Does anyone know anything about design codes for wooden beams/roofs?

I’m going to build a carport, but I’m having trouble finding a design code to apply to part of it.

The roofing sheets will sit on purlins running perpendicular to the fall of the roof, supported by some other beams (rafters?) below.

2018-08-20_10-11-19 by [/url] – Flickr2BBcode LITE

The purlins are straightforward enough to size up using span tables, but I’m struggling to find a suitable design code for the beams holding them up.

Any ideas?

Find an old copy of Part A of the building regs – its full of span tables etc.  Its all been omitted from the current version though.

Find an old copy of Part A of the building regs – its full of span tables etc.  Its all been omitted from the current version though

Cheers for the reply. It’s the building regs I’ve been looking at for the span tables, problem is that the tables assume that spacing is relatively close, and that the ends are supported by walls. That’s fine for the purlins, as I can space them to suit the regs.

As this is a car port, I don’t have any walls – so I need to size the beams holding up the purlins to do the job the wall would do. I can’t find anything in the regs to size beams to take that sort of load. I can’t modify the spacing to suit the tables, as there are only so many posts holding it all up.

That’s a good chunk of roof area, is 8t go8ng to be exposed on 3 sides? I’d also be concerned about uplift if so.

Not much to add other than to make sure you design it so the timber isn’t directly in contact with the floor. They will rot in no time. I built a 3.6m x 3.6m tiled roof gazebo and used 150mm x 150mm treated posts. I used the trada span tables to calculate most of it but sized the posts on what looked right.

You can buy off the shelf products like this

https://www.canopyproducts.co.uk/Stainless-steel-vertical-post-plate200mm-brush-polished-column-326mm-highSSBFH0

but there are many ways of achieving the same result. I designed some, got them laser cut and had them galvanised.

That’s a good chunk of roof area, is 8t go8ng to be exposed on 3 sides? I’d also be concerned about uplift if so.

How would you address those concerns? Is there a pressure due to wind load for open structures defined somewhere?

Is there a pressure due to wind load for open structures defined somewhere?

It is, but it’s not free of charge. It used to be BS 6399 Part 2 (Part 1 would have the gravity loads), now superseded by BS EN 1991-1-4. Usually (see here) a carport doesn’t need approval under Building Regulations, so if you can find either of those standards you’ll be in the right ball park, maybe add a bit on if you’re using the old one.

The requirements are summarised in “The Designer’s Guide to Wind Loading of Building Structures” by N J Cook, which you might be able to find a copy of.

Alternatively, find a structural engineer who has access to the codes and the experience in using them (I’m retired, so don’t have access to them)

What size is the structure? And what pitch is the roof?

Like Greybeard I too am now retired but I can access span tables tomorrow. Assuming England/Wales?

I’m currently banging up another shed at work (100000sqft) and it has a huge canopy that the client wanted to be fully cantilevered out from the portal frame, we just couldn’t make it work due to uplift, so it’s had to have a post on each of the front corners, I’m no engineer but I am a realist builder amd without a load of calcs I knew it wasn’t going to happen. Obviously your car port isn’t going to be quite like that, but it’s hard to say what’s required. Have you much protection from other structures or is literally bolted on the side of a building on the top of a hill?

It is, but it’s not free of charge. It used to be BS 6399 Part 2 (Part 1 would have the gravity loads), now superseded by BS EN 1991-1-4.

Ta, I’ll see if I can acquire those.

What size is the structure? And what pitch is the roof?

Like Greybeard I too am now retired but I can access span tables tomorrow. Assuming England/Wales?

Size TBC, pitch will be 4 degrees at least as that’s the min spec for the roofing sheets! Location is Scotland, east coast.

 Obviously your car port isn’t going to be quite like that, but it’s hard to say what’s required. Have you much protection from other structures or is literally bolted on the side of a building on the top of a hill?

Yea I’m not looking for someone to size them for me, don’t worry! I’m an analysis engineer by day so I don’t mind wading through the codes and doing the maths. Main issue is that I work with pressure vessels so I don’t know anything about the applicable codes for this application.

It’s in a flattish area, but it is rural so I assume the wind loads will be factored to suit being relatively exposed.

I built something similar (but smaller) at my old house, but it was only one bay and I just sized it ‘by eye’ which I’m not happy with doing with a larger one.

It’s in a flattish area, but it is rural so I assume the wind loads will be factored to suit being relatively exposed

Yes, you start with a basic wind speed based on location, then factor in the effect of ground roughness, topography (if relevant) and height above sea level. On the East coast you can often reduce the loads by using the directional method, which allows for the prevailing wind, ie, the strongest winds are SW but crossing the country slows them. Then you get factors for the wind direction related to the shape of the building. Do you have access at work to any of the online code services like IHS, or BSI? Even if the sub has been taken out for pressure vessel codes they are sometimes broader in scope.