Shed/Greenhouse

[cutter12]

Hi. Its not exactly a workshop that am building but a small Greenhouse/shed. (This seemed the most appropriate sub-forum to post in). It won't be the most glamorous construction. I just want the space to store some pots, grow tomatoes and propagate things.


The general plan is:

• Footprint - 3x2m approx.
  
  Plinth - Concrete foundation > single course of 215 x440x100mm block > DPC .
  
  Framing - 3x2 (400mm OC)
  
  Windows and a Door - x3 salvaged
  
  Roof - Single Pitch. 1/3 slope (4/12 Pitch). x5 - 2.4 x 0.6 Mtr Corrugated PVC sheets.
  
  Shiplap - for the wall areas without windows

At this early stage in the planning I have come up with a few issues:

• 1. Rafters. For the Single Pitch roof I was planning on using 4x2 at 400mm OC but the span from (outside) of wall to wall is 1960mm. I am worried that this might be too great a span for 4x2. Would I be better increasing to 6x2 or using a Gable roof?
  
  2. Headers. The largest window is 2032mm wide. I was planning on using a laminated 4x2 + 4x1 above this for the header. I am thinking that this is maybe not beefy enough? the stud work is 3x2 so I cannot go wider but a could go deeper. (the other two windows are 1360mm wide).
  
  3. Bracing.. Three of the walls have mostly window so there will be little space for diagonal bracing. On one wall there will only be space for noggins either side of the large window. Any ideas on how to stiffen up such a wall?
  
  4. Plinth. I was thinking of attaching the P/T 4x2 Sill Plate to the Block plinth with hammer fixings. https://www.screwfix.com/p/fischer-nylon-hammerfix-8-x-100mm-50-pack/33940?_requestid=154229 Are these going to be strong enough? They will also penetrate the DPC. Will a blob of silicone down the hole be OK?

OK I think that's is. Even though I want to keep costs to a minimum I know that this may not sit well alongside the reality of building something useful and enduring.

Anyway, thanks for your patience ( if you get this far through my post. ).

Any input would be greatly appreciated.

Jon

 

[MikeG.]

Jon, have a read of my shed building guides linked to in my signature. They'll answer some of your questions.

As for the roof.....plastic corrugated sheeting.........you don't need rafters, you need purlins. You most certainly don't need rafters at 400 centres. You could easily have a rafter at each end, and one in the middle, and have 2x2s on top of that to fix your roof sheets to.

Use galvanised mild steel straps to hold the frame down. Don't fix through the sole plate into the blocks, as this pierces the DPC. Oh, and you say your sole plate will be 4x2, but your framing is only 3x2. Why the difference? Just make the whole thing out of 3x2s (and the studs can be at 600 centres).

As for bracing, if you aren't lining with sheathing board, then you need bracing. You can either build it into the wall (diagonal 3x2s with the studs cut onto it above and below), or you can plant 4x1s on the face of the studs internally, extending from the corners to the sole plate. Please tell me that you are wrapping the outside of the frame with a membrane, then battening out before fixing feather edge boards.

Headers? You mean lintels, I think. There's no weight on your roof at all. The biggest forces will be upwards, when the wind blows. You can do whatever you like above the windows and doors, so long as you have a continuous wall plate.

This is a bit of an odd building, with solid walls and a clear plastic roof. I'm sure you have your reasons, but the danger is that it falls into the shed category rather than the greenhouse category.

 

[cutter12]

HI Mike.

Thanks so much for replying to my post.

As for the roof.....plastic corrugated sheeting.........you don't need rafters, you need purlins. You most certainly don't need rafters at 400 centres. You could easily have a rafter at each end, and one in the middle, and have 2x2s on top of that to fix your roof sheets to.

What would the max unsupported span of the 2x2 purlins be? If they run the length of the structure (3m) then putting in x3 rafters (4x2) would put the unsupported purlin span at 1.5m (and the rafter span at almost 2m). Do you think the roof will be strong enough?

Use galvanised mild steel straps to hold the frame down. Don't fix through the sole plate into the blocks, as this pierces the DPC.

I agree that fixing with straps is a better idea with regard to the DPC. I am just not sure I want the straps exposed on the inside of the structure. The floor inside will be just soil (for growing in) and therefore the straps will be exposed to moisture and will not be so neat. I am unsure what to do as the strap solution seems the only way I could tie down without penetrating the DPC.

Oh, and you say your sole plate will be 4x2, but your framing is only 3x2. Why the difference? Just make the whole thing out of 3x2s (and the studs can be at 600 centres).

