Flat roof construction help

[ab122]

Building a double garage as shown in photo attached.

 

Id like the fall to be from front to back across the 5.8m span.

 

I'm struggling to find the right roof rafter size to tolerate that span.

I plan to have OSB roof, EPDM (rubber roof) covering, interior insulation in between rafters.

 

Is the 5.8m span doable?

Could I bolt two rafters together across the 5.2m span to reduce down the 5.8m span into two separate sections attached to the double rafters by joist hangers; sitting on 1 end of the 5.8m wallplate and the other ontop of the steel wallplate, therefore allowing me to use smaller rafter sizing?

 

Is it too much hassle, should I just go across the 5.2m span (which is less than ideal as it doesn't help with the garden side view of the garage for my plans.

 

Advice welcome and thanks in advance!

[Temp]

You appear to have have two brick piers in the right place to put another beam across, thereby reducing your rafter span in half.

[Conor]

I'd run a steel beam across between the two piers. Something like a 178x102x19.

[ETC]

I’d get a SE to design the roof and wall panels.

[ab122]

Unfortunately I drew that a bit off, the posts are not in the same place, this is an extension of the previous single width garage.

 

I can't put an rsj between them. But I see no reason why I can't tie in another pillar to align, and then span that. 

 

Could I use a UB?

 

Good shout guys, any thoughts on the rsj/ub to span the garage opening?

[ab122]

On 31/05/2025 at 23:43, Temp said:

You appear to have have two brick piers in the right place to put another beam across, thereby reducing your rafter span in half.

 

On 01/06/2025 at 04:14, Conor said:

I'd run a steel beam across between the two piers. Something like a 178x102x19.

 

Unfortunately I drew that a bit off, the posts are not in the same place, this is an extension of the previous single width garage.

 

 

 

I can't put an rsj between them. But I see no reason why I can't tie in another pillar to align, and then span that. 

 

 

 

Could I use a UB?

 

 

 

Good shout guys, any thoughts on the rsj/ub to span the garage opening?

[ProDave]

Span all the joists the shorter 5.2M dimension?

[SBMS]

Engineered posi rafter could do a 5.8m span. 

[ab122]

2 hours ago, ProDave said:

Span all the joists the shorter 5.2M dimension?

Then my fall has to be in a direction I'd ideally not wanted to be

[ab122]

2 hours ago, SBMS said:

Engineered posi rafter could do a 5.8m span. 

Thought about that but too expensive and too large/tall for just the garage roof

[saveasteading]

48 minutes ago, ab122 said:

too expensive

You can't have big spans without beams or deep joists.

I'd guess that the beams across the door and the midpoint would be more like 250 deep for economy or shallower at a price.

OR posijoists or similar....at a cost.

 

The slopes can be sorted by various means, which can be discussed later.

 

Once (if) you acccept that there are these heavyish members and costs then  the next thing is you need an SE to design it to building regulations.

We don't do calculations and other detailed design on here.

 

I wouldn't use osb. Any tiny leak and it fails , or have a very well built covering.

[ab122]

47 minutes ago, saveasteading said:

You can't have big spans without beams or deep joists.

I'd guess that the beams across the door and the midpoint would be more like 250 deep for economy or shallower at a price.

OR posijoists or similar....at a cost.

 

The slopes can be sorted by various means, which can be discussed later.

 

Once (if) you acccept that there are these heavyish members and costs then  the next thing is you need an SE to design it to building regulations.

We don't do calculations and other detailed design on here.

 

I wouldn't use osb. Any tiny leak and it fails , or have a very well built covering.

Makes sense, while I look into that, how can the slope be handled later? Can furrings go across (90 degrees) as opposed to along the joists then? To change the slope?

[ProDave]

Why can't the slope go either way?  surely whatever wall plate you support them on would be on a slope so the next joist along is a little lower etc.  Is this not allowed?  Are you forced to put them all level and add firrings?

 

And if that is the case, whichever way you want the slope it would have to be dead flat in both directions then firrings added?

[ab122]

16 minutes ago, ProDave said:

Why can't the slope go either way?  surely whatever wall plate you support them on would be on a slope so the next joist along is a little lower etc.  Is this not allowed?  Are you forced to put them all level and add firrings?

 

And if that is the case, whichever way you want the slope it would have to be dead flat in both directions then firrings added?

