Loft access options with 7x2 ceiling joists/trusses at 400mm centres?

[Clarkey]

Hi folks,

First post after lurking for a number of weeks reading up on various potential improvement projects for our 30s semi.

 

I would like to install loft hatches to improve access to a couple of 4.5m x 4m lofts above rooms in a single-story side extension.

 

Unlike the house's main roof, which has purlins, struts and other bracing that made it easy to identify suitable 'safe' joists to cut, for a hatch, these smaller (4.5m wide) hipped roofs have no bracing. Instead, they use simple, fairly chunky triangular trusses, formed from pairs of 7x2 rafters tied with 7x2 joists, and set at 400mm (max) centres. 

 

The attached image shows one side of the kitchen roof. Working from right to left, from the gable end abutment with the house outside wall, six of these triangular trusses support the ridge, and the highlighted rafter, third from the right indicates the truss central to this set that would be weakened by its joist being cut to frame a hatch.

 

My understanding is that the usual loft hatch framing for a single cut joist, of double headers in matching 7x2, is to support the ceiling rather than prevent roof spread. I may be worrying unnecessarily about that risk, as these roof frames seem nicely over-engineered to my layman's eye (they're only supporting plain clay tiles). I'm wondering if I may need to (open the face of the roof to add) sister joists to those either side of the opening? Alternatively, do I just have to accept that I'd need work with narrow access panels between joists?

 

I'd welcome any insights.

[Redbeard]

I imagine @Gus Potter or others may be along soon but a first stab from someone who definitely isn't a SE.

 

I take your point about spread and the 'job of the truss'. I wonder if something can be done with steel (or, perhaps better, timber with 'properly engineered' connections in terms of trimming for the opening but still keeping the strength of the truss in terms of its role in preventing spread. My initial thought was a specially-fabricated 'strap' from the 2 neighbouring trusses to 'give back' the strength to that cut one. On the other hand the steel probably gives you a complicated insulation detail at what is generally a weak point (the hatch and opening) anyway. Timber - properly 'engineered' - seems the best bet. Insulation at that point will still have to be thought out carefully.

[Iceverge]

With the sole exception of Christmas decorations there is no recorded instance of any human in history storing anything in the attic what didn't rightfully belong in a skip. 

 

 

 

 

 

 

 

[Clarkey]

Ha, too true @Iceverge - then again, why dump or hoard stuff in the attic when it's far more amusing to keep those weird and wonderful white elephant purchases visibly in the way! Incidentally, the access isn't for storage - I would have first surfaced in the MVHR forum, but there's no way I can get aVent Axia Econiq S unit through a 🤬 360mm inspection hatch!

 

Thanks @Redbeard for the response - hopefully some additional timber can do the trick, but if it needs to be steel, there's an excuse to buy a welder!