what is meant by "clear span"

[freddiemercurystwin]

Or could be worth considering throwing some steels in spanning between your walls at say 4m centres and then spanning some much smaller timber joists between those, parallel with the length of the building, doubled up beneath the dwarf walls.

Joists to span 5.8m are gonna be ££££s

How much of the 20m are you extending into?

 

[SilverSaloon]

How much of the 20m are you extending into?

pretty much all of it, however a lot of it contains load bearing walls in the centre so reduces the spans to about 4m, its the larger areas that have the big spans.

my plan was to use 6m timber for the areas that require the big spans, and use the cheaper 4.8 timber lengths joining on the walls for the other areas.

putting steels across wall to wall and spanning between those is something i had thought of, but dont want to remove the existing ceiling joists and i dont have the height to spare in having the timbers much higher than the existing joists taking into account the extra height of the new joists.

 

[Hangus79]

How much of the 20m are you extending into?

pretty much all of it, however a lot of it contains load bearing walls in the centre so reduces the spans to about 4m, its the larger areas that have the big spans.

my plan was to use 6m timber for the areas that require the big spans, and use the cheaper 4.8 timber lengths joining on the walls for the other areas.

putting steels across wall to wall and spanning between those is something i had thought of, but dont want to remove the existing ceiling joists and i dont have the height to spare in having the timbers much higher than the existing joists taking into account the extra height of the new joists.

Maybe an engineer can come up with a solution of increasing the span by decreasing the joist centres. Reducing it down to say 300mm centres.

Another difficulty may be getting hold of 6m long lengths of graded timber at all.

 

[SilverSaloon]

i can get the timber (already checked that), but it is 3x the price of the standard 4.8 lengths

 

[tony1851]

Can you put steels across the 5.8m width, and span joists between those (ie perpendicular to the floor joists in your sketch)?

 

[SilverSaloon]

Can you put steels across the 5.8m width, and span joists between those (ie perpendicular to the floor joists in your sketch)?

no as the existing ceiling joists below need to be retained. It would mean the new joists ontop of the existing ceiling joists + a small gap which would leave me with not enough headroom.

 

[freddiemercurystwin]

How small could you go with steels spanning 5.8m tony? Assuming the steels got closer together?

 

[tony1851]

How small could you go with steels spanning 5.8m tony? Assuming the steels got closer together?

152 x 152 x 30. If they were at 3m centres across the gap, the joists running the other way could be 150 x 50. Probably loose 50mm headroom compared with 200 deep joists running across, but the latter would still give excessive deflection unless very closely-spaced.