Floor joists

[the mk1 kid]

Hi all,

Any structural engineers or builders out there: i would like to know if anyone can suggest the size of joists we need for a loft conversion?

Obviously there are plenty of variables to consider, so if anyone steps forward i'll happily oblige.

I've contacted a local timber merchants and await their advise, just thought someone's advice who is impartial would be nice.

Cheers

Tom

 

[sludgeguts]

I'd use 8" x 2" joists at 16" centres for the most part.
HOWEVER
There are also other variables to take into consideration - will any part be load bearing? (what's holding the roof up at the moment? how will this be taken into consideration?)

Sorry, that's 200mm x 50mm at 400mm centres

 

[Focus Owner]

dont you want steel?

 

[the mk1 kid]

Hi, and thanks for your replies.

I was surprised not to find a few definitive answers when googling the topic - it looks like we would need a structural engineer to assess the building and a building inspector to sign the work off once we had put the beams in, but prior to covering it all up with plasterboard.

Victorian house - 2ft thick exterior stone walls, and a central wall equally thick between the ground floor and first floor - the upper partition walls are made from 3 x 2.

The largest span (without any stud wall underneath for support - does that count as proper support??) in the direction the joists travel is approx 13ft.

The loft would have to take the weight of one bedroom (double), its contents and perhaps ensuite with no bath.

Do you need anything else to make approximations?

8 x 2 has popped up a few times, though with the existing joists there to sister up against (and only one room going up there, plus a stud wall underneath) i was hoping for the cheaper (and lighter) 6 x 2's to be an option.

Cheers

Tom

 

[the mk1 kid]

dont you want steel?

Ideally yes - you can stick it in and forget about it.

We can do the timber joists ourselves, but steel.. hmm

Tom

 

[Hellcat]

13 ft span.. similar to ours. At 16" centres you will need 9x2 joists. I did find a table before of required size per span. I'll see if I can find it again.

 

[the mk1 kid]

Thanks, that would be great - 9 x 2's

Cheers

Tom

 

[Hellcat]

http://www.stevenage.gov.uk/stevena...tching-joists.pdf;internal&action=save.action

Hope that works.

Actually 8x2 should be ok. Basically around 4m you can use..

47 x 195 (2x8) @ 400mm (16") centres 4.2m
47 x 195 (2x8) @ 450mm (18") centres 3.89m
47 x 220 (2x9) @ 450mm (18") centres 4.35m


9 x 2 :yum:

https://www.fiatforum.com/gallery/data/500/medium/DSC006951.JPG

 

[Doofer]

Do your existing ceiling joists run across the terrace or along it?

If they run along it then you should be able to use taller joists between the existing joists.

If they run across it then you will need steel joists along each side, with the wooden joists spanning across between them.

You may need steel joists in either case anyway, to support the roof after you've knocked any trusses out.

I'm no expert, just going on what I've seen done in houses I've lived in.

 

[Hellcat]

I think he meant across between the front and rear walls standing on the central wall.

 

[the mk1 kid]

Thank you Hellcat and Doofer.

across the terrace - not quite sure what you mean?

The existing 25ft joists run the full depth of the house and sit on the wall plates - so from front to back. Its a semi, and deeper than it is long.

I should get pics and with paint say make some annotations..

Tom

 

[Hellcat]

Victorian Semi... unusual.

 

[Doofer]

You will need support for the roof as well as the new floor.

How I've seen this done in a previous house was that steel joists were put in at front and back, about 5 foot in from the edge of the roof. These then carry the new floor and have supports up to the roof, which also form the support for the wall between the new room and loft void.

I incorrectly assumed terrace when you said victorian. A semi is easier, as the joists can be posted through two holes, made by removing a few tiles from the end of the roof (if it's hipped), or by removing a few bricks if it has a gable end wall.

See scribble piccy attached. Brown is existing wood, orange new wood and red steel joists.

Don't rely on me - I'm only going on what I've seen and an interest in all things building related.

It might be worth getting in some loft conversion companies to give quotes. You can always save money by getting them to do the structural stuff, leaving the cosmetics to you.