I was going to use a 4x2 sole plate so that it covered the top of the block course and DPC completely. Also if I was going to screw through the Sole plate into the Block then I thought the 4x2 would enable me to screw into the centre of the block for better purchase (if that makes sense?).

As for bracing, if you aren't lining with sheathing board, then you need bracing. You can either build it into the wall (diagonal 3x2s with the studs cut onto it above and below), or you can plant 4x1s on the face of the studs internally, extending from the corners to the sole plate.

Ok. That is what I was worried about. The problem is that the lack of space for bracing. For example the window is 2032mm wide but the wall is only 3000mm. The wall is only the height of the window (plus the block and framing). The windows are strong hardwood ones but without the bracing they would become structural and that sounds like a bad idea. I have seen structures online where people build greenhouses totally out of old windows but it appears the windows are used structurally. Many Greenhouses don't seem to have diagonal bracing but even with research I cannot establish how this is possible.

Please tell me that you are wrapping the outside of the frame with a membrane, then battening out before fixing feather edge boards.

You are not going to like this but no, I was not intending to do that. If it was strictly a workshop or shed then I think it is a very good idea. But as the floor inside is just open to the soil then I thought it might be best to just ensure ventilation on both sides of the Shiplap. Three of the walls are mostly window so I did think that a membrane was not necessary (I am probably wrong here then). As I was not planning on boarding the inside, a membrane would be exposed to wear and tear from inside the structure .Keeping any water ingress through the Shiplap away from the stud is definitely something I am now considering . I could fix membrane only where the framing meets the Shiplap and trimming it so that it isn't seen from inside (if that makes sense) . However there would be no air gap. Would it make any sense to tack membrane strips to the frame on the outside, then put battens on the studs? This would leave the Shiplap not in contact with the stud and also there would be no membrane visible from the inside.

Headers? You mean lintels, I think. There's no weight on your roof at all. The biggest forces will be upwards, when the wind blows. You can do whatever you like above the windows and doors, so long as you have a continuous wall plate.

Yes, you are correct I do mean lintels. I have read one to many American Shed Building books and they refer to them as headers. It is probably now stuck in my head forever. Are you saying that I could actually forgo the Lintels all together? I could downsize them to 3x3 then, as it quite windy where I live at the moment as I am quite near the coast.

This is a bit of an odd building, with solid walls and a clear plastic roof. I'm sure you have your reasons, but the danger is that it falls into the shed category rather than the greenhouse category.

Agreed, it is a bit of an odd building. My motivation for building it is to create shelter for growing and propagating plants. Essentially the windows, door and clear plastic roof are what I want to tie together into something that is durable. I actually have x5 windows but am probably only going to utilise three of them. The structure only really needs light on the three sides and it should be OK that the corner framing casts some shadow. (In the summer it will probably get too hot!). I think you have summed it up when you say that it falls into the 'shed' category, that is probably my problem. I am trying to use the windows to construct a 'greenhouse' but maybe using shed building methods? A greenhouse seems to combine the 'window frame' and 'structure' whereas I am trying to build a frame structure then add windows (like a shed).

I have read your posts on here and Woodhaven (and looked at many of the other builds). They are great. I gather that I am not the only person who appreciates this valuable resource and the time spent giving advice.

Thanks,

Jon.

 

[MikeG.]

What would the max unsupported span of the 2x2 purlins be? If they run the length of the structure (3m) then putting in x3 rafters (4x2) would put the unsupported purlin span at 1.5m (and the rafter span at almost 2m). Do you think the roof will be strong enough?

Yes. As I said, there are virtually no downward forces on the roof, and it is more likely to blow away in a gale. Clearly it has to have a decent enough pitch that it doesn't gather a huge snow load if you live in the Highlands, but otherwise, yes, that's strong enough.

I agree that fixing with straps is a better idea with regard to the DPC. I am just not sure I want the straps exposed on the inside of the structure. The floor inside will be just soil (for growing in) and therefore the straps will be exposed to moisture and will not be so neat. I am unsure what to do as the strap solution seems the only way I could tie down without penetrating the DPC.

Finish the straps above the soil.

I was going to use a 4x2 sole plate so that it covered the top of the block course and DPC completely. Also if I was going to screw through the Sole plate into the Block then I thought the 4x2 would enable me to screw into the centre of the block for better purchase (if that makes sense?).

Makes sense, but it makes constructing the framing awkward, unless you do it from the wrong side. Get rid of the problem by using straps.

