 16 gauge. 18 gauge is normally used to hold things together for gluing, not for fixing. You need a nailer that takes 16ga finish nails.

.andy

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[Steven Briggs]

In message , Andy Hall writes

That is the plan.

Ah, good idea.

They will be.

Yep. Ventilation from eaves, and a small gap over the top of the ridge board.

Yeah, I thought it a little mean for the job too. But 16ga nailers are way more expensive (£175 vs £45). I'll have to see what's in the piggy bank.

Thanks Andy.

Now back to the CAD drawing. Doing a detailed frame design now. Next challenge is to draw the valley rafters.

is this afternoons efforts.

[Andy Hall]

OK. For this, I wouldn't go for a distance between piers of more than a metre, otherwise the floor is going to be very bouncy.

That's going to be a lot of piers (around 15-20). If the existing base is reasonable, I would rethink the timber bearers idea.

Also, it's useful to make a complete frame with the floor joists with timbers across the ends. Then you can make a skirt of TGV cladding and take it almost to the concrete.

You might want to think about two glass panels for the windows and doors or even find an inexpensive source of double glazing inserts.

This will make a big difference. With single sheet glass you are going to lose a lot of heat. Another option might be secondary double glazing inside.

I made a ridge cap out of Western Red Cedar and spaced it off from the roof. There were ventilation strips set into the top of the ridge underneath the cap.

This means that air is drawn in under the eaves into the space above the insulation and out at the ridge.

18 ga really is not going to do a good job of holding the boards.

The time saving of a nailer on a job like this is enormous so it is worth the investment.

Rutlands

have a 16ga nailer for £150.

Top Gun Air Nailers

have a Porter Cable for £150 plus VAT (£176). I have several Porter Cable nailers of different sizes including this one. Porter Cable and Senco are the two leading makes and this is a good deal.

I have also found Top Gun to be really good and helpful on obtaining nails at pretty good prices.

For example, I used cedar shakes for the roof of the cabin that I built last year.

cannot be fixed with galvanised nails because the tannin in the cedar corrodes them, so stainless steel was needed. Not easy to get but Top Gun managed to source and supply some.

If you need some help in the detail of the studs and rafters, there is a good plan for the construction of a shed at

which can be downloaded for $9.50.

This uses more or less exactly the construction that you have described and goes into how to correctly joint and notch the framing.

.andy

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[John Rumm]

That sounds familiar - played the same game myself a couple of years ago...

Oh very posh ;-) Mine was a bit simpler:-

I have a few questions for the collective wisdom of the group on

Fire away...

I did similar - 50x100 floor beams at about 500mm centres on a matrix of half bricks set level on the concrete base. I used 19mm ply for the floor which worked very well.

50x75 for the framing will give a really solid frame and a bit more space to play with for insulating and adding electrics etc if required.

50x100 ridge beam, 19mm shiplap cladding, 12mm WBP ply on the inside - as Andy suggests it is nice and easy to fix things to after.

Not suggesting this is the best way - but I went for 50mm jabfloor (cheaper than celotex - but not quite as good - so your 30mm will probably be as good or better). I fitted it flush with the outside of the framing so that it formed a vapour barrier next to the shiplap - that then left 25mm of space inside the warm/dry area for all the electrics I wanted installed in/under the interior ply.

Bog standard sawn kiln dried timber is nicer to work with and cheaper - just give it a few good coats of cuprinol first.

(as an aside, using a low pressure "pump up" sprayer is a quick way of doing the job without loosing as much cuprinol as you will with the finer atomisation you get with a spray gun on the compressor - really it needs soaking more than it needs a fine finish. I originally did my one with a brush which took ages - got a pump up sprayer from Wicks the other day for about 13 quid, that let me re-spray the whole thing in about an hour)

In no particular order:

Good three layers of felt on top of at least 12mm WBP ply.

Leave a little air gap underneath the whole shed so that it stays ventilated.

Give the floor beams and the underside of the floor a good soaking in wood preserver since you will not be seeing them again for a long time.