 

[the mk1 kid]

I'm really grateful to you, thanks - what you have illustrated there makes perfect sense, rather than running huge great timbers across the whole span when they are only needed in the middle of the loft room.

I also think we could go ahead and complete the rooms below now, knowing the it would in theory be possible to install the steel at a later date by passing it through the roof after lifting some tiles and felt.

Next question (sorry!) - ball park for putting in two steels and nothing else?

I would assume we'd need something like 20mm high RSJ's and approx 7m long, which on their own (well as a pair) wouldn't break the bank, but i guess the fitting would cost a few quid.

Tom

EDIT: gable end, not hipped, so posting them through holes they can then sit in would make more sense as you suggest.

 

[Doofer]

I've no idea on price. With this method you do lose a few inches of head height, as the new joists sit on top of the ceiling joists. Our current house has the floor joists between the old joists, which leaves just the thickness of the joists between the floors. It's a bugger to run pipes and cables through though, as there's more wood than space.

It might be cheaper to span timber across the whole width though - how it was done in the old days. If your roof pitch allows you to get the required height of joist onto the top of the wall without hitting the backs of the tiles.

I personally don't like the steel method, as it means that the entire room rests on four small points of the wall (and you lose height).

I'd get some companies round to give quotes, to see how they would tackle it.

 

[dannygp]

8x2 ftw. But if there was anything wrong with the timber size building reg's wouldn't let you go ahead with the build.....

Daz it would be an absolute nightmare getting gurders in! lol

 

[Doofer]

Daz it would be an absolute nightmare getting gurders in! lol

However you do it, there will need to be a lot of lengths of something substantial. Lengths of 8x2 long enough to span won't fit up the stairs!

 

[the mk1 kid]

I've no idea on price. With this method you do lose a few inches of head height, as the new joists sit on top of the ceiling joists. Our current house has the floor joists between the old joists, which leaves just the thickness of the joists between the floors. It's a bugger to run pipes and cables through though, as there's more wood than space.

It might be cheaper to span timber across the whole width though - how it was done in the old days. If your roof pitch allows you to get the required height of joist onto the top of the wall without hitting the backs of the tiles.

I personally don't like the steel method, as it means that the entire room rests on four small points of the wall (and you lose height).

I'd get some companies round to give quotes, to see how they would tackle it.

Hmm, i see your point - height is OK, but with 8" less..

Thing is with all this, there is nothing stopping me filling the attic with loads of junk that could weight far more than a double bed, wardrobe, chest of draws and a bedside table – for that matter, collapse all 4 items and you’d put them up there for safe keeping (let say) without batting an eye lid.

I think in all honesty we’ll carry on as we are and get builders in to quote nearer the time, its only I guess because the ceiling is down it looks like the opportune moments to add some strength causing us to get ahead of our selves.

Tom

 

[sludgeguts]

If the ceilings are down, why not add some meaty joists to the existing joists?
These can be posted up through a window, straight into the loft.
Instead of raising the floor level, you could reduce the ceiling height - in a victorian house you shouldn't even notice a few feet off the height.
If the supporting wall is in the middle of the house, why not consider putting in timbers which are slightly longer than halfway then overlap them at the middle & bolt together.

 

[the mk1 kid]

If the ceilings are down, why not add some meaty joists to the existing joists?
These can be posted up through a window, straight into the loft.
Instead of raising the floor level, you could reduce the ceiling height - in a victorian house you shouldn't even notice a few feet off the height.
If the supporting wall is in the middle of the house, why not consider putting in timbers which are slightly longer than halfway then overlap them at the middle & bolt together.

Hi,

I would have done just that i think, and not forked out for mega long ones. Lowering the height - never even crossed my mind, but it would work, and you're right, we wouldn't tell! - hmm.

Thing is i suppose i still don't really know exactly what happens when we get around to carrying out the conversion up there - what if Mr building inspector says - nope, those 8 x 2's aren't enough in a couple of years, or if building regs change, and we must use steel for arguments sake.

PS we have no perlins! - checked last night.

Tom