. That is what I was worried about. The problem is that the lack of space for bracing. For example the window is 2032mm wide but the wall is only 3000mm. The wall is only the height of the window (plus the block and framing). The windows are strong hardwood ones but without the bracing they would become structural and that sounds like a bad idea. I have seen structures online where people build greenhouses totally out of old windows but it appears the windows are used structurally. Many Greenhouses don't seem to have diagonal bracing but even with research I cannot establish how this is possible.

Many greenhouses lean. Also, the aluminium ones use the glass as their bracing.

Simply get the maximum bracing you can. If you are really worried, you could double up the studs at the corner so that bracing which may be only half-way up the corner post (to get under the window) is still effective due to the stiffness of the corner.

You are not going to like this but no, I was not intending to do that. If it was strictly a workshop or shed then I think it is a very good idea. But as the floor inside is just open to the soil then I thought it might be best to just ensure ventilation on both sides of the Shiplap. Three of the walls are mostly window so I did think that a membrane was not necessary (I am probably wrong here then). As I was not planning on boarding the inside, a membrane would be exposed to wear and tear from inside the structure .Keeping any water ingress through the Shiplap away from the stud is definitely something I am now considering . I could fix membrane only where the framing meets the Shiplap and trimming it so that it isn't seen from inside (if that makes sense) . However there would be no air gap. Would it make any sense to tack membrane strips to the frame on the outside, then put battens on the studs? This would leave the Shiplap not in contact with the stud and also there would be no membrane visible from the inside.

I'd still use a membrane, personally, as I described.

..... Are you saying that I could actually forgo the Lintels all together? I could downsize them to 3x3 then, as it quite windy where I live at the moment as I am quite near the coast.

Yes. You have virtually no downward load on the roof. You simply have to have something that it beefy enough not to sag under its own weight over the years.

 

[cutter12]

Thanks again Mike.

The straps do seem the way to go and thanks for clarifying the roof and lintels.

Forgive me for not quite grasping here but there are two points in your reply that I don't quite grasp:

Makes sense, but it makes constructing the framing awkward, unless you do it from the wrong side. Get rid of the problem by using straps.

What do you mean by "the wrong side"?

Simply get the maximum bracing you can. If you are really worried, you could double up the studs at the corner so that bracing which may be only half-way up the corner post (to get under the window) is still effective due to the stiffness of the corner.

I apologise for the very simplified non-scale drawing below (red>window, black>framing, blue>bracing. Not showing Jacks/cripples etc) . Currently I plan to locate the Window (as in A) on the base plate (the reduced height on that side of the structure will be fine for growing), so I wont be able to extend the diagonal bracing beneath it. Am I right in understanding that I should frame the window higher to allow the bracing to go beneath it? (as in B).

Thanks, Jon

 

[MikeG.]

By the wrong side, I mean that you build a frame flat on the ground. Normally, you would flush up the surface of the studs and plates to the top, where you can see and feel them. That isn't possible if you have a 4x2 sole plate and 3x2 studs.

"B" is what I would be looking for. Your bracing has to go from sole plate to corner post to do any good.

 

[cutter12]

Ok. I get that now. Thanks

 

[AJB Temple]

On a side note - aluminium greenhouse kits can be picked up in sales very cheaply. They are tedious and fiddly to erect, but do the job of a greenhouse very well. What you seem to be building is pretty much a shed. The thing with greenhouses is that they are often wet as you will be watering and spraying your plants. Is a wooden structure what you really want for this?

 

[cutter12]

Good point AJB,

An aluminium greenhouse would have merit. I will do a bit of a search for prices here in Ireland and see if it is a possibility.

One concern though would be strength against the coastal winds. I might have to add some extra beef to the frame anyhow.

...... And what do I do with the old windows? #-o

 

[Phil Pascoe]

You can often pick up decent sized frames with glass missing for next to nothing. You'll probably have to dismantle it, but it's not difficult - make sure you have plenty of different coloured cable ties so you can mark the pieces as you do it. The glass is standard and easily obtainable. If you do get a greenhouse think about putting on at least one if not two courses of blockwork to raise the height - the extra headroom makes tying up plants much easier (this applies whatever you choose to do, of course).

 

[Benchwayze]

Jon,

My first shop was built onto the footprint of a derelict greenhouse. (It would have been a great little greenhouse when new.) I built the shop nine feet square outside dimensions, with 9 inch solid walls (Although the top half was mostly glass. Roofed with corrugated iron in lean-to mode.
So what's that in metres? 9 square? Near enough. If that wasn't big enough; and it wasn't, then I think you're going to struggle too; regardless of what use you make of your space.

