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Basic Polycarb' Lean-to shelter. Timber sizes and no of supports?

Discussion in 'Building' started by Safety First, 10 Jan 2020.

  1. Safety First

    Safety First

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    Hello and thanks for taking a look.

    Our garden is enclosed with a fence that returns to meet the side of the house and I want to made dry a section around 17 Sq/m with a simple lean to using polycarbonate roofing. Not done this before...

    Can anyone please advise on timber sizes?

    Not sure of centres- will probably go with something that matches polycarbonate widths...?

    It will be enclosed at the drive side- only open at the wider end, (there is a 6' gate to driveway not shown.

    See pics for sizes. I have looked for span tables but cannot find any(thing simple like me).

    At eaves end, I want the gutter to stop along the edge of the current fence so it will keep out the rain. Shame the area is not square....

    Plan view shows the roof only having x2 support posts. With a 5.2m span? Thinking x2 5"x2" nailed and bolted together?

    I want to make it independent of the fencing as I expect it will be the fence that needs replacing next.

    Internet says no need for planning. I think it's a very steep pitch so maybe it can afford to be on the skinny side- timbers wise... but then the gutter up to the fence top design would hold snow... But not much snow in Somerset..

    And if anyone knows peasant logic ways to get the angles and birds mouths correct (as I have no common joist lengths :( ....I'd be grateful of any tips/pointers...



    Any thoughts/pointers much appreciated.
    Thanks, M
     

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  2. John D v2.0

    John D v2.0

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    I'm not a builder but for a <4m span and you're only worrying about snow and burglars in terms of load, 5x2 sounds ample to me.
    You also have to worry about the whole thing falling over like a pack of cards which would be the odd diagonal. On trussed roofs they just put a long timber like an old school floor board diagonally across the rafters in both directions.
    In terms of the birds mouths you can look on youtube for much better explanations. But your other problem will be the varying span, as you would probably want to keep the pitch constant rather than it looking like it's slipped down at the back edge. That means either your eaves need to be higher at the front, or the bit next to the wall needs to be higher at the back. That would make your angles all the same but the lengths would be different still.
    I'll let the pros come up with a more detailed explanation, or just go through on youtube and watch them at work.
     
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  3. Safety First

    Safety First

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    Blimey.... hadn't even thought of that... ( Pitch paradox)... Just saved me some time and money there. Thanks. Most of what I make is a bit heath Robinson meets Beverly Hillbillies. Reckon ?.. I could make it square with the minimum 3m depth to start with where the gutter up against fence, then running away as fence goes off out the extra 0.7 m. Then make another baby pitch roof that brings rainwater from the fence down and into the gutter- making a valley.

    What other options are there if:

    The wall plate (I know it's not called this really..memory fails) has to be level.

    And
    The whole space has to be dry? Rain is driven hard at this area. So gutter meets fence aspect was key.

    Drew a picture, and my wife made 'the face' ... So I am still looking for options...:rolleyes:
     
  4. oldbutnotdead

    oldbutnotdead

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    I think you'll be stiick with varying pitch- sloping wall plate or sloping interface with fence will look very odd. Might be a good idea to mock it up with some screws in the house wall (where the plate will go) & along the top of the fence, run blue polyprop string where the roofing edges will be and see what it looks like. Remember you won't notice the pitch variation inside the leanto, it'll only really be visible when you look from above or to the side. Your baby roof idea with valley would be hard work & give you either lower headroom near the fence or a big hole for the wind to get into.
     
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  5. Safety First

    Safety First

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    Thanks for your input. :D

    I struggle to get my head around the appearance of a sloping wall plate as it is very much on show to the road side our place being semi detached on street corner .If understand you correctly we could instead slope the horizonal that the joists rest on at the eaves end , which would mean that the eaves finish higher as the rafters get longer. (My) Aim being to send the gutter out to meet the fence line, which runs away from the house so keeping wind and rain out of the sheltered area.

    I get it that you're commitment to this nonsense is probably concluded with your input this far (and thank you:D).

    Think I'll set the wall plate level, in final position and do as you say, set string lines as if joists to arrive along another string line as if facia that runs in a good place relative to the fence. Then measure down and away towards working out the height of each support post that holds up the eaves.

    What could possibly go wrong?
    She made 'the face' again.
     
  6. oldbutnotdead

    oldbutnotdead

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    Other way round...with a horizontal wall plate and fixed pitch the eaves (at the fence line) will get lower as the rafters get longer. Which will look very odd, almost as odd as a sloping wall plate
     
    Last edited: 11 Jan 2020
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  7. John D v2.0

    John D v2.0

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    I second the mock-up idea, I cant really get my head round it either, I just feel like it's a strange situation! The valley idea does sound like a nighgtmare given it's just a simple shelter.
     
