How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
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How much of a roof truss can I cut out to make a storage platform in the attic?
Sunday, September 30, 2018
None. Roof trusses are an engineered assembly of pieces of lumber and metal connector plates, the sum of which is much stronger than the individual parts alone. The top sloped lumber piece on which the roof sheathing plywood gets nailed is called the top chord, and the bottom horizontal piece is the bottom chord. The connecting lumber pieces between them are named webs.
A truss is carefully designed so that the loads are transferred efficiently through the chords and webs to the bearing points at each end. Some parts of the truss are in compression (pushed inward) and other adjacent parts are in tension (stretched out) as the weight above and below the truss moves through them. Removing any one pieces disrupts the intended direction of the transfer of loads and weakens the truss.
The fact that the truss is still intact after web or two have been removed is not a verification that it’s alright. A truss must be able to withstand the wind loads of a hurricane and additional weight of workmen on the roof or working in the attic, for example, and removal or damage of even one piece of wood or metal connector in a truss can significantly reduce its structural strength. Don’t do it.
The top photo shows an attic where the homeowner cut several webs away to create an open platform for storage around the attic ladder opening. The fix for this blunder is not simple. An engineer must specify the size lumber, connectors, and nails for replacing the missing webs, and it must be inspected to verify that the work meets the engineer’s specifications before the trusses are acceptable again. Spending $1,000 or more is the typical cost for engineering, materials and labor.
Sometimes an HVAC contractor is responsible for removing truss webs, usually because of the relocation of a furnace or air handler to attic, which requires a clear area to place the equipment and a service platform in front of it. The photo below was taken in a one-year old luxury residence where the center webs of three trusses were removed to make room for a last-minute decision to move the air handler up into the attic.
Even if you are willing to accept the reduced structural integrity of your roof when chopping into roof trusses for a “remodeling” project, any competent home inspector will nail you for it, causing additional expense and aggravation when you sell your home.
Also, see our blog post What are the most common problems with wood roof trusses?
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