Garage Conversion - Large Span

[TheEasterBunny]

Hi,

We are converting a double detached garage at the back of our garden. It will need building regs approval because of the size (45sqm) and being at the boundary on 3 sides.
I am struggling with the roof - The open span of it (5.79m) is not even in the span tables so I am imagining we will need to get a steel installed. A builder said they'd use 7x2 timbers but that wouldn't satisfy building regs. Would we be better off getting a structural engineer to design it? Has anyone had this done and estimate a cost?

Alternatively, I have been looking at posi joists but they would be £400 each for the span we need.

Would one posi joist the whole width of the building with joists running to it work? (Shown in image 1).

Some drawings/photos for details. (can you tell I got bored of drawing straight orange lines? Only for reference, not to scale)

ETA: The orange line on the front of the sketch is an internal single brick wall with a rotten, wooden framed window in it. The window will be filled in but i'm not sure if that wall is at all helpful in the construction of the roof (weight bearing etc).

 

[TheEasterBunny]

Some more photos for condition and layout

 

[23vc]

Is the internal wall staying? Could inspect its foundations and if adequate span the 3m and 4.5m off it? Still chunky timbers for the 4.5 tho

Otherwise just get a steel calc done by an SE, I’d imagine it’s a pretty straightforward one

 

[TheEasterBunny]

Is the internal wall staying? Could inspect its foundations and if adequate span the 3m and 4.5m off it? Still chunky timbers for the 4.5 tho

Otherwise just get a steel calc done by an SE, I’d imagine it’s a pretty straightforward one

It is staying. It doesn't have to, I guess. I just never thought about it being removable... Hmmm

Will be checking foundations after Xmas. If good enough, the 3m and 4.5m isn't too bad. 7x2 for a 4.5 span.

 

[robinbanks]

Alternatively, I have been looking at posi joists but they would be £400 each for the span we need.

5.8m is a relatively small span. Have a look at JJI (OSB webbed I beam, Steico etc) joists if you're finding the metal webs pricey; I've got some 245Cs at that span and I think they were 70 quid a pop (3 years ago mind, but I did find similar at about a tenner a metre so maybe that's about right price wise)).

If you split a span, use a steel; it might well come in handy for hanging things from (a car/engine) in the garage. You can choose either to use long joists in a one-er over it or you can pack it out with joist hangers and hang joist ends on it. Lot more faff but means you can bury the steel in the ceiling.

Personally, I'd just get joists to go the span - forget splitting it. Your BC might want an SE to design the roof, snow loading, blah blah blah but a decent SE will be able to save you enough to make a good dent in their fee

 

[TheEasterBunny]

5.8m is a relatively small span. Have a look at JJI (OSB webbed I beam) joists if you're finding the metal webs pricey; I've got some 245Cs at that span and I think they were 70 quid a pop (3 years ago mind). If you split a span, use a steel; it might well come in handy for hanging things from (a car/engine) in the garage. You can choose either to use long joists in a one-er over it or you can pack it out with joist hangers and hang joist ends on it. Lot more faff but means you can bury the steel in the ceiling.

Personally, I'd just get joists to go the span - forget splitting it. Your BC might want an SE to design the roof, snow loading, blah blah blah

Interesting.

Just been on their website and we would require 300x72 at 400 centres. I'll contact them for a price. I like the interactive span table on the website - great for late night research!

I'll have a conversation with BC tomorrow about their expectations (we've gone private as our council is apparently abysmal and we're having a few other things done too).

Thanks!

 

[robinbanks]

What loading have you been given/are you assuming for the roof?

 

[cdbe]

What are you converting it into? Presumably not a garage, looks like a candidate for demolition and rebuilding, especially that bit on the left.

 

[TheEasterBunny]

What loading have you been given/are you assuming for the roof?

I've been looking at the span for 0.5kn pr sqm. Not sure if that's correct. I will need a structural engineer to confirm.

 

[TheEasterBunny]

What are you converting it into? Presumably not a garage, looks like a candidate for demolition and rebuilding, especially that bit on the left.

It'll be a craft room/room for the kids to play in and a store room to the left.

I had thought demolition but our budget is small, it's already taller than 2.4m so we can convert without planning and a new build would be subject to height restrictions so close to the boundary.

I've watched Ali Dymock build his garden building on YouTube multiple times over and have often thought about doing that but the reduction is size would be massive - 2m either side would leave us with a building under half the width.

We've have 4 builders round who have all said the building is fine for conversion

 

[robinbanks]

I've been looking at the span for 0.5kn pr sqm.

A 245B at 600 will give you that.

https://www.jamesjones.co.uk/assets/downloads/subdir/Technical%20manual%205th%20edition.pdf

Maybe make yourself a spreadsheet, get some joist costs etc. It's a balancing act between height (taller are stronger so you can use fewer), width (wider are stronger so you can use fewer), common size (more common are cheaper), spacing (wider means fewer means cheaper and also if installing insulation fewer means less bridging of insulation)

You don't just have to go for a 300C because the calculator says so, but maybe it does look at the most cost effective option - I worked it out myself after the SE had said "use blah blah spec" I was arguing back "but if I use the wider one I can have a 600 spacing instead of a 400 so instead of 24 I need 18, so even though it's a tenner more per joist for the extra width it's 6 joists at 70 quid each I don't have to buy". Yes, you have to factor in buying 22mm boards instead of 18mm etc but all in..

For the garage, which didn't matter so much about a sign off, I made my own I beams, which cost about half what it did buying them (but took a long time. For another project I modified the design to a box beam, which was a lot faster, but more material intensive)

EPDM rubber weighs about 12 times its thickness in N/m2 and OSB and plasterboard about 6.5 times their thickness. A cubic metre of fresh powdery snow apparently weighs about 500 N/m2, but remember that it self compacts and adds to its own weight over time so if you're in a part of the country that frequently experiences freak weather, designing for 500N may not be enough even if the roof structure itself is only knocking on for about 250..

..but all that said, take stock of what is there already, what its design strength is, as it's clearly proven it's case by standing/not collapsing for x years already

(I did wonder why this is even a question; the existing structure clearly has a working roof; are you enlarging the building? Why is the existing roof not serviceable for the new purpose?)

 

[cdbe]

Interesting - a conversion to "habitable" will require a fair bit of insulation both in the floor and the roof (the current warm roof method involves it sitting on top of the roof deck). Hopefully you have sufficient headroom or can raise the finished roof height.

As said, If you can keep the wall (or a pillar of it - to carry a couple of timber beams where your red line is) you'll be in the realms of standard timber span tables - SE would need to approve the pillar and design the beams. Timber is generally cheaper and easier to work with than steels.

I still think it's "dog rough" for a conversion (especially if solid wall/single skinned) and I would probably submit a planning application - I can't see them refusing a nice new build of the same size.

Many builder's will happily agree to whatever you want if you pay them.

 

[BlueLoo]

I'd knock it.