I know aircrete coursing blocks should not be used for loadbearing, and probably not as the first course on top of a lintel, but is there any reason they should not be used directly under a roof plate (2x4 the rafters join onto) ? Thanks, Simon.
Simon,
If the blocks are not classed as "load bearing" then that is a very good reason for not sticking around 20 tons of roof on top of them (by the way, there is a distinct lack of information here as to the type of roof that you are talking about) - and it's that weight the wallplate transfers to the wall.
Cash
TWENTY tons? - what roofs have you seen that got to even half that weight?
To the OP: - it's the crush value that matters here, not load bearing (the entire weight of the roof is bearing down on the blocks anyway as that is what the walls are built with!) - you can use normal bricks, either 1,2 or 3 courses at the top of the internal walls to sit the wallplate on (this normally requires a course of block being knocked off if you've already reached ceiling height)
Stick on a heavy, wet snow loading, and 20 tons is very obtainable on a roof!
Crush value and load bearing are in the same context here - the weight of the roof is transferred to the inner skin via the wall plate - and then straight down to the foundations in a compression (crushing) load - and this can cause problems with low, or non-load bearing materials, particularly in buildings of more than 1 storey.
Now if Simon will tell us whether he is taking about a 'main' roof (and it's construction [which will also have bearing on its weight]) or a simple flat or lean-to roof of an extension, then perhaps a definitive answer can be given.
BTW, with regards to your first paragraph, there are roofs that weigh around the 20 ton mark - usually on older houses using traditional materials.
Cash
It's a single storey extension, he's been building it at his own house
I find that impossible to accept, unless you are talking about huge mansions or stately homes?
An average 3 bed semi takes about 500 tiles (slates are much lighter, but people don't usually have these nowadays) and depending upon which tile you choose, they weigh in at about 3.5 to 4.5 kilos each, but to work things out easier, lets say they are 5kg apiece. Felt will weigh approx 18kg a roll and it will take about 3 rolls
3kg of nails Timber for a main roof (remember the purlins are built into the wall and are not resting on any wallplates and are therefore not part of the roof) is normally around 7X2 and say 5m long - these will weigh in at around 40kg each and there will be about 15 on the front and 15 at the back, not to mention a ridge board @ say 15kg. Lead flashing is quite heavy though and a normal roof will take a few 25kg rolls and not forgetting the wall plates themselves, say 2 X 6m lengths of 4X2 @ about 15kg each, so lets recap:Tiles = 2500 felt = 36 nails = 3 main timbers = 1200 ridge board = 15 lead = 50 wallplates = 30
Total is 3384kg, or 3.3 tonnes, even if you round it up to 4000kg for sundries like ridge tiles, velux windows and roof battens, it's still well short of 20 tonnes.
And if you are talking about a 5 or 6 bedroom detatched house roof, there is still nowhere near double the amount of materials as mentioned aove and even if there were, it would still only get to 8 tonnes, but all this is academic anyway as roof trusses weigh very little and a new build roof even on a 5 bed det would be closer to 3 tonnes overall weight, and I can't remember ever seeing 17 tonnes of snow on a roof
Folks, I'm not talking about crush strength for uniformly distributed loads, more for concentrated loads under lintels etc. I read somewhere (can't find it now !) that if you need to use aircrete coursing blocks, dont put them on the course on which is bearing a lintel, joist hanger etc. Thats what I meant by "load bearing" in this situation - some type of concentrated load. I guess the reason is that coursing blocks fairly easily snap, since for aircrete, its strength is in its homogeneous nature. Just like a very thin sliver of brick should not bear point loads, neither should a moderately thin piece of aircrete. If the mortar bed under a coursing block is slightly difficient, a point-type load on it could cause it to snap, but a whole block would never snap in that situation.
If you look at buldings in construction around the place, you often see a row of coursing blocks below the top row of whole blocks at lintel height for standard height windows and doors. But you don't see the coursing blocks above the whole blocks. That's what lead me to believe that what I read was correct.
With a roof, the roof plate on top of the blocks will even out the load, so I don't think there would be a problem, but the question is what is good practice ?
Simon.
What do you mean by 'coursing' blocks? - are you talking about the smaller blocks used to get heights correct? - these are usually cut on site by the brickie, or more often his labourer, and no, they should not be used under lintels, what you are probably seeing are in fact concrete bricks, which have a higher N rating than blocks at around 20N, wheras aircrete are 2.9N.
Coursing blocks are what you say. I cut a whole lot when I needed them, but you can buy them ready cut. I am talking only about aircrete. Cavity wall lintels are usually sat on aircrete one side, brick t'other. But the question is: why is it OK to sit a lintel on a whole aircrete block, but not on an aircrete coursing block ? I agree aircrete is often not sufficient to carry a heavy beam without a padstone, but cavity wall lintels do not usually need a padstone. Cheers, Simon.
He's not wrong. 20 tins is not that much really.
He's right there too.
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