I bought my first machine (A Coronet major, lathe-based, combo machine. ) After that there was hardly room to swing a cat-o-nine-tails! )

I wish you the very best; but you'll have to keep yourself organised and disciplined, to make the space useful. I'm struggling with 17 feet x 8 feet; believe me!. I can't move in the place!

I bought a shed in which to store timber, but when my Lady became ill, that had to be left half-done. But we never give up do we? I don't suppose you will either Jon! (hammer)

Best of luck again.

John

 

[cutter12]

Use galvanised mild steel straps to hold the frame down. Don't fix through the sole plate into the blocks, as this pierces the DPC.

Mike, as the wall is single skin how effective will the straps be regarding lateral wind pressures (I live by the coast)? Maybe I am worrying and over thinking this but am I right in thinking that the straps on the windward side of the structure prevent uplift but not inward lateral movement of the wall framing?

BTW I have now got the concrete perimeter strip foundation down and am going to use brick instead of block for the pier. They were left over from another project and when i counted them up I found I had just enough

 

[sunnybob]

You can buy an aluminium and glass greenhouse for a couple hundred quid, with auto opening windows, how much is this costing you? :shock: :shock:

 

[MikeG.]

Use galvanised mild steel straps to hold the frame down. Don't fix through the sole plate into the blocks, as this pierces the DPC.

Mike, as the wall is single skin how effective will the straps be regarding lateral wind pressures (I live by the coast)? Maybe I am worrying and over thinking this but am I right in thinking that the straps on the windward side of the structure prevent uplift but not inward lateral movement of the wall framing?........

Timber framed buildings are treated generally as "acting as one", rather than each wall being considered in isolation. It isn't the strapping in the wall which faces the wind that is really doing the work, it is that in the 2 walls at 90 degrees to the windward wall which are taking the strain. This is always the case, no matter which way the wind blows. That's one of the reasons why the straps should start fairly close to the corners on each wall, to provide the longest lever possible against the wind on the adjacent wall.

Obviously, if wind is a big issue, you should be strapping your roof down to the walls.

 

[eezageeza]

You might also want to consider how much light you're going to get through the roof and windows. Greenhouses are nearly all glass for a reason - I think you'll have disappointing results growing stuff because your plants wont be getting enough light.

If you want a wind resistant cheap growing structure, have you considered a domestic sized polytunnel ?

 

[cutter12]

You can buy an aluminium and glass greenhouse for a couple hundred quid, with auto opening windows, how much is this costing you? :shock: :shock:

Hi sunnybob, I am not sure how much it is costing. No more than a cheap greenhouse hopefully :shock: (The cheapest aluminum greenhouse I have seen in Ireland is around €400). I have the windows and the bricks already so its makes good use of them. It might go against logic but now the idea is in my head I feel the compulsion to try and build something. On the plus side it will be so much stronger than a greenhouse, I get to practice laying concrete, bricklaying and the challenge of building something (although I am not sure if 'challenge' is exactly the right word )

Timber framed buildings are treated generally as "acting as one", rather than each wall being considered in isolation. It isn't the strapping in the wall which faces the wind that is really doing the work, it is that in the 2 walls at 90 degrees to the windward wall which are taking the strain. This is always the case, no matter which way the wind blows. That's one of the reasons why the straps should start fairly close to the corners on each wall, to provide the longest lever possible against the wind on the adjacent wall.

Obviously, if wind is a big issue, you should be strapping your roof down to the walls.

Thanks Mike. That makes so much sense. I hadn't visualized that before. I will put them close to the corners.

You might also want to consider how much light you're going to get through the roof and windows. Greenhouses are nearly all glass for a reason - I think you'll have disappointing results growing stuff because your plants wont be getting enough light.

If you want a wind resistant cheap growing structure, have you considered a domestic sized polytunnel ?

Hi eezgeeza, I take on board what your saying. I think the light will be sufficient for the propagating/growing on/hardening off that I want to mainly use it for. The clear Pvc roof will not all the rays through but it is surprising how much light is brought in through the roof rather than the walls. I think it should be OK for a couple of tomato plants in the summer too. As I get better equipped its role may slide towards more of a shed and I might swap out the roof for something opaque.

Unfortunately I don't have a high enough shelter belt yet for a poly tunnel but when I get one it will be most likely one of the Polycarbonate style ones: http://www.ksbgarden.com/index.php?rout ... duct_id=67