 

I'm self building, so I guess I could do whatever. But the way I was advised is fully flat and then firring ontop of joists, didn't know I could put them across, nor that I could put the wall plate on a slant. Though the walls are level so I'd probably find that hard to do

[SBMS]

8 hours ago, ab122 said:

Thought about that but too expensive and too large/tall for just the garage roof

Cheaper than putting in steel maybe.. you could do a reduced depth joist (maybe an 8 inch) with larger chords to beef it up. If you want larger spans without a steel you’ll have to get something engineered… don’t think you’ll get away with anything less than 7 or 8 inches tall anyway spanning that much?

[SBMS]

5 hours ago, ab122 said:

 

I'm self building, so I guess I could do whatever. But the way I was advised is fully flat and then firring ontop of joists, didn't know I could put them across, nor that I could put the wall plate on a slant. Though the walls are level so I'd probably find that hard to do

You could always ply all the way over first then put your firrings on whichever direction you want then ply on top of them to form your sub deck. 

[ab122]

24 minutes ago, SBMS said:

Cheaper than putting in steel maybe.. you could do a reduced depth joist (maybe an 8 inch) with larger chords to beef it up. If you want larger spans without a steel you’ll have to get something engineered… don’t think you’ll get away with anything less than 7 or 8 inches tall anyway spanning that much?

 

So to make sure I understand, what you are saying is put the steel above garage door opening and use the posijoists to span from front to back without the need for steel.

 

And to use an 8" posijoist?

 

Thanks

[Temp]

On 02/06/2025 at 22:21, ab122 said:

Could I use a UB?

 

For the mid span ChatGPT suggests a UB 203 x 102 x 23 with 3m C24-grade timber joists 47 x 170 mm at 400 mm centres. 

 

I would shot nail or bolt a timber infill either side of the shear web and use joist hangers. Probably long tail joist hangers wrapping the tail up over the top of the UB and down the other side. Nail both sides.

 

I'm not a structural engineer though!

 

[ab122]

23 hours ago, Temp said:

 

For the mid span ChatGPT suggests a UB 203 x 102 x 23 with 3m C24-grade timber joists 47 x 170 mm at 400 mm centres. 

 

I would shot nail or bolt a timber infill either side of the shear web and use joist hangers. Probably long tail joist hangers wrapping the tail up over the top of the UB and down the other side. Nail both sides.

 

I'm not a structural engineer though!

 

Thank you! Is it not just easier to get the steel drilled with bolt holes to bolt a wall plate to the top of them. Saves me messing with heights of differing steels and rafters to get it all in the right dimensions for hanger fitting. 

 

In the case of wall plate on top of steel at far end, centre and then the outer brick wall, it's as easy as can be right?

[Temp]

2 hours ago, ab122 said:

Thank you! Is it not just easier to get the steel drilled with bolt holes to bolt a wall plate to the top of them. Saves me messing with heights of differing steels and rafters to get it all in the right dimensions for hanger fitting. 

 

In the case of wall plate on top of steel at far end, centre and then the outer brick wall, it's as easy as can be right?

 

Yes sure you can put the rafters on top of the beams if the headroom isnt an issue. 

[ab122]

46 minutes ago, Temp said:

 

Yes sure you can put the rafters on top of the beams if the headroom isnt an issue. 

Thanks, should be okay hopefully. All brick courses are currently level, that's why I think it might be hard to use hangers.

 

If I used your method, what would I do at the wall end where rafters need to rest on the back wall? Assume this course would need to be lower than steel height to allow them to sit on it? Or can they also attach to hangers on the brick wall?

[ab122]

Edit: except the posts where I would place steel, currently slightly lower pending steel (may need to be adjusted but expecting to be flush and level)

[Temp]

22 hours ago, ab122 said:

If I used your method, what would I do at the wall end where rafters need to rest on the back wall? Assume this course would need to be lower than steel height to allow them to sit on it? Or can they also attach to hangers on the brick wall?

 

If the walls are already up to height I'd put the rafters on top.

 

You can fit joist hangers to a wall but they normally require a minimum number of brick courses on top of the hanger (to prevent the forces prying the hanger out) so this is normally only done for the first floor in a two storey house or two storey extension. Alternatively you can bolt a wall plate to the side of a wall and put rafters either on top or flush using hangers. Again I'm not sure it's wise to bolt so close to the top of a wall?

 

Another option would be to notch the top of the wall with an angle grinder to allow rafters to be inserted at the right height.

 

I mentioned headroom issue because Permitted Development rules limit the eaves height of an extension or outbuilding near the boundary. Think its limited to 2.4m?