Carport structure

[Benpointer]

Hi All, now that the house is done and we have been in 8 months (very nice, thank you) I have turned my attention to something much simpler - a carport. 

 

Broadly what I am looking to do is a 5.25 x 5.75m carport with two open sides, oak post in each corner and a shallow (8.7deg) mono-pitch roof in single ply  membrane or Greencoat on a ply deck, to look like standing seam zinc, echoing the zinc roof on the main house.  

 

We have planning for that as a freestanding structure next to an existing timber workshop but have now put in a new planning app to integrate the roof with the workshop roof, for aesthetics.  But fundamentally the carport needs to be its own structure.  We don't need Building Regs as <30m2 area.

 

Some sketches attached but the question I have now have is how to design and size the posts, beams and rafters to cover the span safely and deal with wind loadings etc?  I am currently thinking a glulam lintel across the 5.25m front opening and open web joists from front to back (5.8m) with the back supported on a timber stud wall and suitable wall plates.  Or possibly a second glulam spanning the opening midway to act as a purlin and then smaller solid rafters from front to middle and then again from middle to back.

• Pay a SE to do the calcs, and if so what do I need to provide?
• Approach a rafter/glulam company with the design constraints and ask them to design and cost it for me?

 

I am very conscious that whatever I build will need to be very well tied down to the pre-existing 6m x 5.5m x 150mm reinforced concrete slab, which I assume is going to be enough to stop it all taking off in a gale.

 

Here are some pics, and thanks for your help!

 

Views of the proposed carport (existing workshop not shown but abuts the west side of carport)

 

Floor plan (not that there's much to see - confusingly West is to the top of this plan(!))

 

Extract of block plan showing the newly proposed integrated roof (North to the top).

 

[ToughButterCup]

Hello Ben . Happy Monday Morning

Glad you are enjoying your house : and have the courage to think of even more building. We are at the same stage - well almost. A few SWMBO requests seem to have surfaced.

 

I haven't got much time this morning, so to help my thinking I did a quick AI search ( yes, it makes mistakes). Here's the result.

 

I treat AI as a starting point for my own thinking and always check it using a different AI, and - if merited - always ask for professional advice

This is from Gemini

 

Your points about beam sizes is about half way down.....

 

Quote

Designing a carport requires balancing practical space requirements, structural frame integrity, and local planning thresholds. Below is a practical design framework to guide your layout.

 

1. Key Dimensions & Sizing

Ensure the design accounts for vehicle clearance plus room to fully open doors and walk comfortably around the vehicle.

 

Layout	Minimum Width	Minimum Depth	Recommended Eaves Height
Single Vehicle	3.0 m	5.5 m	2.2 m – 2.4 m
Double Vehicle	5.5 m – 6.0 m	5.5 m – 6.0 m	2.2 m – 2.4 m
High-Clearance (Van / Roof Box)	3.2 m	6.0 m	2.6 m – 2.8 m

Tip: If mounting against an existing wall (lean-to design), ensure the ledger board attaches to solid brickwork/masonry rather than cladding or render alone, and check that the pitch line does not obstruct ground-floor windows or upper boiler flues.

2. Structural & Material Options

Timber Framing (Posts & Rafters)

• Freestanding Post Size: Minimum 100 mm × 100 mm (4"×4") pressure-treated C24 timber for lightweight roofs; 150 mm × 150 mm (6"×6") or green oak posts for heavier or wider spans.
• Ground Anchors: Metal post shoes or steel stirrups set directly into concrete pad footings (minimum $450\,\text{mm} \times 450\,\text{mm} \times 600\,\text{mm}$ deep) to keep post bases elevated out of standing water and prevent timber rot.
• Cross-Bracing: Triangular knee braces at the top corners of posts are essential for racking resistance against crosswinds.

Metal / Aluminum Systems

• Powder-coated extruded aluminum frames offer low maintenance, slimmer profiles, and integral rainwater drainage within the support legs.

