Javen Simon

Archive for July, 2009

OSB Vs Plywood

Monday, July 27th, 2009

When you receive a roofing estimate which involves re-sheeting your roof you may get some quotes proposing to use plywood and others to
use OSB. Which one should you choose? Let’s start with the similarities. Building codes use the phrase “wood structural panel” to describe the use of plywood and OSB. Codes recognize these two materials as the same. They are both made by compressing and gluing small pieces of wood together.

Plywood is made from thin sheets of veneer that are peeled off a log. Picture a giant pencil sharpener. These sheets are laminated together in a hot press to make plywood.

OSB is made from wood ground into thin wood strands. These strands are mixed with wax and adhesive and then hot pressed. Approximately 50
layers of strands make one sheet of OSB.

OSB has some advantages over plywood:

*  OSB is generally more square and has smaller dimensional tolerances

*  There are no soft spots such as those that can occur in plywood

*  OSB is made from smaller (often farmed) trees reducing the demand for old growth timber

*  OSB has greater shear strength than plywood; the span rating, nail pull and screw hold are all roughly the same

*  It can be $3 to $5 a panel less expensive than plywood. For a typical 2400 square foot home, OSB will save about $700 if used instead of plywood.

The major disadvantage of OSB is that if it gets exposed to significant amounts of water or moisture, the edges expand by up to 15%. . .especially if they are cut edges. This swell will then telegraph onto the shingles . When plywood gets wet, it expands evenly throughout the panel, dries more quickly and shrinks down to its original size more rapidly than OSB. At the end of the day if your roof is properly ventilated and free from any leaks, OSB will hold up just as well as plywood. However in anything less than an ideal environment plywood is much more resilient and will perform better.  Port Alberni in general tends to favor plywood, so the decision to have plywood or OSB installed under your roofing is not only personal one, but can also be controversial. Rather than taking sides I will typically give a price for both, explain the facts, and allow the homeowner to chose the product they feel more comfortable with.

Can’t We Shingle Over The Old Roof?

Tuesday, July 7th, 2009

I hear this question a lot and the answer is: Sometimes it is possible, but not advisable. The original roof and roof deck has to be in extraordinary condition to meet the standards necessary for the new roofs warranty to apply. If the original roof meets the necessary criteria, certain procedures have to be performed, such as removing the ridge capping, nailing down or replacing any loose, missing, or curled shingles. Cutting back the eaves. Inspecting all flashing, vents, and plumbing stacks, and ensuring the rain water will correctly flow into the gutters due to new height of roof edge.

Another factor worth considering is if the supporting structure of the roof can support the additional weight. For an average 2000 square foot roof one layer of roofing will weigh approximatly 5000lbs. Most roofs will easily handle the extra weight, but I have seen some roofs that can barely handle the roof they already have. This reinforces the fact that each roof is unique and needs to be treated as such. For resale value of your home, warranty purposes, and peace of mind I always recommend removing the original roof before installing your new product.