Storage / Access to attic in a truss roof

tails_naf

Hi All,

Near the end of the build now, and I am looking at what to do to finish off the attic. We never intended to convert it, and only maybe use it for access to HRV/light storage, so never paid much attention to it.

However, I have since learned that truss roofs (timber frame build), often use smaller timbers, in our case 5x1.5 timbers, 600mm spacing. While strong timbers, walking on them could cause cracks in the ceiling below. Plus, with 300mm of insulation over them, we won't know where to walk anyway!

So I had the following ideas -
1) insulate attic as normal. For areas I intend to store/walk, lay rigid-board insulation over the joists, and 9mm osb over that, should give a walk-way, as well as required overall insulation (5inches of rockwool, and 120mm of rigid board).

2) insulate the attic entirely with rockwool to 300mm depth. For areas to walk on, add new timbers (say 400mm above cieling level) and attach to the vertical trusses. Then lay ply on top of this for the walk way.

I do have a certain amount of the rigid board lying around (spare), and option 1 seems like it would be easier to do - but I don't know if it will make the surface resistant enough to walking so to alleviate any movement and cracking of the cieling.

Any advice on how to deal with truss roofs would be greatly appreciated!!

dan_d

I'm a bit confused about what you mean when you say truss roofs.

I live in an ordinary semi-d house, and the roof is supported by timber trusses....it's about 11 years old, so they're fairly solid. Then obviously, there's a gable wall and a party wall in blockwork.

We've laid ply between the trusses, and store stuff up there and have no problem with the ceilings cracking.

From your description, do I take it that you don't have blockwork up at that level anywhere? Genuine question, I'm not sure what the implications are. Otherwise, every house I've ever seen has timber roof trusses, and all of them have access for attic storage, using ply if you want.

tails_naf

hi dan_d,

Well I'm still gettting my head around it also, but from what I've heard there are 2 main types of roof - a cut roof and a truss roof.

my understanding is:
Truss roofs are engineered, are pre-manufactured and dropped into place on site. They use tension and special framing techniques to make them strong, and as a result can use lighter materials and wider spacings between the timbers.

Cut roofs are 'cut' and assembled on site. Usually rely on very strong materials and less engineering.

One of the differences is when cutting an attic hatch - you cannot cut a truss roof joist and install the hatch. the hatch must fit between them (which is why I had to go to some lengths to find a hatch 550 wide instead of the usual 600 wide. I think this is because the joist is actually under tension in a truss roof.

Another difference is truss roofs often have very limited 'open space' in the attic, as there are vertical timners for strength, etc. So my attic is difficult to navigate.

If I had my time back, I would have asked the TF company for alternatives, but ces't la vie.

Anyway, long story short, the lighter timbers means there is a slight deflection when standing on long spans of timber (say in the middle of a room), As some of our rooms are quite large, My concern is that walking would do damage.

Anyone able to offer any advice here?

mickdw

Yes you are quite right about the truss roof structure. The engineered truss is specifically designed to act as a complete unit with various components in each truss being in tension or compression. You therefore cannot simply cut into them for access etc as the frame will automatically fail given that some of the truss members rely on tension for sufficient strength.

In your case, Im sure the trusses would be reasonable to support basic household storage but the walking area would be your biggest issue.
If you have a hall area with 2 walls reasonably close together & if these wall are constructed in a load bearing manner, the ceiling at that area might be suitable to walk on due to the significantly reduced span. It would need investigation though as it is likely that the truss would be floating over those walls and gaining no support.
To strengthen an area you could introduce additional timber to the span that you wish to walk on. The span would determine the timber sizes.
Alternatively, you can create a walkway isolated frm the ceiling. This would be done again if you could identify 2 load bearing wall close together then span them with say 6x2 but having these 6x2 bearing slightly higher than existing ceiling thereby separating walkway from ceiling completely.
You should get a local Engineer who would happily come up with a cost effective solution if it is not possible to walk directly on the trusses.

Mellor

Tails naf,
you are spot on regarding truss roofs and the issues you now have. not all truss roofs have this issue, it depends on size, centrees and level of insulation.

Mick,
I think you are over complicating the issue.
The trusses are (maybe) designed for storage loading, but the weight of one person walking wouldn't cause problems. They have to be design to at least take this additional load, as roof load, snow loads and wind loads would be similar magnitude.

I think the rigid board is a reasonable solution. Jusst to create walkway for access to HRV etc. Keep storage to a min.
Its worth pointing out, that if you lay rigid insul directly on joists it will break when you walk over it as the joists will simply compress it every 600mm. A sheet of OSB laid first, then insul, then OSB will spread the load and prevent. Ply can be used instead of OSB also

mickdw

Mellor wrote: »

Mick,
I think you are over complicating the issue.
The trusses are (maybe) designed for storage loading, but the weight of one person walking wouldn't cause problems. They have to be design to at least take this additional load, as roof load, snow loads and wind loads would be similar magnitude.

While I agree that truss will support the loading from a person accessing the attic area on occasion for storage or maintenance, I would not agree that the trusses are sufficient to be used as a regular walkway. This would IMO almost certainly crack the ceiling beneath unless as previously stated that you were walking on a section between 2 very close support points. The additional spacing (600 as opposed to 400mm) being a critical issue too.
Ive seen cases where plumbers have opened up cracks in ceilings while working in truss roof attics. Heavy plumbers I admit but the construct is borderling tbh.
If the OP had something like a folding attic stairs allowing easy access to the attic, I would almost guarantee that the ceiling would become cracked over time. Having siad that, a cracked ceiling isnt a major issue anyway.

tails_naf

Thanks Guys for the responses.

I did notice that some of trusses actually 'float' over the walls - not sure why they always don't sit on the walls directly as I would have imagined that would give extra support, but I'm sure it was designed that way.

I could indeed locate these walls myself and build supports on them. Fairly involved effort but would give a solid structure.

As for the idea of osb+rigid board + osb - the only 'fear' is that a large weight could cause a bowing along the span of the truss - although it should be distributed somewhat over at least 2 trusses with the osb+rigid board combination. Maybe I could first cross-batten the trusses (2x1?), then osb, then rigid board, but that is getting complicated ...

mickdw

The truss is designed to carry from outside wall to outside wall in general & so the walls that they are floating over may well not be suitably constructed to support anything so you would need to be careful there.

Board, insulation ,board running perpendicular to trusses as suggested by Mellor would spread the load and if only for occasional use might well be sufficient.
If you planned on using the area everyday for storage, well I would look at creating a strengthened or isolated floor