Get a decent respirator if spraying cuprinol (the 15 quid 3M jobbies from Screwfix work well and will also block the smell of the stuff completely)

Stick some diagonal cross braces in the wall stud work as it will make the whole thing much more rigid and will prevent any of the framing leaning or skewing. I ended up using only half the amount that you see on the drawings - as that seemed to be enough - but doing some is well worth it. While on the topic, stick a couple of cross braces between some of the rafters - this will convert the load of the roof (and anyone on it!) into a downward thrust rather than a "splaying" thrust that would otherwise tend to push the walls out. Note also three layers of roofing felt is heavy!

My tools / materials shopping list was:-

are a bit on the light side - having said that - they are what I used. I used 30mm brads which are cheap enough you can afford to use loads. Hence I stuck between three and five into the lower quarter of each plank every time it crossed a joist. It has been up a couple of years now and seems to be holding tight - although it is in a reasonably sheltered location.

Anyway if you want the whole saga you can read it here:-

[Steven Briggs]

I've just been down the timber yard, sawn 25x75 is a bit scaggy. 2nd choice was CLS, but they only have 2.4m in stock. So 50x75 regularised carcassing it is. It'll work out the same cost as CLS when wastage is taken in to account.

I think if it were my shed, I'd use 12mm, but I don't think there'll be much, if anything, hung of the walls in this case. 6mm is much easier to handle single-handed too.

I thought Jabfloor was polystrene, i.e. not to be in contact with PVC wiring?

Good tip, I hadn't thought of diagonals, but it will make a huge difference. There's already a divider wall that'll brace the side walls, and I was thinking of another one or two cross braces as well.

[Peter Crosland]

Take a look here before you start to make sure you don't run into future problems.

[Mike Mitchell]

"...we can see that the drains must have been leaking for a while.... not to mention a crack in the soil pipe....", as that is pretty much what happened to me when I lifted *my* chunk of concrete aka patio! I replaced the elbow (clayware) and had to pop down to Tesco's for Number Twos for a couple of days while the cement set. (No way would I use the public bogs in the village!)

MM

[Andy Hall]

Which is extremely unlikely.

Regarding sheds the issues are:

- Size

- Proportion of garden covered by all outbuildings and additional developments.

- Height

- Distance from house. material used may be relevant here.

- Distance from road

Unless an absolute monstrosity is constructed, a typical shed or summerhouse will not fall foul of any of these.

It also has to be said that there are large numbers of instances where a shed may fail on one or other of these technicalities and the planning authority won't bother.

For example, near me there is an instance where the bottoms of the gardens of some houses back onto a road but with normal 2m fencing. All of them have at least one shed or greenhouse at the bottom of the garden. Technically, these are too close to the road, but it's a non-issue to the planners.

.andy

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[Andy Hall]

I was able to get 5.4m delivered by Jewsons if it helps. Pretty cheap as I remember. One point is that it is worth going to a place like this with your entire shopping list. They will give you a project price with quite heavy discounting when you include the Celotex and boarding as well.

Don't forget that the ply will form part of the strength unless you are going to diagonally or cross brace the framing. I think that it's better to screw the ply to the frame if you are going to use 6mm.

For wiring, in any case, I used PVC conduit with singles run in it. You can then run additional wiring if you need it later and it's also pretty cheap.

.andy

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[David Hemmings]

What about distance from adjoing properties boundaries, if it is wood construction over a certain size the minimum distance should be at least x m ? Unless of course you have it treated with a flame retardant.

[Andy Hall]

Basically the rules are this:

Planning ~~~~~~

Greenhouses & Sheds - (and garages more than 5 metres from the house) are permitted if they cover less than 50% and are less than 3m high or

4m, with a ridge roof.

