I’m planning to build a carport using the design found here. However the wood sizes quoted look way over the top to me. To keep the cost down I’m thinking that several of the parts could be downsized as the roof does not need to support any weight (suspect its a US design that’s designed to hold large snowfall build-up). The image below is the rough design.

using the following parts

(a) Posts 100mm x 100mm (4″ x 4″)
(b) Beams 200mm x 50mm (2″ x 8″)
(c) End rafters 200mm x 50mm (2″ x 8″)
(d) Rafters 150mm x 50mm (2″ x 6″)
(e) Blocking 150mm x 50mm (2″ x 6″)
(f) Galvanized bolts
(g) Joist Hangers
(h) Galvanized Angle Bracket

The ones I’ve highlighted in bold are the bots I’m thinking could be smaller. The carport is going to be 4500mm by 3500mm. Roofing will be corrugated PVC (I think), or maybe chipboard covered with roofing felt.

I’m thinking that parts (b),(c) would be fine as 150mm x 25mm, or even 100mm x 25mm, and parts (d),(e) as 100mm x 25mm.

Are these sensible downsizes?

Is it going to be flat or are you going to have a fall on it ?

youll need to take into account snow loading and wind loading, but Im sure there are guidelines available.

You may be able to downsize from 8×2 to 6×2 but I personally wouldn’t go any smaller, defo not down to 4×1 that is asking for trouble.

Going to have a fall. Design is for a fall over the longer length, however I’m going to do it over the shorter legnth as this will then drain over our fence into the wood next door (meaning no guttering initially). Will be giving it a 100 – 200 mm fall I think.

Wind loading is the only one that worries me, the most snow we have had in 10 years was only an inch or so. We are at the bottom of 2 hills sheltered by trees so not building in an exposed location.

Cool, thanks, going down just that little bit saves £1.20 a meter which is useful. Would applying the same logic to the rafters work, so 6×2 to 4×2.

As slimraybob says, you can go to 6×2 but i wouldn’t go for anything lower than 2…too much wibble wobble.

I’m not sure you’d find joist hangers to take anything 1″ anyway (but I could be wrong).

Would applying the same logic to the rafters work, so 6×2 to 4×2.

I’d still be sticking with 6×2 myself.

Ok, thanks :D.

over a 3.5m span, you’ll be fine with 4×2 (95×47) timber. I’d stick with the 4″ posts though. Where abouts are you? Selco is the best place I’ve found for construction timber by a long way. You will get everything you need in there of the shelf. Only use Tanilised timber though

edit, looks like the nearest selco branch to you is reading

Selco is the best place I’ve found for construction timber by a long way.

+1 on this too. They’re great value (though you need a trade account with them to buy which can make it a bit awkward).

I would go for a more significant pitch than you propose, and I wouldn’t downsize anything significantly if using that design (though it isn;t clear the spacing of elements). You could get away with smaller members all round if you used a different design based on either a triangular rather than rectilinear grid, or used a smaller scale structural grid (more frequent, but smaller members). Both of these would give considerations regarding access from the sides, and greater number of connections, but these may not be important.

Why not use matches and pva??
I’d stick with 6×2 for the bearing timber (b) and as funky said 4×2 for the rafters at 400 centres tho (d). End rafters have no need to be of the same size as (b) other than for aesthetic reasons. If you want to save further money do away with the three inner posts and fix a ground to the wall. Allow for a fixing at each rafter location then just sit on top and fix down. It’s one of those jobs that can be done to whatever spec really but only time will tell. Oh and for what it’s worth I’d fix two diagonals of 4×1 to the underside in a v just to stop that house of cards affect when you climb up on the top to fix your poly carbs!
Oh and that’s the other thing make sure you set out right to lap the timbers.

Shirley the addition of some strategic diagonals would strengthen it significantly, no?

Errr as above!!

Thanks for all the replies.

edit, looks like the nearest selco branch to you is reading

Yeah, quite a way from me in Southampton unfortunately, suspect the delivery will be prohibitive at that distance.

I would go for a more significant pitch than you propose

I’m planning on 2400mm posts on one side and 2100mm on the other, so can easily go to 300mm drop and maybe 400mm if I trimmed the shorted posts a bit (without loosing too much headroom).

You could get away with smaller members all round if you used a different design based on either a triangular rather than rectilinear grid, or used a smaller scale structural grid (more frequent, but smaller members). Both of these would give considerations regarding access from the sides, and greater number of connections, but these may not be important.

Side access is important due to positioning so the effectively rules out smaller scale I think.

If you want to save further money do away with the three inner posts and fix a ground to the wall.

I think this isn’t an option for me unfortunately. The carport is going out in a parking area not next to a building so needs to be completely free standing.

Oh and for what it’s worth I’d fix two diagonals of 4×1 to the underside in a v just to stop that house of cards affect

I’m going to use concrete in metposts so will definitely be doing this on the sides near the existing fence. Also going to brace some of the other joints near the top to get some triangulation. Will probably brace only the existing fencing to (partially as reinforcement for the fence).

Doing something similar-ish for my parents – as mentioned above, I agree that it looks a little oversized but do not underestimate wind loads!

I have gone to town a bit on the design calcs for them (because I can), working out the wind loads to BS6399 and Eurocode and assessing this against the design strength of the timber, foundations and fixtures (their site is on top of a hill, with minimal scope of vertical posts and adjacent to other building, so the ‘blocking’ out of the wind out from under the canopy is quite high) and the effects of the wind loading is dominating the selection of the section sizes….

I’d love to do that, but have no idea how (programmer not architect :D).

We do have a similar lean to against the house that was there when we moved in that seems solid enough, don’t think that has any wind issues, its is sheltered on 3 sides though by building and fences.