Floor joists

6 Jan 2016
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United Kingdom
hi, we are about to buy a new house, the house has a detached double garage with large roof space and window.... my plan is to convert it into a games room with pool table... while it won’t have constant use, when it is used there may be several people up there along with a 6” pool table weighing around 80kg.
It is a new build, there are joists and a floor already installed but I’m sure the joists are too small for the project I have planned.
The room is 5.5 m long by 4.1 m wide, the existing joists run the 5.5 length and are sat on the breeze block wall. They are 600mm apart and 170mm x45mm....
Based on current regs I think I would need joists around 400mm apart and measuring 220x75....
As I don’t want to screw around too much, could I sister joists to the existing joists, this would mean joist sizes of 170x90 at a distance of around 550mm....
I’m open to sistering both sides meaning joists would be 170x135 at a distance of around 500mm...
Would this be OK? If not, how should I go about it without removing the existing joists?
Thanks in advance...


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I believe the joists would be made stronger by making them deeper rather than wider so it may be a bigger job than anticipated; but there are experts on here who may offer an opinion.

Ultimately you would need an SE to do calculations, or consult an experienced builder.

Thanks for the reply.... it’s not what I was hoping to hear though!!!... what you’re saying is logical, but much more expensive and time consuming!! I was really hoping it could be done without taking up the existing floor... I may have to consult an SE... does anyone know how much they would charge just to give advice/opinion?
I may have to consult an SE... does anyone know how much they would charge just to give advice/opinion?
Their 'advise/opinion' would be to just tell you what needs to be done for something like this. As you say the existing joist sizes and spacing are not adequate for this span and the floor thickness needs to be at least 22mm for joists at 600mm centres.
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If I were to put in an rsj across the centre of the garage and support this with brick/block columns, would this effectively half then length of the joist span therefore reducing the need to increase joust sizes?
installing a support beam would mean re-arranging the garage door equipment.
what type of roof does this garage have - gable or hip etc?
where do you propose to put the staircase for this new first floor?
I was going to put a staircase on one of the walls, having it reach the first floor around the middle of the room, the roof is an apex type roof, it is attached to next doors garage on one side and an open gable end on the other... ive attached more photos....
I was hoping that I could put up an rsj just behind the electric door unit which would be roughly halfway between the existing joists.... this way I wouldn’t need to mess with the door and mechanism... I was going to get the stairs to finish on top of the rsj....


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thanks for the reply and new pics.
i dont think there's any call for a SE.

what you propose seems perfectly doable and thought through.
a couple of buts: maybe the RSJ will freely slide in - thats a site call, and will a 2 - car garage now become a 1 - car garage?
Yeah, the garage won’t be used for cars in any event..... in relation to the support pillars for the rsj, could I just use acrow props (meaning I get a tight fit) or should I use block pillars? The acrow props would save time and hassle....
Using Acrow props as permanent support to a steel beam doesn't seem a good idea to me , aren't they just meant to be used as temporary support :?::!:
Your 170 x 45mm joists at 600mm crs will span about 2.9m according to the old Approved Document A table for floor joists so you will be ok with a steel beam at about mid span as additional support to the existing joists.
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Ok, thanks for all the answers and help... stupid question, if I was to build pillars out of block, how would I get the rsj “tight” underneath the existing joists? The idea of the acrow props was so I could tighten the props to get a snug fit so the rsj actually supported the joists....
Assuming some form of padstone at the top of each masonry column then either slate or steel shims would do the job
You should be able to cut a hole in one side and slide the beam in that way you will only need to support the other side, and maybe you could slide it back so you don't need any pillars. You can use an acro to support the rsj if you like just make sure once the beam is in place you fix it to your joists and then fix your acro to the rsj.

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