3 timbers on top of timber frame wall?

[glenboy]

Hi there, I'm not sure if this is posted in the correct section or if I need to post it in the timber frame section.

My question regards the 3 no 150 x 50 timbers which seem to be vertical in the drawing at the top of the walls. This is a first floor timber frame extension on top of an existing brick and block wall. Reason for timber frame is the cavity is only 50mm. So has to be done this way for U value. Is it normal for the timbers this way Vertically as I have normally on all the programs and books is Horizontal and the timbers overlapped at the corners. The wall studs are 400 centres as well. 

 

Thanks

 

 

[ProDave]

Nope, not seen that before. Usually only 2 and laid flat.

 

A stack of 3 like that vertically is usually used as a lintel over a door.

[Temp]

Got any windows high up?

[Gus Potter]

Just roughly.

 

The drawing looks like you have a roof load.

 

The 3no 150 x 50 vertically is fine and this is what I would expect to see. Using three timbers in this orientation gives you more capacity; strength, deflection and so on. You''ll often see this when you have a 6 inch TF kit (old money)  metric sizes - 145mm deep studs or similar. If you have a 4 inch kit (89 mm deep studs or say 95mm) then you'll often see just the two lintel timbers vertically aligned, but they need to be deeper to carry the same load as 3no 145 mm timbers forming the lintel on your drawing.

 

Using three timbers means you get a shallower over all lintel depth, all other loads and so on being equal. The thickness of each timber is often 45 mm (actual timber size as opposed to nominal size) so you need to pack out by 10mm on the inside ( 3 x 45 = 135mm but the studs are 145mm hence the 10mm) , you carry your inner insulation envelope round the lintel on the inside and this is a recognised way of mitigating the cold bridging.

 

The horizontal lapping and the orientation of the timbers you may be thinking about is to do with how you connect each timber frame panel together at the top, bottom and where you have a corner. This is essentially to do with tying the panels together rather than supporting a roof load say.

 

There is other stuff to do with TF shrinkage. Wood shrinks by differents amount in each direction relative to the grain. In summary you are best to put the timbers the way your drawing shows rather than on the flat, although they still may be strong enough.

 

 

 

[glenboy]

18 hours ago, Gus Potter said:

Just roughly.

 

The drawing looks like you have a roof load.

 

The 3no 150 x 50 vertically is fine and this is what I would expect to see. Using three timbers in this orientation gives you more capacity; strength, deflection and so on. You''ll often see this when you have a 6 inch TF kit (old money)  metric sizes - 145mm deep studs or similar. If you have a 4 inch kit (89 mm deep studs or say 95mm) then you'll often see just the two lintel timbers vertically aligned, but they need to be deeper to carry the same load as 3no 145 mm timbers forming the lintel on your drawing.

 

Using three timbers means you get a shallower over all lintel depth, all other loads and so on being equal. The thickness of each timber is often 45 mm (actual timber size as opposed to nominal size) so you need to pack out by 10mm on the inside ( 3 x 45 = 135mm but the studs are 145mm hence the 10mm) , you carry your inner insulation envelope round the lintel on the inside and this is a recognised way of mitigating the cold bridging.

 

The horizontal lapping and the orientation of the timbers you may be thinking about is to do with how you connect each timber frame panel together at the top, bottom and where you have a corner. This is essentially to do with tying the panels together rather than supporting a roof load say.

 

There is other stuff to do with TF shrinkage. Wood shrinks by differents amount in each direction relative to the grain. In summary you are best to put the timbers the way your drawing shows rather than on the flat, although they still may be strong enough.

 

 

 

Thanks for the other guys replies.

@Gus Potter thanks. There is only 1 window in the front I will post a bit more of the plan. A couple of questions. I take it you would coach bolt the 3 timbers together maybe at 400 centres staggered? Also how would you fix these to the 150 vertical wall studs. Would you drive a long screw down through the 150 high timbers into the wall studs or metal strap them. I guess we cannot use the nail gun to gun in at an angle.

Also how would you tie the front aspect timber wall into these side walls? as we cant use the plate overlap method? Once again thanks for your guys replies.

Thanks

Edited July 27, 2020 by glenboy

[Gus Potter]

Hello Glen.

 

Hope this helps.

 

Coming off the main house you have a gabled portion (call this gable 1) then you have another bit on the end of gable 1.. call that gable 2. When you are forming timber lintels in a TF kit you can generally nail them together. Common spec would be " nail timbers together with pairs of nails at max 250mm horizontal centres, for deeper lintels you use three nails vertically. You need to keep an edge distance between the nail and the edge/ end of the timbers. In other words you if put the nails too close to the edge / end of the timber it is no good. Also, if you put the nails too close together this is less helpful as you start to encourage the timber to split and so on.

 

To support the ends of the lintels you commonly use what is called a cripple stud arrangement. The window you have on gable 2 looks fairly wide so you may have two shorter timbers under the lintel to hold it up, these are the cripple studs. Smaller openings tend to have just one cripple stud each end of the lintel.. the reasons for this are a bit lengthy to go into detail here.

