250mm cavity

[JackofAll]

Hi all,

In planning stages for new build bungalow and was wondering if any of you who have a 200mm plus cavity used 4" blocks for inner leaf or had to use 6" blocks or 4"block on flat. Reason I ask, have heard with a wide cavity and a vaulted ceiling the chances of the roof pushing out the 4"block on edge ad it wouldn't support the roof load? Pitch roof BTW.

Edited October 26, 2021 by JackofAll

[gravelrash]

Your architect would advise on basic principals. If using scissor trusses or King/Queen  for vaulted roof it should not create a problem but Building control would want Structural Engineer to ensure any design is suitable. 

[Iceverge]

We have a 250mm cavity. 215/250/100 downstairs and 100/250/100 block cavity block upstairs. 

 

Structural engineer will have the final say on this. 

[Gus Potter]

53 minutes ago, JackofAll said:

Hi all,

In planning stages for new build bungalow and was wondering if any of you who have a 200mm plus cavity used 4" blocks for inner leaf or had to use 6" blocks or 4"block on flat. Reason I ask, have heard with a wide cavity and a vaulted ceiling the chances of the roof pushing out the 4"block on edge ad it wouldn't support the roof load? Pitch roof BTW.

Hello JackofAll.

 

Firstly all the best with your project and a tantalising question.. if you are that frame of mind. Hope the following is of some use to BH members.

 

As a bit of background and as you haveJackofAll identified. Vaulted ceilings can introduce a sideways force into the head of the wall as the bottom part of the roof acts like a bit of a cantilever and bends like fury not just downwards but also moves sideways.. Masonry walls are good at carrying vertical loads but less able to resist horizontal loading particularly when the load is applied to the top via say a raised tie roof truss.

 

JackofAll. To get the best out of BH post some drawings. But for now say you want to create an open plan space 6.0m long (call this the side elevation) with the trusses (a wooden roof) spanning 8.0m onto the 6.0m long walls. In other words the 6.0m long walls are 8.0 apart.

 

I have picked a good size of room (but a bit of an odd shape) for ease of discussion. Let's also say that the 6.0m wall (side elevation) is 2.4m high and has no openings. 

 

The above is a stick diagram of one truss.

 

And in the spirit of BH one of the things we are interested in is how much will it cost and what are the options.

 

To cut a long storey short first look at how to mitigate the horizontal loading on the wall head. Say you have some decent gable walls tying into the side elevation walls  and that these gable walls will support a beam at ridge level or somewhere along the gable length.

 

Let's look at the economics of putting in say a steel universal beam(UB). Well all other things being equal I would say.. the room is 6.0m long.. 6000mm. My first stab at a sizing steel beam depth for a standard roof may be  6000 / 20 = 300mm.. draw a 305 x 146 x 46 kg per metre weight. It's got a nice wide flange for connecting timber to. May work but deflection, how much it bends by may be an issue but it's a start. I think that the beam is optomistic in depth (may need to increase the size) but.. it's for demo purposes at the moment. Maybe span / 15 would be a safe place to start.. 6000 / 15 = 400mm say a 406 x 178 x 60 UB.. ugly though! if you can't hide it.

 

The beam is just a cut steel, say shot blasted and primed with a zinc rich primer. Say you are a first time builder buying a beam like this. Beam weight 60 kg x 6.0m = 360 kg @ 2K per tonne = £ 720 delivered.. prices will vary but compared with building much heavier walls and the loss of say floor space it's a drop in the bucket.

 

And if that does not suit the mood music or the forces are a bit big then what about this..

 

Now it may be that putting in steels as above may just not be acceptable visually and we need the walls to resist the horizontal loading. Here it gets a bit more complex. As an aside a cavity width up to 300mm when using say the BS design codes say does not have that much effect on the strength of the walls. Even using block on the flat does help a bit but not as much as you would hope and.. it's a lot of money to spend building an extra thickness of wall just to resist the horizontal loading. Also, you can lose floor space which is expensive when you come to value a property.

 

For all.. the above techniques can be used to convert your attic too! Sticking to the thread. 

 

Suppose we can't have any steels as Architecturally it just not acceptable. I deviate but it helps to explain a point.

