Securing wall plate to existing solid wall

[jayc89]

As part of installing IWI to our house, we're thinking about what we can do with the first floor joists that are currently pocketed into the external walls. We're considering shortening them by fitting a wall plate and hangers to bring them within the thermal envelope. Something like this;

 

 

From what I understand the new plate should be bolted through the wall to the outside, which I'd rather not do. Are there any other options?

[Russell griffiths]

What on earth gave you that idea. 
chemical anchors drilled into the brickwork, I would put a bolt every 600mm but some will say 1200 is plenty, you can stagger the bolts top and bottom. 
as this is carrying a fairly substantial load I would seek professional input. 

[jayc89]

1 minute ago, Russell griffiths said:

What on earth gave you that idea. 
chemical anchors drilled into the brickwork, I would put a bolt every 600mm but some will say 1200 is plenty, you can stagger the bolts top and bottom. 
as this is carrying a fairly substantial load I would seek professional input. 

 

Interesting, thanks. Would those be staggered top/bottom every 600mm rather than always in the centre? We have a SE onboard for other reasons so I'll mention it to him when I next see him.

 

I'm pretty sure I read the fixing methods on a Gov white paper. Either bolt through or install a steel joist to rest the existing joists on. I'll try dig it out again.

[markc]

1 hour ago, Russell griffiths said:

What on earth gave you that idea. 
chemical anchors drilled into the brickwork, I would put a bolt every 600mm but some will say 1200 is plenty, you can stagger the bolts top and bottom. 
as this is carrying a fairly substantial load I would seek professional input. 

+1 to this

[tonyshouse]

I would build joists into pockets 

[jayc89]

1 hour ago, tonyshouse said:

I would build joists into pockets 

 

And risk the thermal bridge/chance of moisture damaging the joist ends?

[tonyshouse]

Wood is likely a better insulator but I would do EWI too 

[George]

On 17/04/2022 at 17:45, jayc89 said:

 

Interesting, thanks. Would those be staggered top/bottom every 600mm rather than always in the centre? We have a SE onboard for other reasons so I'll mention it to him when I next see him.

 

I'm pretty sure I read the fixing methods on a Gov white paper. Either bolt through or install a steel joist to rest the existing joists on. I'll try dig it out again.

Probably a fair few more bolts than that. Your SE will need to look at the load rating for the fixings when put into the wall type you have. I'd happily bolt a ledger plate direct to the wall - but I assume in that photo they had internal wall insulation to run down behind.

[bassanclan]

I would leave the timbers in the pockets and insulate around them. The cost of doing the work will far outweigh the benefits

[jayc89]

8 minutes ago, bassanclan said:

I would leave the timbers in the pockets and insulate around them. The cost of doing the work will far outweigh the benefits

 

That was my original plan, although concerns were raised around; 1) thermal bridging and 2) the joist pockets now being in a colder and potentially damper wall.

[Gus Potter]

On 17/04/2022 at 17:15, jayc89 said:

As part of installing IWI to our house, we're thinking about what we can do with the first floor joists that are currently pocketed into the external walls. We're considering shortening them by fitting a wall plate and hangers to bring them within the thermal envelope. Something like this;

 

 

From what I understand the new plate should be bolted through the wall to the outside, which I'd rather not do. Are there any other options?

Hello jayc89

 

I agree with George, quoted below.

 

On 19/04/2022 at 09:08, George said:

Probably a fair few more bolts than that. Your SE will need to look at the load rating for the fixings when put into the wall type you have. I'd happily bolt a ledger plate direct to the wall - but I assume in that photo they had internal wall insulation to run down behind.

 

Resin anchor fixings, in fact any fixing has often has quite a lot less capacity than you think. You look at the fixing and think.. that looks sturdy..but it's the things round about that generally cause the issues. Some of these are:

 

1/ What you are fixing into. Concrete, natural stone, modern solid brick, modern bricks with holes in them, old say London brick clay brick.. a long list. The variation in load bearing capacity is enormous so please be careful.

 

2/ What you are fixing to the wall. Take a timber wall plate / bearer / ledger piece. The joists are connected via hanger to the timber, the load in the timber plate has to be transferred into the fixing. The timber starts to crush locally.. as you have a small bearing surface between the timber and the fixing.

 

3/ How much stand off do you have. You photo shows packers behind the wall plate. The actual behavoiur is very complex so will not explain in detail but touching on a few key points.  The fixing deflects (bends) and this causes over stress at the fixing / timber interface. As the fixing bends it causes higher stresses where it enters the masonry as the fixing starts to act more "like a lever" rather than just acting in shear. One consequence of this is that the fixing develops additional tension and this causes other issues.

 

4/ Often the mortar is weaker than the masonry units / size of the stones if natural stone. The distance between the fixings and mortar joints is critical.

 

For a bit of fun here is a real world example of how you design the fixings. I have screen shotted parts of the design.

 

Let's take a house extension to a 1930's house.. old bricks, maybe some soft mortar. The key point is that for the fixings to realise their capacity they must be fixed nearly the centre of the brick and well away from the mortar bed. You can't control the floor height and you can't be certain that the brick coursing will be level. Thus you have to make sure that luck is on your side and you do this by adding in extra fixings. It gives the builder (maybe you) a fair chance to get it fixed safely, not slip later cause cracking, squeeky floors and so on. Remember that the new timber will shrink so while it may appear "rock solid" at the install it won't be quite so when it all dries out in a year or so.

