Narrow Lean To Shed Project

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10 Jun 2024
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Hi all. So, I had a plan for a lean to shed at the side of my house to give us an option of a garage conversion. I had a plan….then watched enough videos and read up on enough topics to give myself a degree/headache.

After stumbling across this forum with a wealth of experience and expertise I’m hoping someone can help me choose the right path to go down.

Bottom line I only have a 2 foot 8 inch/33.5 inch/851mm alleyway. Measured from the house brick to the fence posts. Measure to the fence post as this is where the frame would have to go, presumably.
I wish to use most of the length of the house. Around 24 foot to maximise storage. I’m aware nothing huge can go in here unless I step over it. I wish to maximise height as well, and as per regs I believe this to be 2.5M/8.4 Foot at its highest point. The square yardage will be in regs to and on that note apparently, I need the outer material to be code 0, or what is currently coded B. All this will avoid any planning permissions required.

Now this area will store things I had in the garage. I want it watertight, but I understand and accept it is basically a very long and narrow shed. However, I want it built right and last for many many years. I want it to feel like it’s a room as opposed to a shed without losing to much space! I’ve looked into water barriers etc. That is a rabbit hole of info!

Now as I’m so close to the fence I’m going to have to build the frame in 2 halves, maybe 3 and place it in situ with the outside material (looking at basic shiplap – Code pending) already screwed/nailed to the frame. Then build the other sides and everything else around it. So, it’s a bit more forward planning then a shed where you have easy access to all 4 sides. The wood will all be pressured treated with regards to the frame and roof rafters (get to that shortly).
This will be on cemented down flags as you can see in the picture. I’m going to raise it slightly. Can only really see this point to allow any rain going down the outer material to run off on to the floor and to add another layer of damp proofing. Also, there’s around the thickness of ½ brick of slope from one end to the other with regards to it being level so a raised floor will fix this minor issue.
The roof will be sloped with the aim of the slim guttering (again having to plan backwards) running exactly in the middle of the fence line, if anything slightly towards my side. The roof will be OSB3, felted, and finished off with code 3 maybe 4 flash leading then sealed. Approx. pitch 10-13 degrees or 2.5/12.
I’m sure I’ve missed plenty out but I’ll start with a few questions to get me going…

1) Is shiplap (Thinnest possibly really) a good call for the outer material. Bear in mind you will only actually see around 2 foot of this from the highest point to the top of the fence line. Just seems to work with the runoff of rain water

2) As far as space saving is the attached a good set up? Fence line side.

3) Membranes? Should I put one between the frame and the shiplap (or other recommended outer material)
4) With space saving in mind I wasn’t going to frame the brick wall side and instead have a Girder (Anchor bolts?). Maybe 1 support in the middle? Although a frame would be easier but I will end up losing to much space. Then again it is quite along run!

5) I intend to use 18mm OSB3 for the floor. 9mm for the roof and fence wall side?

6) Membrane under the raised floor, sat directly on top of the flags.
Going to leave this for now otherwise I’ll write a book. Obviously I’m on here for any opinions and ideas. If you have read this far and can help thank you so much! Off to B and Q now to weigh my options up.
A decent wall that will last and doesn’t feel like a shed will be at a minimum 125-150mm thick. From inside to outside plasterboard/plywood, vapour barrier if insulating, 75mm framing with insulation if required, breathable membrane, battens, cladding. On the house side you’d also need some framing to stop it feeling like a shed, likely 50-75mm minimum thickness. From your 851mm space that leaves you 675mm at best which is honestly not wide enough to comfortably walk down.

In my opinion I think you’ll have to accept it feeling like a lean-to so that you can leave the brick bare and gain the space of open framework.

If this is a shared boundary then nothing can overhang the boundary, even half the guttering width, without agreement from your neighbour. Without this agreement you can have real issues if you come to sell, or if you get new neighbours they can force you to remove it.

Some specific answers to your questions.