 

@saveasteading Saveasteading for example has posted a photo or two of a conversion that has solid stone walls. These massive walls can resist quite a lot of sideways load as they are so heavy.. it takes quite a lot to tip them over and this extra spare capacity can be used to resist the horizontal loading from the trusses in certain cases. That is why when you are doing a barn conversion that has solid stone walls you may wonder why is the old roof not spreading?.. it's because the walls are say 2 foot thick and very heavy.

 

But modern construction is a different animal. We just can't afford to build walls 2 foot thick just to resist horizontal loading from the roof.

 

Now lets turn to a modern cavity wall. Say the roof is supported by the inner leaf, could also be the outer leaf. We have a horizontal load from the roof. Now to resist this we could turn one of the leaves of the wall into a reinforced beam that spans horizontally between the gables.. along the top of the side elevation wall, by introducing say bed reinforcement. Looks great on paper but the brickies are not that keen on it and will charge you more. For the novice self builder it's hard to argue with a brickie, they are often grumpy. Also even with a 215 thick block on the flat the beam is too shallow.. only 215mm thick less the cover to the bed reinforcement.. it's hard to make something like this fly.

 

Also, as soon as you start to form openings in the walls, put in lintels it all starts to get a bit complicated, often stops working in practice, you really need to control the quality of workmanship.. more money. Below is a sketch, in effect you are turning the top courses of masonry into a reinforced beam spanning gable to gable which resists the sideways loads from the roof.

 

 

 

Turning back to / as an aside for the barn converters on BH @saveasteading Sometimes you can do this if you are faced with a choice between conservation and preservation. Solid stone walls can be a bit weak / weathered.  You be may faced with a choice; conserve or preserve. Sometimes on balance you may conclude that the best route is to cast a perimeter concrete beam on the inside of the wall, you could also do this in a modern context but it is not cost effective.

 

Lastly but not least JackofAll.

 

There is another trick that you can look at if you just can't bear the thought of steels, glulam beams performing the functions shown in the above.

 

For an overview. You'll see in the diagrams above I have indicated horizontal and vertical forces.. but we have a sloping rafter. There are a number of things going on in the rafter in terms of forces. For another day, but one major / dominant force is compression of the rafter (call this axial load) this is the compression load along the length of the short bit of rafter ( there is also a bending force and another force but ignore that for now) between the ceiling tie and the wall head. What I have done is to split this load into two forces.. horizontal and vertical.

 

 

Now ignore the vertical and horizontal lines, imagine you are looking up from the gutter to the ridge, up the slope of the roof. Also imagine that the rafters are clad with a good thick well nailed sarking board or say an 18mm OSB / ply board. What you are looking at is a big deep beam from gutter to ridge that spans from gable to gable. Provided this beam is fixed to the gables then you don't load the side elevation walls horizontally so you can have a standard cavity wall. It takes a leap of faith and you need to detail the gable properly. This all works fine in principle provided you don't then go putting in massive skylights, velux in the roof..  we know if you cut holes in the web of a beam it weakens it. Small apertures carefully positioned.. fine.. be sensible.

 

In Scotland they use sarking boards which help turn the roof into a deep beam in the plane of the roof as this helps for example stiffen the roof against the wind. Old steadings / barn roofs use the same principle. The sarking / bracing helps to turn the roof into a big beam in the plane of the rafter and this stiffens it all up and reduces the horizontal loading on the side elevation walls.

 

What I describe is nothing new, these old builders have been doing this for hundreds of years. The key is to appreciate how you can construct roofs to shift the loads to where they can be easily dealt with.

 

JackofAll. Hopefully you can take some of the info above and adapt it for your project.

 

All the best.

 

 

 

 

 

 

 

 

 

 

 

 

 

[saveasteading]

I hadn't thought of the sarking as a top flange to the roof trusses. Probably because we do have some outward leaning walls.

 

This is now a clincher in choosing to use OSB rather than traditional 200 wide timber boards with the 2mm ventilation gap, wherever we have to rebuild. The 2mm gaps are enough to allow the rafter to bend upward and outward and to lose the flange benefit.

 

From my current homework, the guidance for a new timber structure is to have a structural pod of a controlled area, and to further stiffen by building cross walls as stiffening. It is not there to stop the roof from splaying, but it will.

For normal sized rooms this is straight forward but requires more foundations and OSB on the partition walls.

