The efficiency of 2x4 stripping on metal roofs

We have installed hundreds of metal roofs on residential homes for over the past ten to fifteen years now. The picture to the right is one of the roofs we installed.

We have installed the panels several different ways. Some we have laid the metal straight down on the shingles (we later found out this was not a good idea), some we have laid it on tar paper. Some we have laid it on 1x4 stripping, and some we have laid on 2x4 stripping etc.. All we install now is laid on wood stripping with very few exceptions.

The reasons not to install your panels directly on top of your shingles 1. he shingles can rub the bottom of the metal panels thus making holes to more easily appear. 2. This would be not air space.

You can note also that you don't have to remove our shingles if you are stripping out roof with wood.

There are some important benefits to 2x4 wood stripping that you might what to know. For one, the spacing between the metal and the original roofing material acts as a insulation space ( like the air between the two pieces of glass on storm windows ). The separation or air space between metal and existing roof material does not allow the heat from the sun to penetrate your attic space. This in its self allows easier cooling during summer months.

We actually did our own myth buster test on this theory. We built 2 identical boxes of wood with pitched roofs, completely sealed up from the outside. Both roofs were covered with singles.

On one roof we laid the metal roof straight down on the singles.

On the other roof 2x4 stripping was added, then the metal roof was laid on top of the 2x4's. By roofs were finished out with all the trim with ridge caps

A digital thermometer was installed in both boxes. On the very first test during a 85 degree sunny day the box without the 2x4 stripping rose to 112 degrees, while the box with the 2x4 stripping maxed out at about 95 degrees. That's about a 17 degree difference. Thats' plenty of justification for spending the extra dollars to strip your new roof with 2x4's.

The other reason for installing your metal roof on wood stripping is the metal suppliers are more likely to honor there warranty's sense that singles are not rubbing the bottom of your panels. Panel naturally swell and contract with the hot and cold weather although not as much as other construction products.

The other reason to use wood stripping under your metal roof in that on the raised rib panels there 3/4 inch by one inch space every nine inches (on the traditional detailed rib used on most residential roofs) and a one inch by 2 inch every twelve inches on the r-panel ( used on barns and metal buildings such as warehouses). This allows for the hot air to rise from all the way down at your eve to all the way up to your ridge and out the ridge vent made by the metal ridge cap. Thus you have created a vented roof system.

To install insulation is a choice you need to make, but here are some considerations.

*An important note for ventilation and air flow here is not to overfill the space created by the wood stripping with insulation thus blocking the air flow from your eve to your ridge cap.

*Another thought on using insulation between or over boards is moisture. The metal sometimes will sweat causing insulation to get damp. This becomes a problem if there is not ventilation to dry out the areas between wood stripping. Damage and mold could accumulate under your metal undetected by you. The air space is there to keep the area dry and to remove old hot air.