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  9. oldbutnotdead

    oldbutnotdead

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    I think you'll come unstuck if you try and keep to a constant pitch- the sloping effect on the fence line will look very strange. The purpose of the mockup was to use straight and level for the fence top and wallplate and simulate the varying pitch for you to gauge whether it looks really weird or not (I don't think you'll notice it from the inside and who cares what anyone looking from outside thinks- unless you're trying to sell the place in which case don't even bother starting it- won't add value & could put a lot of people off
     
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  10. John D v2.0

    John D v2.0

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    Not wishing to question your preconditions but does it need to be steep? If you do it at 1:20 or so with a bit more overlap it might work better? I'm also concerned how well the sheets would tesselate, maybe theyd end up on the wonk as well
     
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  11. oldbutnotdead

    oldbutnotdead

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    I was assuming the thin flimsy plastic corrugated stuff (which I think has got enough flex in it to get away with the varying pitch). Twinwall or wrinkly tin would be a bit of a mare
     
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  12. Safety First

    Safety First

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    Thanks all.
    What a minefield!
    Was thinking of twinwall polycarbonate as might look neater and less Hillbilly than the corrugated. Last longer too?. A none square layout with changing pitch would almost certainly be a disaster for me with twinwall as I appreciate the corrugated is a tad more accommodating.

    It's not meant to add value, but I have done a lot of DIY indoors and wouldn't want anything outdoors to look too shonky- would alarm people!

    Pitch is very steep indeed. Going to take some pics tonight and mock up on MS paint to see if it just looks really weird. Reason for such height is that we are aiming to fit a long clothes dolly/ line along the whole 4m length so you can walk under the clothes to reach side gate. That's where the height comes from.

    We have another lean to porch monstrosity that I didn't build around the back and it's similar height with a clothes dolly and pulley set up. It goes up above the back door and inspired us to make this larger outdoor lean to.
     
  13. Safety First

    Safety First

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    Ok. So not sure if I should start a new thread as it's a new question really, but from the same project...

    Thanks to input so far I am clear on the following..
    5" X 2" rafters will probably do fine. Roofs are more complicated than I thought.

    A sloping wall plate is beyond my skill and I think would look weird. I cant imagine the trouble I would get into making a variable pitch roof.

    Current plan is to set wall plate level. Set eaves support rail level and square, i.e. rafters will be common length from high end to birds mouth.....but...at the narrow end of the lean to the eaves will overhang by a whisker, and at the wider end by 600mm.

    Only this will take the roof line over to the fence which runs out away from the house by 600mm.

    It will also make the facia drop approx 200mm end to end but as it will be against a fence no one will see it....

    Providing my birds mouths aren't too sloppy, 5"X2"s won't sag, or will they?
     
  14. oldbutnotdead

    oldbutnotdead

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    Hadn't done the sums before but if you've only got 200mm to lose across 5200 then I withdraw my objection to twinwall or corrugated iron (the curved stuff) on a variable pitch roof. The joint bars on the twinwall might be a chew to fit but apart from that you'd be fine (200mm across 5200 comes down to 40mm per metre which is nowt).
    With your new scheme the vertical support post 600mm from the fence is going to reduce your usable area and be tricky to brace in any meaningful fashion without reducing usable space even more. And you'd be committed to the gutter running down towards the wide end at a somewhat steeper angle than guttering normally runs at.
    Yes that is a fairly steep pitch but not unheard of- you might have problems with rainfall overshooting the gutter and hitting your fence (which will accelerate the deterioration). Various ways of dealing with that- low-rent is run some big beads of silicon on the roof 200mm above the gutter (slows the water down), give the gutter a high back with some Code 3 lead or felt support trays.
    If the fence is nearing end of life I'd suggest fixing it now and using the fenceposts as the support for your eaves (fix blocks level with the top of the fenceposts, sit your rafters and eaves board on top of the blocks, gutter runs above fenceposts).
    Go with the variable pitch roof- no common birdmouths but that isn't the end of the world. You could work them out using trigonometry but it is much easier to set wall plate and eaves board then offer rafter, cut to length (remember angled cut at top and bottom), straight edge, mark, cut (remember same drop both ends) and fix.
    5" x 2" will be fine for the rafters, one bonus of your steep pitch is snow load becomes a bit theoretical- it might snap the gutter off but it isn't going to overload the roof.
     
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