Building an outdoor shelter - Advice on Timber Sizes and Spacing

I have an outdoor cooking area with a BBQ and Pizza Oven. It generally only gets used in good weather as preparing pizza dough when it's ******* down isn't a good idea.

The area I'm looking to cover is 4m wide by 2.2m deep. The roof will slope on the short length. I intend to cover the roof with 2440mm x 840mm Corrapol Stormproof Low Profile polycarbonate sheets.They need to be fixed at 500mm. I'm not looking to build a shed so the sides and front will be partly open.

I'm guessing that given the width I'll need 3 4x4 posts along the front and back. However, due to the current paths (I'm not planning on changing the layout) I will be taking a notch off one corner.

This is what the roof 'could' look like but I may be giving myself more work than necessary.

NOTE: these drawings are not to scale.



So I see the roof as being more like this:-



As I need to be able to fix the roof sheets every 500mm I will need 4 cross beams. To act as the main roof support I think I'll need jist/rafters at c.600mm spacing.




The questions I am hoping people more knowledgeable than me can answer are:-

1) Do I need the middle posts or could i do a single span across?
2) Do I need as many main beans (in green) as I have shown or could I use less?.
3) What size of timber will I need for the frame and beams? I had thought of 6x2 for the outer frame with 4x2 running 'vertically' (green lines) on my drawing with 2x2 running horizontally (green lines) at right angles on top of the 4x2.

I do have the option to make it a 4000mm x 2200mm and therefore remove the need for the 'notch'.

Am I over-engineering the roof or am I risking making it very heavy and it collapsing under its own weight?

DiscoGaza said:

1) Do I need the middle posts or could i do a single span across?
2) Do I need as many main beans (in green) as I have shown or could I use less?.
3) What size of timber will I need for the frame and beams? I had thought of 6x2 for the outer frame with 4x2 running 'vertically' (green lines) on my drawing with 2x2 running horizontally (green lines) at right angles on top of the 4x2.

I do have the option to make it a 4000mm x 2200mm and therefore remove the need for the 'notch'.

Am I over-engineering the roof or am I risking making it very heavy and it collapsing under its own weight?

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It sounds like you're over engineering it.

Lets pretend it's a first floor in a house, so you might want to put a bed on the roof, maybe a wardrobe.

http://nhbccampaigns.co.uk/landingpages/techzone/previous_versions/2010/Part6/section4/appendix.htm

If you did joists going across the short span, 2.2m you'd be looking at something like 6x2s with 600mm centers or 4x2 with 400mm centers. In practice you're not going to be walking on the roof so you can probably drop that down to 4x2 at 600 and still be overbuilt.

A lot of conservatory roofs use joining bars that also serve as the structural support, so you'd just need the frame around the outer edge. Is there a reason you're not looking at those? Edit: just looked at the ones you're looking at, I was thinking of the flat sheet style ones.

IT Minion said:

It sounds like you're over engineering it.

Lets pretend it's a first floor in a house, so you might want to put a bed on the roof, maybe a wardrobe.

http://nhbccampaigns.co.uk/landingpages/techzone/previous_versions/2010/Part6/section4/appendix.htm

If you did joists going across the short span, 2.2m you'd be looking at something like 6x2s with 600mm centers or 4x2 with 400mm centers. In practice you're not going to be walking on the roof so you can probably drop that down to 4x2 at 600 and still be overbuilt.

A lot of conservatory roofs use joining bars that also serve as the structural support, so you'd just need the frame around the outer edge. Is there a reason you're not looking at those? Edit: just looked at the ones you're looking at, I was thinking of the flat sheet style ones.

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My concern was that it was over-engineered and I think you agree.

As the roof only needs to support itself and the Corrapol polycarbonate sheets I think I can cut down the number of joists/rafters to have approximately 1000mm centres but I'll still need the same number of pulins as the sheets need to be fixed at 500mm intervals.

I think need to keep the double joist/rafter in the middle as I can't find any 2x2 longer than 3000mm .

Do you need 2x2? Looking a the sales brochure it looks more like you just need battens than structural Purlins.