 

Example: Floor joist span 3769mm, joist spacing 400 mm

 

 

The above gives the load on each joist. 0.81 kN/m is about 80 kg/m run of joist. Each end of the joist is supported by a hanger attached to the wall plate. The dead load is the self weight of the floor. The live load is you, furniture, book cases, having a party and this equates to about 150 kg per square metre of floor This is a code requirement that BC expect to see.

 

The above works out the maximum spacing of an M12 fixing in a typical 1930's brick with the plate hard against the brick face.  Each fixing can carry a permissible shear load of ~ 157 kg. But the test data is based on a European masonry size, not an imperial size! Thus while we can adjust do other calculations we apply some conservative Engineering judgement. The result is a detail like this. The detail below is a ground floor, vented solum below with PIR insulation.

 

 

You can see that in actual fact you need quite a lot of fixings to make it fly and safe for the design life of the building.

 

If you have read this far then thank you and I hope it helps.. a bit.

 

 

 

[bassanclan]

On 19/04/2022 at 12:05, jayc89 said:

 

That was my original plan, although concerns were raised around; 1) thermal bridging and 2) the joist pockets now being in a colder and potentially damper wall.

Yes there is thermal bridging, but you can insulate around it to mitigate that to some extent, joists have been fitted into pockets for 100s of years.

 

If you were starting with new joists then you would avoid thermal bridges etc, but you have to take a pragmatic approach and go for the easy cheap big wins rather than expensive marginal gains.

 

I doubt the cost of the works will ever be recouped by the savings you make on your heating bills in your lifetime.

[George]

11 hours ago, Gus Potter said:

Hello jayc89

 

I agree with George, quoted below.

 

 

Resin anchor fixings, in fact any fixing has often has quite a lot less capacity than you think. You look at the fixing and think.. that looks sturdy..but it's the things round about that generally cause the issues. Some of these are:

 

1/ What you are fixing into. Concrete, natural stone, modern solid brick, modern bricks with holes in them, old say London brick clay brick.. a long list. The variation in load bearing capacity is enormous so please be careful.

 

2/ What you are fixing to the wall. Take a timber wall plate / bearer / ledger piece. The joists are connected via hanger to the timber, the load in the timber plate has to be transferred into the fixing. The timber starts to crush locally.. as you have a small bearing surface between the timber and the fixing.

 

3/ How much stand off do you have. You photo shows packers behind the wall plate. The actual behavoiur is very complex so will not explain in detail but touching on a few key points.  The fixing deflects (bends) and this causes over stress at the fixing / timber interface. As the fixing bends it causes higher stresses where it enters the masonry as the fixing starts to act more "like a lever" rather than just acting in shear. One consequence of this is that the fixing develops additional tension and this causes other issues.

 

4/ Often the mortar is weaker than the masonry units / size of the stones if natural stone. The distance between the fixings and mortar joints is critical.

 

For a bit of fun here is a real world example of how you design the fixings. I have screen shotted parts of the design.

 

Let's take a house extension to a 1930's house.. old bricks, maybe some soft mortar. The key point is that for the fixings to realise their capacity they must be fixed nearly the centre of the brick and well away from the mortar bed. You can't control the floor height and you can't be certain that the brick coursing will be level. Thus you have to make sure that luck is on your side and you do this by adding in extra fixings. It gives the builder (maybe you) a fair chance to get it fixed safely, not slip later cause cracking, squeeky floors and so on. Remember that the new timber will shrink so while it may appear "rock solid" at the install it won't be quite so when it all dries out in a year or so.

 

Example: Floor joist span 3769mm, joist spacing 400 mm

 

 

The above gives the load on each joist. 0.81 kN/m is about 80 kg/m run of joist. Each end of the joist is supported by a hanger attached to the wall plate. The dead load is the self weight of the floor. The live load is you, furniture, book cases, having a party and this equates to about 150 kg per square metre of floor This is a code requirement that BC expect to see.

 

The above works out the maximum spacing of an M12 fixing in a typical 1930's brick with the plate hard against the brick face.  Each fixing can carry a permissible shear load of ~ 157 kg. But the test data is based on a European masonry size, not an imperial size! Thus while we can adjust do other calculations we apply some conservative Engineering judgement. The result is a detail like this. The detail below is a ground floor, vented solum below with PIR insulation.

 

 

You can see that in actual fact you need quite a lot of fixings to make it fly and safe for the design life of the building.

 

If you have read this far then thank you and I hope it helps.. a bit.

 

 

 

Agree entirely.

 

(If it was a cantilever (ignoring non-shear forces), if the anchor fails in any way then it is falling out of the sky. But for a floor confined and bolted to walls on either side, it isn't actually going to fall, it's going to locally crack the masonry but the bolts themselves and the resin will be fine, which will hold it in place. So there will be movement, cracking and possibly a loss of restraint on the wall. But it won't look like a catastrophic failure and no-one is going to die. Which is probably why builders happily put a single anchor every 600mm. That plus safety factors plus unlikely to see full live load means they get away with it.)