Fitting restraint strapping ceiling joists?

[Tyrone Thompson]

Hi guys/gals,

I’ve recently had my building regs drawings through from my architect for a loft conversion where the truss roof is coming down and new roof put up, it states “restraint strapping - ceiling joists and rafters to be strapped to walls and gable walls, straps built into cavity across 3 timbers with noggins”

Installing these for the rafters is pretty straightforward, they would simply sit over the inner leaf into the cavity, spanning 3 rafters with noggins and a Packer. The problem is where it requires straps for the ceiling joists, especially as it’s an existing cavity wall. My thinking is that I would either have to remove a block, sit a strap in and mortar block back in. Alternatively channel out a slot in block/mortar, thick enough to slide strap in, and turn 90 degrees to hook on the inner leaf.

can I talk to my architect or BCO about using alternative strapping/fixings? Like some sort of resin anchor. Has anyone had any running’s with this instal? Or any advice from their architect?

thankyou

 

[noseall]

channel out a slot in block/mortar, thick enough to slide strap in, and turn 90 degrees to hook on the inner leaf.

This how we would do it. Only encumbrance is remains of mortar snots on the inner surface, though these should fall off in the strap zone, as you do chop out your slot.

 

[paulrockliffe]

How are you fixing the floor at the gable walls? If you resin-fix a wall plate to the gable you should be able to then use that to affix the restraint straps to. You haven't said which way your floor joists are running, but either you need a wall plate to hang the joists from, or you need one to sit the floor on at the edge. Either way, it's quicker to do that than smash out blocks. If the wall is old you might find that hooking over two bricks will mean that those bricks will be restrained, but the rest of the wall can still move if it wants to.

But it's worth bearing in mind that you don't have those straps there now and the house hasn't fallen down. Are you imposing any new lateral loads? Probably not. Have you got restraint across the full width of the floor? If not, what are you restraining the wall to? If that can move, what's the point?

I'm heading towards the end of the structural side of a loft conversion and have had that conversation with Building Control, they agreed that restraint straps at the gable can't provide any meaningful restraint unless the floor joists are then restrained against two perpendicular steels and between the steels. But my joists are perpendicular to the gable, you would get some restraint if the joists are horizontal, so long as the joist ends can't slide sideways.

 

[Tyrone Thompson]

How are you fixing the floor at the gable walls? If you resin-fix a wall plate to the gable you should be able to then use that to affix the restraint straps to. You haven't said which way your floor joists are running, but either you need a wall plate to hang the joists from, or you need one to sit the floor on at the edge. Either way, it's quicker to do that than smash out blocks. If the wall is old you might find that hooking over two bricks will mean that those bricks will be restrained, but the rest of the wall can still move if it wants to.

But it's worth bearing in mind that you don't have those straps there now and the house hasn't fallen down. Are you imposing any new lateral loads? Probably not. Have you got restraint across the full width of the floor? If not, what are you restraining the wall to? If that can move, what's the point?

I'm heading towards the end of the structural side of a loft conversion and have had that conversation with Building Control, they agreed that restraint straps at the gable can't provide any meaningful restraint unless the floor joists are then restrained against two perpendicular steels and between the steels. But my joists are perpendicular to the gable, you would get some restraint if the joists are horizontal, so long as the joist ends can't slide sideways.

hi thanks for the response, my joists run front to back of house, wall plate to wall plate, so parallel to gable walls either side.

it requires straps at both gables, it’s a semi-detached, end of terrace bungalow. I haven’t been in contact with a BCO at all yet so I’ll see what they say...

I know it’s a different subject, but do you have any experience with vertical DPC’s at a starter kit profile for an extension, my options are either, cut into existing cavity to make continuous or simply fitting dpc behind started which doesn’t seem to solve the problem.

if not I’ll start another thread or look for a relevant one

 

[paulrockliffe]

Ha ha, only that my Dad has just had an extension done and I know BC asked him to cut up the bit of the existing wall that was left bridging the new cavity to fit a vertical DPC. For some reason they didn't ask for the wall to be cut fully through, which I don't understand. I'd just ask BC what they want, mine insist you can't start an extension until they've visited to look at any nearby trees, so tackle them then.

For your new floor I would resin fix a wall plate and fix your restraint straps to the new timber only. It's easier, you need something to sit the floor on there anyway, and you're spreading the load across the width of the wall rather than relying on point-loads where you can't really assess the suitability of the wall to take them.

 

[noseall]

But it's worth bearing in mind that you don't have those straps there now and the house hasn't fallen down. Are you imposing any new lateral loads? Probably not. Have you got restraint across the full width of the floor? If not, what are you restraining the wall to? If that can move, what's the point?

The straps are there to protect the wall and not the other way around.

 

[paulrockliffe]

The straps are there to protect the wall and not the other way around.

It's the same thing. If both ends of the strap can move then there's no restraint.

 

[noseall]

If both ends of the strap can move then there's no restraint.

Eh? How is a strap that is fixed across three floor joists going to move?

 

[paulrockliffe]

If there's enough load to move the gable wall then there's a lot of load on the floor joists. Most joists aren't adequately stiff in that direction or adequately restrained at the ends to resist a large lateral load as they're simply not designed for it. If they're mortared into blockwork at each end and sufficiently nogged then they might do something. If there's a set of stairs coming up against the gable wall then they'll likely just move with the wall. I'm not going to run the numbers, but there's probably as much restraint being provided by the floorboards running across the full width and twice as many joists as a couple of small metal straps.

 

[noseall]

If there's enough load to move the gable wall then there's a lot of load on the floor joists

Wrong.

 

[^woody^]

If there's enough load to move the gable wall then there's a lot of load on the floor joists. Most joists aren't adequately stiff in that direction or adequately restrained at the ends to resist a large lateral load as they're simply not designed for it.

I'm confused.

Surely the theory is that it is the wall that needs restraint, not the floor. There is no lateral load on a floor, and neither do you design for it. Its a floor, it holds things up not sideways. That is why the wall is strapped to the floor because the floor is not going anywhere.

But in reality there is no significant pressure difference to cause probelms with an un restrained house wall with rooms on one side. The concern is only really with lofts or garages with gables and no ceilings

 

[tony1851]

It's not so much the floor joists themselves which provide restraint. It's the flooring (and to a lesser extent the ceiling) which diaphragm the floor joists and give the floor its overall rigidity.