attic: pre fabricated attic roof trusses or a builder/roofer

[Burnet]

Does anyone have any thoughts on the benefits and drawbacks of these 2 methods of roof construction? Or alternatives?

The duty officer in Planning seemed to think prefabs were obvious choice but roofer/builder said we could 'find' more attic space using own construction design, starting from scratch.

The aim if to gain as much space as possible under the roof to create attic living space where there was none being used before. Its a 4 room cottage. Dormers are a possibility.

Its a 45 degree pitch, with double corrugated tiles to finish. Before the fire it was lathe and plaster beneath roof.

The opportunity has arisen because the whole roof has gone, following a fire at the end of last year. The bad news is that the insurance lapsed a few weeks before the fire, so now cost is a great big factor.

 

[^woody^]

If you can construct a traditional roof without massive beams/purlins and deep rafters, and without needing internal supports below, then you may be able to get a touch more space. But not all space is 'usable' space in a loft.

An attic truss will have speed and simplicity on its side, plus no need for beams/purlins and potentially a more open interior with no boxing around purlins etc

You may find that material prices and labour cost make a traditional roof more expensive.

Remember to have a warm roof construction with the insulation on top of the timbers and some in between - then you get more head room

 

[Shytalkz]

Downside of an attic truss roof is that, once it's up, your outside walls in the loft space are fixed, you can't go changing them on a whim. Also, truss manufacturers can only normally get to about 4.5m width in the room using sensibly sized timbers (if the trusses are only supported by the outside walls) and the height of the perimeter wall can sometimes be a bit strange, depending on the roof profile, eaves arrangement etc.

If you've got dormers, Tee-shaped roof, wide rooflights, stairs running at right angles to the run of the trusses etc, all of those areas will need to have a cut roof infill anyhow. Much more than 30% of the roof plan area and you might as well do the whole lot as a cut roof.

As Woody says, a cut roof depends on the structural layout at ground floor and if there are load-bearing walls at sensible positions, there are not going to be big, heavy steels in there somewhere. But, it does give you more flexibility with the internal layout, if you want to change it around a bit, once the main roof structure is up and you get a handle on the space.

 

[AtoZ]

the fire brigade have an educational/training video called 'seven minutes to collapse'
it shows how trusses pass all structural requirements until a fires involved.
the gang nails used in trusses transfer heat more quickly and destroy the joint causing the roof to fall inwards.

i dont like trusses and would not buy a house with them.
the skill/cost is higher with a cut roof but speed is not that' much of an issue if you use a roofing carpenter.
all carpenters should be able to cut a roof but most of them cant do it, even good ones.
good luck with it.

 

[^woody^]

Do they also have a video on how PVCu frames melt, the glass blows out and more air is available to combustion and fire spreads? Or the same principle with PVCu fascias and soffits leading to fire spreading in the roofspace and neighbouring properties?

Or how by simply closing a door can confine a fire to a single room and not the whole house?

The point is, anything will succumb to fire if the conditions are right. A trussed roof, with typical levels of plasterboard linings is no worse risk than a traditional roof.

I find that fire service personal are very alarmist in their descriptions of how bad things could be. I've seen the aftermaths of fires and of fires in properties with trussed roofs, and not seen and worse damage to trussed roofs - and I don't think there is any either. If the fire gets into the roof space, then the roof is shot whatever construction it is. If the roof or room in the roof is protected by plasterboard, then the timbers are most likley to survive unless the fire brigade take a long time to get there.

I'm not aware that trussed roofs are a recognised risk in everyday situations. I'll look at the BRE reports on this to see if there is any notable risk/research on this, but I'm doubtful

 

[JerryM]

duty officer in Planning

a good man to speak to on planning but not on build questions - only the building inspector will do (the bco) and they are quite different people.

the other posts seem to have sorted anyhow/hopefully

 

[Burnet]

Thankyou - this is all very helpful.

For the record, in case it helps anyone - the fire did get into the roof area very quickly as the room with the woodburner where the chimney fire started was open to roof height (already done when I moved in).

Closed doors did slow the fire travelling through the ground floor but in this situation because the burning roof came down from above into all the rooms, bringing ceilings with it, every room got burned, nothing salvagable (although a little of this was water damage to papers in a metal cabinet which would otherwise have survived)

The fire brigade did take quite a while to come, being retained, then found no mains water supply, as the place is offgrid.

Big learning curves.

In terms of the new roof space, could a brick dividing wall be sensible in terms of fire, with door between?

 

[^woody^]

A brick wall is possible, but is it feasible or practical?

A 100mm timber stud wall with 15mm plasterboard and skim and rockwool inside will do the same job.

Plasterboard is a really good fire barrier. I've seen rooms totally gutted by extremely hot fires, with adjacent rooms and roof spaces saved by just the plasterboard. But once the plasterboard fails, then thats it.

It's also a shame about the insurance. I see lots of fires where the homeowner or tenant has no insurance and looses everything. No-one thinks it will happen to them, but it must happen to someone.

Good luck with it.