If you want longer lengths of wood then your friendly local timber merchant may have some. Also, rather than double up the beam you could just butt the two purlins against each other on the central rafter.

Slating battens (25 x 50) will do for the 'purlins' as will 50 x 100 for the rafters, 50 x 75 would probably be OK, that sheeting looks fairly stiff (should be at £12/sq m, you could put cement slates on for £13/sq m). Don't rely on the DIY sheds for timber- look at builders merchants and/or timber merchants. 4800 is the standard length for most construction timber.
Is this thing freestanding or is there a wall somewhere? You'll need some quite serious anchoring if any of the post runs are freestanding to prevent the whole structure from lifting. Again if it is completely freestanding you'll either need some crossbracing between the posts or excessive buried length of post to prevent the whole thing from racking, also if freestanding some diagonal bracing across the rafters would be wise

oldbutnotdead said:

Is this thing freestanding or is there a wall somewhere? You'll need some quite serious anchoring if any of the post runs are freestanding to prevent the whole structure from lifting. Again if it is completely freestanding you'll either need some crossbracing between the posts or excessive buried length of post to prevent the whole thing from racking, also if freestanding some diagonal bracing across the rafters would be wise

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My plan is to use 3 x 3000mm posts for the rear and 3 x 3600mm for the front and concrete them in at around 600-900mm depth.

The area I'm building it in is quite sheltered. The wall is about 20 feet high but as it is Grade 2 listed I can't fix anything to it. In any case, it goes off at an angle of about 110 deg from the fence so would make the shape awkward.

How much heat does the Pizza oven kick out?
My local pub landlord wasn't the sharpest knife in the box and built a thatched BBQ.

Are you actually going to cook in the pouring rain?
You might consider something like reeds/cane as a covering (note above) as it will provide shelter without being a hot box in sunny weather

Tigercubrider said:

How much heat does the Pizza oven kick out?
My local pub landlord wasn't the sharpest knife in the box and built a thatched BBQ.

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The LL sounds like a prime contender for a Darwin Award.

The oven gets up to 400-500c internally. Not a great deal comes out of it. The chimney will be going through a flashing in the roof so there won't be an issue with heat of smoke.

Tigercubrider said:

Are you actually going to cook in the pouring rain?
You might consider something like reeds/cane as a covering (note above) as it will provide shelter without being a hot box in sunny weather

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I do tend to use it year round although last winter I took to using my small Ooni Koda gas oven in the garage as I didn't have shelter or light.

I had considered the effect of sunlight through the roof but at the same time I wanted something with lots of natural light. I do need something that will stop rain as it destorys the pizza dough and/or makes it impossible to get the pizza on to the peel and in to the oven.

You will need a specified clearance around the chimney where it goes through the roof and some thermal insulation to help protect the polycarbonate catching fire/melting.
Not sure who you would need to contact to find the exact details though.

conny said:

You will need a specified clearance around the chimney where it goes through the roof and some thermal insulation to help protect the polycarbonate catching fire/melting.
Not sure who you would need to contact to find the exact details though.

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I've looked in to it and there are flashing designed for taking the flues from wood burning stoves through polycarbonate roofs. You also need a double flue to insulate it.

DiscoGaza, just found this thread and it saved me a headache!! I had 8 joists/beams then got wobbly thinking it may be too heavy and over engineered so based on info here i now only have 4 joists. How was the roof sheets to handle? any tips? Have you finished your project and was it a success! Just wanted to say this thread helped me out

I just filled a corrapol roof on a tomato growing shelter and the sheets are no problem to handle, even on your own (2.4m sheets) and they give you plenty of fixings in the box compared with the listed quantity.
You can pretty much use any timber, we used 4no 3m long 3 by 2s from a timber merchant and they are not going anywhere.
You're supposed to support the edges of the sheets but we do have an overhang of 30cm or so at the top and bottom which I'm not worried about.
My suggestion would be to get the edge of your roof at 90 degrees and mark a right angle at the point where each sheet ends, as otherwise if you screw it down tightly you'll end up going off at an angle! The marks will keep you straight.
Good luck!