 

Then you have another stud which is the same height as all the other studs and this is nailed to the cripple studs. You nail through this full height stud into the ends of the lintel. Over the top of the whole thing (unless you are tight for height) goes what is called your top rail and on top of this you have a head binder.. this is the bit of timber that ties all the panels together.

 

Once you nail all this up the lintel is held in place. If you search internet for timber frame cripple studs you''ll see drawings and so on as to how this all fits together.

 

Some of points I make above are conservative in nature, you can fine tune stuff later.

 

Turning to the tying. It's good to get this out the way early on as it's just as important to stop things moving sideways as it is to stop things falling down vertically. Well done picking up on this commonly missed feature.

 

As a further word of encouragement! There are a few ways you can approach tying gable 2 into gable 1 and gable 1 into the main house. The starting point is to determine the ceiling height.. are the ceilings flat or is there some vaulting going on? There are trusses called raised tie trusses - see internet. Here you would line through the ceilings and tie them all together at this level with wind bracing, usually 100 x 22 timbers.

 

Lastly I see the are some UB's (universal steel I shaped beams) on your drawing. It could well be that unless you have a very big main house roof that you can swap these out for timber beams, solid timber or laminated type. I'm a big fan of trying to mimimise the trades, simplify the material procurement process and so on. Here, if you can use timber instead of steel then it's easier for the joiner. You need them for the kit anyway. It's often a lot easier to connect old/ new timber to other timber than connect it to steel.

 

Also, if you get / measure the steel wrong it's often harder to fix. If you cut a bit of wood wrongly.. nip to the mechants, buy another bit and try again.

 

You can use the " reclaimed" timber on the wood stove if you are lucky enough to have one..or cut it into dwangs (Scotland) noggings (England and mostly else where?) That's what I do "occasionally"

 

 

 

 

[Hobbiniho]

First thing i will say is i have never seen timbers stacked vertically like that to make up the top plate detail on a timber frame, also the "roof truss" sppears to be missing a considerable amount of structure, and appear to be drawn as if they are going to be cut on site and overlapped at the eves and the drawing doesnt show a ridge board, the first drawing indicates it is section "B1" however on the plan drawing i dont see the corresponding line to show where "B1" is showing the section of. i am struggling how you will connect the tripple top plate to the studs, also 400 centres seems excessive as there isnt exactly a massive amount of load on the roof to hold up, in my opinion it has been drawn by someone with little timber frame experience

[Gus Potter]

 

Hobbiniho makes some good points. I’ll weave some other TF stuff in as I go which I hope helps all.

In reverse order. Yes, when you look at the configuration of the roof it appears to be fairly standard. Generally, truss designers start out with 600mm spacing as this fits with the spanning capability of standard tile battens and so on.

You can then close the spacing / double up if need be when you start to get stuck, say where you have extra localised loads – dormers etc. What you try and avoid is to start changing the depth of the members as this starts for example to cause problems with the alignment of the roof bracing.

If you have a good height and want to convert the attic later and the ceiling joists / rafters are different depths then how do you floor / line it out easily?

A lot of TF houses throughout the UK don’t have sarking or boarding on the roof. This sarking/ boarding stiffens the roof and often stops people from falling though during construction and maintenance.

An SE will often take advantage of this stiffening (called diaphragm action) to shift horizontal loads (loads cause by the wind for example) to where they are more easily dealt with.  When you go in the loft you may just see a vapour / moisture control layer and you can feel the tile battens and so on through this. If so, you should see more diagonal type timbers bracing the roof.

In old money you’ll often see roofs in Scotland say with rafter spacing of 18” (~458mm).. this keeps the roof member sizes down a bit and lets you use a thinner sarking board. If you are slating onto sarking you don’t want “bouncy” sarking as it’s really difficult to drive the slate nail to just the right depth to keep the slates tight. The closer truss spacing can help here. In other words you need to look at things holistically and not just the trusses in isolation.

As a general point. If you are building a single storey extension on a house that has big roof you need to think about snow drift loading. The snow can blow off the main roof and overload the extension roof below. It can drive against any wall above the extension roof and fall back. Snow drifts can be heavy. Another  great example is where you have an large span, high eaves height agricultural shed..lots of snow available to pile up on a lower lean to roof.

Glenboy posted a couple of drawings. There may well be others that show more detail.

While some features of Glenboys drawings may look a bit odd under scrutiny they look to me like concept / early stage provisional drawings. Often a designer will just add rough stuff or notes when a Client is just trying to get a budget price for the works. What I think these drawings are doing is to say to the builder... We need some kind of beam in here, we need some trusses and so on and this is the size of the thing... so give us a rough price for that. The detailed design comes later in the process.

In terms of the timbers stacked vertically. I would like to see the rest of the end detail and the TF panel drawings before exploring further.

For all. Every house is different and has it’s nuances. Sometimes, where you are tight for height for example you need to go back to first principles of design and this can throw up some less commonly seen details.