Max weight limit on garage loft board out

[mattwilk]

I'm in the process of boarding out the garage roof space to add some extra storage.

I've upgraded the joists to the correct dimension for the span but I'm just a little concerned on how much weight the walls will safely take as its a single skin brick with breeze block pillars halfway down and the weight of just the joists and chipboard is quite a lot before I start adding anything extra.

Is there anything I can do to strengthen the walls?

Span width ways is 2.75m and length ways its approx 5.5m and there are two wall straps on each side.

Thanks

 

[tony1851]

It depends mainly on how high the walls are from the foundation to the underside of the joists.

 

[mattwilk]

It depends mainly on how high the walls are from the foundation to the underside of the joists.

Its 2.37m

 

[tony1851]

Even allowing for the piers, that's a fairly slender wall; the more slender a wall is, the less weight it can safely support.

It's not helped by the timber joists on top; when you load the joists, they will bend very slightly and this bend will bring the load closer to the inner face of the wall and risk producing tension in the mortar joints, which would cause the wall to fail.

Realistically and allowing for the weight of the roof, I'd be inclined to put not much more than 20 - 30 cwt up there, evenly distributed.

 

[mattwilk]

Even allowing for the piers, that's a fairly slender wall; the more slender a wall is, the less weight it can safely support.

It's not helped by the timber joists on top; when you load the joists, they will bend very slightly and this bend will bring the load closer to the inner face of the wall and risk producing tension in the mortar joints, which would cause the wall to fail.

Realistically and allowing for the weight of the roof, I'd be inclined to put not much more than 20 - 30 cwt up there, evenly distributed.

Thanks again

Agreed it is slender - is there anything I can do to mitigate the risk? I'm not against some sort of prop in the centre of the garage is that would help?

 

[^woody^]

The joists will fail before any wall

 

[tony1851]

The joists will fail before any wall

Fail in what way?

 

[^woody^]

Fail in no way. It was illustrative

What the OP proposes is not going to impact on the wall anymore than it will impact on the joists. But if he was going to be storing car engines up there, the joists will go before the walls.

 

[tony1851]

OK, let’s start with the basics.

First the wall. 2370mm high from the foundation, and 1/2 brick thick, with a pier in the middle. Because of the pier, we can assume the wall is slightly thicker than the usual 102mm brick – say 110mm to be conservative. Then the ‘slenderness ratio’ (ie height ÷ thickness = 2370÷110 = 21.5.

The tables in the Brickwork Code tell us that for this slenderness ratio, the allowable compressive stress on the brickwork must be reduced by approx. 50%

Next the joists. Timber joists bend noticeably under heavy loads. The bend means that the end of the joist turns up slightly where it bears on the wall. Because of this, the load does not act centrally down the wall, but is offset – the usual assumption is that the load acts about 1/3 in from the inner face. This puts an eccentric load on the wall which reduces the allowable stress even more.

This is not to say that the wall will fail, but common sense says that the additional load should be reasonable and well-distributed. With a little time and further info., it would be possible to work out the maximum permissible load per meter length of wall, but then if there was a gale during the night…….?

 

[^woody^]

The walls and the roof are working as one, bracing each other. None of it it's going anywhere and no exaggerated bearing image, code or theory is going to make any difference.

Is there actually any record somewhere of a load on a ceiling causing a building to collapse via the walls?

 

[tony1851]

The little diagram was exaggerated to show the principle.

I was 'following the science' as they say. But if in future any checking SE asks me to justify anything I'll just give them your sound advice, which is that it all 'works as one .... and no code or theory is going to make any difference'; that should satisfy them

 

[^woody^]

Be like Isambard and just do it.