Saturday, June 18, 2011

Ask the Architect - Configuring an attic for storage


Photograh of stick framed roof - don't do it like this
Photograh of stick framed roof - don't do it like this
I know NOTHING about construction, but I DO know that I need more storage space. With that in mind, I keep looking up at the entrance to my attic, which is in the upstairs hallway. I have used a ladder to have a look, and see what appear to be rafters everywhere! In doing some research, I have learned that these are “trusses”. In a remodeling show on television, I saw an attic makeover wherein they built supports between the attic floor and the roof rafters (the support looked like a ladder lying on its side). In addition, they put supports horizontally across the peak of the roof, joining the already existing rafters (creating an A shape at the peak). Can these things replace some of the existing trusses, to open up the center space? I am not looking for additional living space, only storage space. I do not need to be able to stand; I only need to be able to put boxes, etc. up there. I would also like to replace the existing small, square hatch, with a long, rectangular drop-down hatch and ladder to facilitate access. Last on my “wish list” would be a light in the attic with the on/off switch in the hallway below.?
Obviously this is coming from a real novice vs. someone who has some sense of architecture and construction. Can this be done? Will it be SAFE?

 The Architect’s Answer

Wendy, the modifications you can make to your attic construction are dependent on how your roof is built and what supports it.  The short answer is that you need an architect, engineer or someone very knowledgeable about wood frame construction to look at your specific situation and evaluate it.  By very knowledgeable I mean someone who is capable of beam span calculations with varying point and uniform loads, that is someone who can do the math.  I’m a residential architect in the Seattle area and I’d be glad to come out and look at it and tell you what’s possible.

 Anatomy of a stick framed roof

Roofs with attics are generally constructed one of 2 ways.  A stick frame roof is constructed with individual pieces of lumber cut and fit on site.  The sloping members that support the roof are rafters.  The flat members that support the ceiling are called ceiling joists.  In addition to these main members you often see a ridge board, purlins, struts and collar ties.  My guess is that in the example you saw on TV the ladder was made of purlins and struts.  The horizontal support that created the “A” was a collar tie.  Typically most purlin/strut supports are as I’ve shown, at an angle, projecting up from a bearing wall.
Typical stick frame roof configuration in the Pacific Northwest
Typical stick frame roof configuration in the Pacific Northwest

 Altering a stick framed roof

This kind of attic space can be altered to provide more efficient access to storage space by reconfiguring the struts and purlins.  However, this reconfiguration still needs to be designed in a way that won’t cause the system to fail.  This means you still need an architect or some sort of an expert to design it so that the weight of the room and the new storage still finds its way down to bearing.

Anatomy of a truss roof 

The second type of roof attic construction uses pre-manufactured trusses that are designed by an engineer, built in a factory, trucked out to the site and installed without field cuts or modifications.  The second illustration shows a typical queen post type truss configuration.  The sloping member that supports the roof is the top chord.  The flat member that supports the ceiling is called the bottom chord.  The rest of the internal members are called webs.
Typical truss roof framing configuration
Typical truss roof framing configuration

 The problem with field altering a truss

This type of attic construction generally cannot be altered once it is installed.  Each piece of the truss is absolutely necessary for the proper functioning of the truss and cutting any member would cause the truss to fail.  It is possible for a structural engineer to figure out how to cut and reinforce a truss in order to accommodate the space you’d like to have.  However, it is likely to involve substantial reframing to make it work, which could result in a great deal more expense and effort than the space is worth.

 Pull down attic stair

As long as you can install the drop down stair without altering the either the trusses or the ceiling joists you can add it by enlarging the current attic access hole.  If you need to alter or modify the framing in any way, I would suggest you have an expert look at it first.
 I hope this helps clarify your options for attic storage.  Please feel free to ask follow up questions if you need more information.

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