Restoration Guide: Roof Design
Editor's Note: This is article 2 of 13 in Chapter 3: The Roofing Guide of Old House Web's Home Restoration Guide. This guide was developed and edited for old homes from original materials in the U.S. Housing and Urban Development (HUD) Rehab guide.
Roof failure: what can be more disruptive than to have the cover over your head fail? Whether it be catastrophic roof failure--hurricane damage, fire, snow, falling trees--or lesser woes such as leaking flashing, there is no such thing as a minor roof failure. The roof is your home's protector, and it needs to be done right, even if it means major home renovation.
This section of the Housing and Urban Development (HUD) Roof Guide probes problems in roof design regarding framing errors, wind resistance, seismic durability and decay that can increase the risk of roof failure.
Problems with Framing in Roof Design
Many roof woes are simply a case of poor construction, whether it be your builder's fault, your own remodeling or home renovation mistakes, or as a result of an outdated building code. Unfortunately, the correction may not be as simple as the error.
A roof's ridge can tell you a lot about its condition--a sagging roof indicates a number of possible problems, including:
- Undersized beams and/or rafters
- Improper ties
- Lack of solid blocking at the rafter heal (where the rafter meets the wall)
- Interior supporting walls not doing the job (perhaps because footings have settled)
- Insufficient rafter bracing or bracing that is not transferred to a supporting wall.
Typical problems with trusses include
- Trusses cut to accommodate ductwork or other equipment in the attic (a cardinal no-no)
- Trusses bearing atop interior walls instead of exterior walls (interior wall is too high)
- Trusses lacking proper lateral bracing
Sometimes ridge sagging can be remedied by jacking up the ridge to take most of the load off the walls, followed by new anchoring, supports and braces, and possibly repositioning the walls. If this method works, it is a relatively simple, cheap and unobtrusive fix. If it doesn't work, you may be facing extensive demolition and home renovation.
If you have sagging between rafters or trusses, you probably have inadequate sheathing for the rafter spacing, or deteriorated sheathing. In this case, you need to remove the roof and sheathing and redo (See Section 3 on sheathing).
Roof Design Should Provide Wind Resistance
Most old homes--and many not-so-old homes--simply are not built to take extremely strong wind. Properly built, the roof of a house should be tied to the walls and the walls tied to the foundation so that the structure has a continuous load transfer from top to bottom, with all elements well connected.
A properly connected roof includes sheathing fastened to the truss/rafter with the proper nailing scheme; rafters/trusses attached to the wall top plate with "hurricane straps" or other appropriate fasteners, and the trusses/rafters need appropriate bracing to prevent them from "dominoing" onto one another.
The fix for a roof improperly tied to the walls is complex:
- The best solution involves removing the roofing and the bottom row of sheathing--a labor-intensive and expensive task unless it's time to replace the roof anyway. Removal of roofing and lower sheathing leaves you with easy access to the wall's top plate to attach the proper ties, and lets you check that the sheathing is nailed properly. The type of ties and the sheathing nailing pattern depends on the maximum winds for your area.
- For a simpler, but partial fix: Ties between trusses/rafters can be added to the outside of the house by removing soffits and a top row of siding, but this method does not address possible improper nailing of the sheathing.
- There has been some experimenting with modern urethane-foam adhesives to tighten sheathing to trusses/rafters with, though there is some concern that the stiffening that it causes could affect load transfers to other structural members.
If you live in an earthquake-prone area, you should have the same ties and nailing that protect against destruction by wind. Particular attention should be paid to lateral bracing in order to protect against the whipping action of ground movement.
While you want your roof properly reinforced for seismic stability, reinforcement of the foundation and shearwalls is more effective protection against earthquakes than roof reinforcement.
Avoid Rot in Home Restoration
Nothing but bad comes from water getting into your structure--even small amounts. Moisture reduces the strength of the wood, corrodes metal tie-downs, braces and truss plates. And with time--something that an old house has had plenty of, water will reduce the timber to a pathetic, crumbled pile of compost.
Any sign of moisture should be addressed immediately--it could be coming from deteriorated roofing, for which the fix is obvious: install a new roof including repair of any existing rot--a major home restoration. Your flashing also could be leaking and need repair. Moist air rising from the living spaces through openings in ceilings also causes dampness in the attic; this problem is addressed in the insulation section, Section 6.
Jim Mallery, a semi-retired journalist and onetime registered contractor, has extensive experience remodeling, repairing, and rebuilding homes.