Building Control ~~~~~~~~~~~

In order to be exempt the detached building must satisfy the following criteria

No sleeping accommodation Internal floor area must not exceed 30m2 More than 1m from any boundary or substantially non-combustible Can be any distance from boundary if not exceeding 15m2 floor area

.andy

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[John Rumm]

I had two solutions to that problem - first: use the plastic collars with rubber O rings for joining the pipe - that way as soon as it is assembled it is water tight. Second: get the next door neighbours son to climb down in the hole and fit the pipe while I promise to not flush the loo! ;-)

[John Rumm]

Don't know if your preference for assembly is screwing or nailing, but I found that two 4" twinthread quickscreews (Screwfix) per joint made for very strong framing even when screwed into the end grain of the wood.

It is - hence why I put all the wires in oval conduit.

[Steven Briggs]

In message , Peter Crosland writes

Its in the back garden, well away from the road. The grey block its sat upon in the .jpg I linked to is a concrete base of a soon to be ex-garage. Even measured from the natural grade, I will just be under the 4m height limit. Actual height will be about 3.05m from the base level. I don't know which level the planners measure from, but I'm OK.

[David Hemmings]

Ta, Yes that is what i was on the lines of thinking about.

[Pete C]

Hi,

Nice plans! Might be worth considering 4" rockwool beween 4" joists, would be cheaper, better and easier to install than 30mm Celotex I'd expect, though a vapour membrane could be necessary.

If you set the bricks supporting the floor timbers in cement this will allow you to even out some variation in the concrete base.

As far as timber goes, it tends to rot at the ends where exposed to water so dipping these pieces in wood preserver then coating the ends would be a great help.

Getting everything pressure treated seems like a bit overkill and CCA treated wood is nasty stuff to work with.

cheers, Pete

wrote:

[Andy Hall]

It isn't - I've tried it. The problem is that the rockwool tends to hold on to any water that gets in for whatever reason (e.g. driving rain against the side for a long time, among others). It is difficult to support it away from the outer cladding to permit ventilation. A vapour barrier is definitely needed and will need to be fitted additionally.

For insulating below a floor it is a PITA because there is no good way to support it effectively.

I made shed in this way some years ago and ended up ripping out all the rockwool and replacing it with Celotex.

I've now done three buildings with it, including the garage workshop and it is extremely easy to use because it stays in place and is light. A given thickness of polyisocyanurate foam has approximately 4x the insulating property of glass fibre, and the foil on both sides provides the vapour barrier. It is also a lot more pleasant to work with because there are no fibres to stick in the skin.

THe only slight disadvantage is the slightly higher cost, but in the context of a project like this and the time taken, it's a no-brainer.

It should certainly be treated with some form of spirit based preservative if it is to last a reasonable time.

.andy

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[Pete C]

It's doable. One way would be to staple support netting on the outside of the studs, then tack a batten on the studs to give the required spacing. You could also add tyvek/housewrap as well as the netting for extra protection if required.

Another way would be to cut the netting to the fit the gap between the studs and staple it away away from the outside to give a gap.

If the shed is in a well sheltered position you might just be able to put housewrap on the outside and leave out the gap.

You wouldn't have to use a vapour barrier but if you wanted one then some poly stapled to the inside of the studs would do. It would give you a better vapour barrier than celotex fitted between the studs.

Again, you can get support netting for this. The netting could be stapled to the bottom of the joists as they are laid down, or stapled to the side of the joists.

I take it the rockwool was directly against the cladding, and leaking or blown in water was soaking into it.

AFIAK celotex is 60% better for a given thickness, so 100mm rockwool is about twice as good as 30mm celotex.

True, although if you space the studs correctly you only need to cut the rockwool to length.

True, celotex would only work out £3/m2 dearer.

I think it also depends on how you rate the different types of insulation, and what sort of deal you can get on what's available.

True, any wood that might get repeatedly wet should be treated. Having a look at some old sheds that are starting to rot gives you some idea of what areas need particular attention.

BTW my neighbour has a _really_ nice shed, I'll try and see if he'll let me put a picture of it up at sometime to give the OP some ideas.

As shed styles go a lot depends on the surrounding environment, a Wickes type shed would't look too good next to a 17th century thatched cottage.

cheers, Pete.

[M. Damerell]

I would like to suggest a DPC between bricks & wood. My apologies if you already thought of this.