The same logic will apply with masonry walls.

JackOfAll....do you have normal or big rooms?

 

[tonyshouse]

The horizontal force needs to be resolved, f = m x a . It needs to be zero otherwise things move  

 

you may need a ridge beam 

[JackofAll]

Thanks guys for your responses, Gus very detailed reply, took a while to type(if you're anything like me) nice piece of info to digest. Ultimately it will be the engineer who makes the call but I would like to be in a position to offer alternative ideas incase he offers a potentially over engineered solution which will add more cost to project. I'll throw up a sketch at some stage tomorrow. The kitchen/living area is 34ft×17ft.

[George]

There's a great long topic on eng-tips about vaulted roofs.

 

You do need a ridge beam.

 

When I design them, I always specify sarking as although the ridge beam does work fine globally, live loads (point loads and lateral loads from wind and snow) are harder to control. Sarking sorts that out.  

[Gus Potter]

On 27/10/2021 at 07:16, tonyshouse said:

The horizontal force needs to be resolved, f = m x a . It needs to be zero otherwise things move  

 

you may need a ridge beam 

 

For all the above all sounds a bit technical but if you are a self builder and want to get your head round the structure start with a card board box. Tape up all the sides and tape onto the top the sloping sides or your flat roof. Stick the box to the top of the table just like it is sitting on you foundations. Next push it from side to side and put a bit of weight on the roof. Observe.. get the kids involved.

 

Now start to cut out your windows and repeat. Observe.. Then cut out say dormers or add a floor.. it's like say that game Jenga! You can then start to understand what is going to happen to your house. This will then let you communicate with the Architect and SE and enable you to ask some searching questions! It is then up to them to respond.. you have captured the high ground from the start! Make them expain!

 

Spot on Tony with your appreciation that unless you resolve all the forces things will move. In a structure all the forces need to be in balance.

 

12 hours ago, George said:

There's a great long topic on eng-tips about vaulted roofs.

 

You do need a ridge beam.

 

When I design them, I always specify sarking as although the ridge beam does work fine globally, live loads (point loads and lateral loads from wind and snow) are harder to control. Sarking sorts that out.  

George the ridge beam... I chucked that bit in at the end of my long rambling post for the very keen and as a bit of "last resort stuff" in the spirit of BH and.. I was on a roll.. typing away like mad you see...technically feasible but to prove it calculation wise you need several sheets of paper a lot of pencil lead!

 

@saveasteading Having read some of your posts you know a fair bit about industrial building design and building say B&Q sheds. You'll know (as a contractor) that the wriggly tin roofs provide diaphragm action.. so they don't deflect at the wall head as much as the SE says they will from their basic model in the office... same applies on a smaller scale with a steading roof with timber sarking.

 

 

@saveasteading save points out rightly that sarking has series of boards with gaps between to ventilate the roof. OSB does too! just the gaps are further apart...OSB needs a 3.0mm gap that is why the size of the sheet delivered (1197mm ) on say the nominal width is not 1200mm metric size. With a fair wind (very fair wind) you can make this fly.. if you have a long rafter and not daft holes for velux. That is one reason (one other is that they are heavy and thick) why these old steading roofs are still sound and the walls are not spreading after 100 years plus. Also, the old nails had a bit more poke to them, near the sea.. story for another day.

 

For all. The above all sounds a bit technical but if you are a self builder and want to get a feel for things structural wise then start with a cardboard box (also could be your ICF basement). Tape up all the sides and tape onto the top the sloping sides or your flat roof. Stick the box to the top of the table just like it is sitting on you foundations. Next push it from side to side and put a bit of weight on the roof. Observe.. get the kids involved. Now start to cut out your windows and repeat. Observe.. Then cut out say dormers or add a floor.. it's like say that game Jenga! You can then start to understand what is going to happen to your house. This will them let / help you communicate with the Architect and SE and ask them to explain what you have observed playing about..

 

I may not sound too convincing here but the cardboard box thing is a great way of learning by observation. We actually use this concept to train SE's about  more complex theory for example shear centre behavoir.. we make a cee section beam (as opposed to an I shaped beam) out of cardboard load it up and watch what happens.. then we learn the mathematics behind what we have observed.

 

@saveasteading Don't give up on the solid timber sarking just yet to restrain your old stone wall heads !