Structural question (reinforcing garage roof)

bainbridge · 31 May 2020 at 09:22

Hi all

Here's a sketch the garage roof:

There's a large loftspace which I'm going to utilise, but the 4x3 joists are widely spaced and because they run the length of the garage they're not continuous, they consist of 2 pieces of wood joined with what I understand is called a biscuit joint. I'm going to strengthen the roof so I can stow stuff up there and also scuttle around up there getting stuff down and organising.

The plan is to double up the existing joists and have a large supporting timber across the middle. I've indicated where the new timbers will sit using dotted lines in the sketch. I've measured up but have few questions I wonder someone can help with.

Firstly, what is the correct method to attach the central timber to the blockwork (concrete) either side? There is a blockwork pier on one side I could build up a couple of courses and use as a bearing but the other side (which is the gable end of our house) is a clean blockwork wall. I've researched this and everything points to using something called a multi truss hanger, but the central timber isn't just a joist, it's the central supporting timber. What can I bolt to the wall as a support?

Secondly, what size should the central timber be. I was hoping to get away with a 6x4, will this be adequate?

Lastly, what is the correct fixing for "sistering" the new joists to the old? Long screws? Nuts and bolts?

Thanks for reading, any comments welcome.

DXP55 · 31 May 2020 at 09:39

I can't help but others can- They will probably ask you for the size of the garage.

LOAM · 31 May 2020 at 10:50

Do you mean how do you support the ends of the joists at the wall? Heavy duty joist hangers would do for the joists but I'd be looking at steel for the mid span but without the dimensions I'm guessing.

Give me the span and I'll have a look on the trada span tables.

bainbridge · 31 May 2020 at 11:32

LOAM said:
Do you mean how do you support the ends of the joists at the wall? Heavy duty joist hangers would do for the joists but I'd be looking at steel for the mid span but without the dimensions I'm guessing.

Give me the span and I'll have a look on the trada span tables.

Hi

The span is 5520mm, which will obviously be halved when the centre support is in.

At the ends of the joists I'll just sit them on the plate next to their sister joists.

fobose · 31 May 2020 at 11:56

My bro has a double garage, he has used some hangers and some very beefy timbers so he can store stuff up there. You can see the new timbers next to the smaller (50mm) old ones. Ignore the two cross beams, that's for another job.

LOAM · 31 May 2020 at 11:57

How long is the centre support? Out of everything it's the one member that is taking the most load

LOAM · 31 May 2020 at 12:09

100*150 is a non standard joist size so no span info available got that size. The tables suggest a 220*75mm would span it without the centre support but it's close. I can't see how the centre support can be anything but a steel beam or 3 timber beams coach bolted together but again it depends on the span

bainbridge · 31 May 2020 at 13:43

LOAM said:
100*150 is a non standard joist size so no span info available got that size. The tables suggest a 220*75mm would span it without the centre support but it's close. I can't see how the centre support can be anything but a steel beam or 3 timber beams coach bolted together but again it depends on the span

The span across the centre is 2700mm.

Hedge · 31 May 2020 at 14:07

This might help you.

LOAM · 31 May 2020 at 16:38

2700mm I would probably double/triple up a 47*170C16 joist down the middle then span 47*145 onto it. Use an upside down jiffy strap or a small 90 trust strap to fix them to the centre timber.

bainbridge · 31 May 2020 at 16:48

LOAM said:
2700mm I would probably double/triple up a 47*170C16 joist down the middle then span 47*145 onto it. Use an upside down jiffy strap or a small 90 trust strap to fix them to the centre timber.

Great thanks! How would the central timber be attached to the wall?

LOAM · 31 May 2020 at 16:59

Is there a wall plate at that end or it just bare block on the top? A photo would help because some hangers are face fixed nd some turn over at the so and can either be chased in or fixed down into the wall plate.

bainbridge · 1 Jun 2020 at 12:29

Here's a picture of one of the sides

PlacidCasual · 1 Jun 2020 at 17:32

What is the centre truss going to sit on. I'm guessing it runs from back wall to above the door? Will the centre span sit on an existing beam at the front because otherwise the wall above the door probably isn't load bearing (assuming a single double width opening).

If this is a double garage is it acceptable to have a post for the centre truss to sit on at the front?

LOAM · 1 Jun 2020 at 22:39

The centre beam is bearing onto the side walls (2700mm), the truss bottom chord member is bearing onto the opening wall.

sdavlaws · 2 Jun 2020 at 00:17

What you planning to store up there? as from the image it looks like standard fink trusses, it will have a 250N/m2 light storage loading and a man on bottom chord loading already. The only loading we normally take off a garage compared to a normal house is the weight of a plasterboard ceiling.

If you are worried the trusses will have a label pressed in with one of the plates somewhere, if you know the original name for the site call who ever made them they may be able to tell you what its already designed for depending on the age.

bainbridge · 2 Jun 2020 at 08:00

PlacidCasual said:
What is the centre truss going to sit on. I'm guessing it runs from back wall to above the door? Will the centre span sit on an existing beam at the front because otherwise the wall above the door probably isn't load bearing (assuming a single double width opening).

If this is a double garage is it acceptable to have a post for the centre truss to sit on at the front?

The joists run front to back so the central support will be halfway along in the middle,.there's a sheer wall either side.

Hades · 2 Jun 2020 at 14:10

I have a double garage with usable loft space. It is very similar to your drawing but the width of 2.5 cars. The central span is steel.

DB_SamX · 2 Jun 2020 at 21:22

sdavlaws said:
What you planning to store up there? as from the image it looks like standard fink trusses, it will have a 250N/m2 light storage loading and a man on bottom chord loading already. The only loading we normally take off a garage compared to a normal house is the weight of a plasterboard ceiling.

If you are worried the trusses will have a label pressed in with one of the plates somewhere, if you know the original name for the site call who ever made them they may be able to tell you what its already designed for depending on the age.

Correct, modern trusses are indeed designed for 25kg/m2 (250N/m2) uniform load or 90kg point load for storage to meet British Standards.

If you give me some dimensions, I can run some calcs for you tomorrow:
Need a ceiling plan with dimensions
Size and spacing of existing joists
Photos of existing top of existing wall/ceiling interface to all 4 sides to see how you would support the new timbers.