1. I think your cladding needs to be non-combustible as you are within 1m of a shared boundary.

2. No image attached to look at.

3. Yes, a breathable membrane so any rain penetration is kept out of the frame.

4. A 4x2 bolted to the wall will be more than sufficient for the rafters to sit on.

5. 9mm osb is fine for internal wall but is ugly and will look like a shed. 9mm osb is insufficient for the roof you need 18mm on 600mm rafter centres.

6. Yes a damp proof membrane would be required between flags and floor.


PS experienced DIYER opinions not qualified builder opinions.
See pic


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Thanks for reply. I quite like the idea of open framework actually. When you say 4x2 bolted to the wall. Do you mean sitting on frame work or a 24 foot 4x2 bolted directly to the brick work?

The fence side then what's best as far as material? You literally won't see about 5 foot by 24 foot. So really just want it water tight really. Maybe something half decent just so it looks OK on the neighbours side. Again they won't see it really.

Nothing will be over hanging but thank you for the advice.
You are not going to find access very easy - it will be a narrow corridor with space only for fairly shallow racks along the wall. A few thoughts:
  • is it possible to split the space and get access from both ends. This way you could end up with two "sheds" each ~11ft long.
  • alternative would be door through from garage so that the external ends would not need doors, just panels - more secure??
  • if you want it to last it may be better to go for something more substantial than thin shiplap - even though it will get minimal weathering
  • consider whether construction would be easier with a flat roof - draining to both front and back from the centre would require a fall of ~2-3".
  • a flat roof would be easier to install unless you have access to next door - working on a 2' 8" sloping roof whilst trying to lay felt and fix guttering would not be my choice.
  • to simplify construction - consider making in sections (say) 4ft long using a frame of 3x2, T&G cladding. Roof battens could be pre-attached to sides.
  • Sections could be bolted together rapidly with simply rubber "gasket" between each to ensure weather tightness.
Not a professional builder but reasonably competent DIYer.
Hi terry.

The house side is actually my livingroom and dining room. Otherwise a good idea. No access from the other side unfortunately. When you say substantial, what alternatives are there where the rain can easily run off? I do indeed intend on using 3 sections. Gasket? Didn't think of that. Yes i do have access to the neighbours side to with this project so the guttering etc is fine to work on
Red shiplap
Black Membrane

As no one will see about 4 foot of the side is there anything else i can use that will do the job. It appears cheap enough to just do the whole lot in shiplap to be fair. Just seems a waste. Any one recommend the membrane for the floor and walls?


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What is that sticking out the wall, looks like a boiler flue:

If it is you can't close that in.
Any recommendations of a company/timber yard i can get all my material from around the Bolton/ wigan area?
Hi. That's going to be extended to a raised flue above the roof
That's definitely not a diy job. Do not be tempted to just add a bend and a bit of flue pipe. The existing balanced flue must be replaced by a complete, new, correct flue from your boiler manufacturer. There's no such thing as a generic flue in the UK.
Rather than using timber cladding you could use corrugated iron sheet. It will probably work out cheaper than wood and you can buy it ready painted in various colours.
I'll have a look into it thanks. Everything i try and price up it seems b and q are cheapest. Timber merchants for example are around 10%+ dearer!
Hi. That's going to be extended to a raised flue above the roof
Hi Chris, my main job for over 30 years was for b. Gas as a service and installation engineer. I assume that’s a balanced flue fire and if so I’m not aware of any fire of this type having any sort of additional flue bends or extensions. They work on the principle of allowing air to enter the appliance via that flue and the flue products to leave from a 2nd pipe within the flue . They are designed to be straight and as short as possible. It all depends on the type of appliance balanced or open flue and as per @HOJ if the fire manufacturer allows such a modification and if so are the relevant parts available.. you might find that you will have to have the appliance relocated or simply removed. Hope this helps but take no chances with gas appliances.
Hi. Thanks for the advice. Definitely doing it the right way. If it can't be done or its silly money then I'll have to rethink things. Thanks

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