Roofing Coverings

• Twin-Wall / Multi-Wall Polycarbonate (16mm–25mm): Lightweight, economical, lets natural daylight through.
• Corrugated Bitumen or Steel Profile Sheets: Highly durable; low-pitch metal roofing sheets require anti-condensation felt backing underneath.
• EPDM Rubber / Felt on OSB3 Decking: Best for flat or low-pitch roofs (minimum $1:40$ fall to ensure water run-off).

3. UK Planning & Regulatory Constraints

In England and Wales, a carport is generally treated as an outbuilding under Permitted Development rights, provided it satisfies these core limits:

 

• Location: Must sit behind the principal elevation (front wall) of the original house.
• Boundary Proximity & Height Limits:
   
  • Within 2 metres of a boundary: Maximum overall height must not exceed 2.5 metres.
  • More than 2 metres from a boundary: Maximum height is 3 metres for flat/pent roofs or 4 metres for dual-pitched roofs.
• Land Coverage: The structure, plus other outbuildings/extensions, must not cover more than 50% of the total garden/curtilage area.
• Building Regulations: Open carports (open on at least two sides) under $30\,\text{m}^2$ floor area are typically exempt from standard Building Regulations approval.

To help tailor the specific structural layout, beam sizes, or roof pitch for your build:

 

1. Will this be freestanding or a lean-to attached to an existing wall?
2. What are the dimensions or types of vehicles you need to accommodate?
3. Where on the driveway or boundary line do you plan to position it?

Downloaded 05/10/2026

[Benpointer]

Aha! Yes, I have also consulted Gemini's besty: Mr ChatGPT and he says... well, he says a lot tbh but he ends up with:

 

 

 

 

[Russell griffiths]

Haven’t read this all but just as a quick one, instead of a central glue lam, change all rafters to kerto beams, they will span longer, will not warp and twist, will come straight. 
they will need painting as they may not be that attractive unfinished. 
just a quick thought to chuck in the mix. 

[Benpointer]

23 hours ago, Russell griffiths said:

Haven’t read this all but just as a quick one, instead of a central glue lam, change all rafters to kerto beams, they will span longer, will not warp and twist, will come straight. 
they will need painting as they may not be that attractive unfinished. 
just a quick thought to chuck in the mix. 

I'd not heard of kerto beams before, will take a look.  Thanks.

[JpManning]

Nice project. I'd go down the SE route, but you can keep the fee sensible by giving them a really clear brief. A small timber frame like this isn't a big job for them if the information is all there.

What I'd send: a dimensioned plan and section showing the post positions, the 5.25m opening, the 5.75m depth and the 8.7 degree pitch. The full roof build up (ply deck, membrane or Greencoat, any battens) so they can work out the dead load. Your postcode, rough height above ground and how exposed the site is, because the wind and snow loads come from location and terrain. The slab details: thickness, mesh if you know it, any edge thickening, and how far the posts will sit from the slab edge. And your preferred materials, so oak posts plus glulam or LVL, solid or open web rafters.

With a shallow mono pitch and two open sides, uplift is usually the governing case rather than the weight of the roof. The critical bits are the connections all the way down: rafter to beam, beam to post and post to slab. Expect the SE to specify raised post shoes so the oak end grain stays off the concrete, fixed with resin anchors or through bolts sized for the uplift, with proper edge distances. Worth getting them to confirm the slab edge can take those anchor loads, not just that the slab is heavy enough. With two open sides you'll also need racking resistance, so knee braces at the posts or a braced back wall.

On the beams, it's worth talking to a glulam or LVL supplier in parallel. Many will size a single member for you, but they normally won't design the whole frame or the connections, which is where the real risk sits. A midspan purlin with shorter solid rafters keeps member sizes down. Full span LVL rafters give a cleaner soffit, but deflection over nearly 6m is what will size them.

If you do tie the roof into the workshop, I'd keep the carport structurally independent and use a flexible flashing detail at the junction so any differential movement doesn't crack or tear anything.