HTM high thermal mass log purlin roof deck and insulation details

HTM high thermal mass log purlin roof deck and insulation details

with log purlins and a vented cold roof SIPS panel design

Passive solar roof details do not have to be unique, but cold vented roof design with vented SIPS panels and layered insulation is a natural with log purlins. A lot of attention has been given to our passive solar house plan choice of log roof beams purlins, but you can certainly use dimensional lumber and trusses in your roof details if you wish. Square timbers are easier to build with than trying to use round logs. The main advantages to log purlins are in their availability since you may have to drop trees to create your home site and their cost since raw logs are cheaper and always more environmentally sound than processed lumber. But what you notice after the home is finished is the way the round logs greatly soften the angular nature of an HTM. They truly add a great deal of beauty to the final product and are a natural choice for a flat roof deck. I think you’ll agree that they are worth the effort in the long run.

There is no ‘one best way’ to build any structure in any one climate and or soil type. As with any design question, the answers vary from project to project and need to be addressed in a more holistic manner to function properly. You can build your own roof panels, as shown below, sandwiching EPS blueboard atop the ceiling planks. Or, you can order the SIPS panels readymade. It all boils down to initial cost versus labor time savings. Insulated structural roof panels SIPS are a great way to save time during construction. Get a quote on vented SIPS panels locally to compare cost. Sometimes the quality is good and the engineering they provide very helpful. Look for R30 minimum and extruded EPS blueboard or the newer generation expanded insulation boards, not white Styrofoam bead board, since it absorbs water more readily.

SIPS panels come with faux finishes on the ceiling side, sometimes perfectly imitating tongue and groove or stucco. Always go with the cold vented roof option. ‘Cold roofs’ are a design that will cut your cooling bills dramatically by allowing heated air to vent out the roof peak. Using a layer of Mylar reflectix atop the felt in a cold roof design is a recommended to most designs and climates. This Mylar layer gives your roof the advantage of reflecting heat back up and not absorbing into the insulation underneath.

We’ll work with you to create any type of roof you wish, but we have found this system to have a great many benefits. The beauty is hard to argue with. With so much stone, block, and stucco, the use of wood on the ceiling to soften the look is a design imperative. This choice of material is also more natural and healthier than drywall. Drywall ceilings are so very sterile and, well, just so very white. Think twice before you commit to that type of ceiling construction.

Our consultation concentrates on simplicity and ease of construction for the homeowner-builder. Deck roofs fit this need to a T. As you can see in these photos, construction simply consists of building a box and filling it with EPS foam sheets. There are several design details not shown here, like the moisture barrier between the foam and the decking, but the basics are evident. You can see where attention is given to overlapping joints and caulking gaps. Please also note that this is a vented, ‘cold roof’ design with the plywood sheeting laid upon the 2by4 sleepers atop the final layer of foam. This allows the roof to vent, preventing overheating and ice dams. Vented, cold roofs are very important in both hot and cold climates. You just can’t afford to allow heat buildup in your roof to penetrate into the home. Always design with a vented, cold roof.

For most clients, the ‘sandwich’ style at right is easier to construct with minimal skills. The conventional joist approach shown above is often more popular with contractors who wish to use fiberglass batting insulation.

The ceiling planking shown below is classic penny pinching. As the 2by6 tongue and groove ceiling planks neared the peak of roof, we ran out. The last 10 T&G boards are actually 2by12 planks we notched with a router to mock the look of T&G grooves. Give it a close look and you will see the change in planking material about half way up. Curiously enough, the tongue and groove does not add strength to the deck — it just keeps the boards from wanking and warping as they dry. If you use well-seasoned 2by12 lumber, you can plane 45 degrees off both sides of the plank and router Vee cut the middle for much the same structural and visual effect, at a pretty healthy savings.

With any home roof design, it is key to note:

  • pre-Fab roofing panels are a way to speed construction if you do not wish to build your own roof. Structural panels are available (no need for roof beams). You will find a good introduction at: James Dulley’s site
  • cover the plywood roof decking with Grace Ice and Water Shield roll bitumen and not just 30 pound felt. This product will ensure a very long lifetime for your roof. It fully adheres to the plywood keeping moisture from eventually deteriorating the decking: Grace Ice and Water
  • do not use dark colors on your roof in a hot climate because you are increasing your cooling problems by subjecting the home to a greatly increased amount of solar gain. Use as close to white as your personal taste will allow in a hot climate.
  • if you want a roof that will last for generations spend the money now for a standing seam metal roof. This new generation of metal roofing does not have any exposed fasteners (the panels interlock, hiding the mounting brackets and screws). James Dulley’s site has a short article for reference. Your homeowner’s insurance may even be reduced with a metal roof.
  • if you live in forest fire prone areas, you should go with a lifetime rated architectural shingle. ‘Architectural’ shingles are heavier (up to 540 pound per square), often lifetime (100 year rated) and fireproof. Metal roofs are not fireproof — despite what you might hear, a fire atop a metal roof will reach flashpoint and eventually set the plywood underneath ablaze, unlike thick shingles.
  • if you cannot afford a standing seam metal roof, modified torch down is the best for roofs which slope less than 3/12. We would suggest not installing an EPDM (pond liner) type of roof. The chance of puncture is too great. Especially if you walk around on it much.
  • to save money, use roll roofing in a double overlap pattern instead of shingles. It is easy to install, has no exposed edges or fasteners, and the materials are very reasonable. Double overlap or half-lap roll roofing has been the standard for commercial buildings for over a century.
  • place 3/4 inch kicker boards along the fascia to send the runoff out and AWAY from the roof. Without these kickers, water tends to run against the fascia, behind the gutters, and eventually rotting and staining your fascia boards.
  • always install gutters everywhere around the roof to take water runoff at least 20 feet away from the house and/or to planterbeds.
  • Metal roofing has its strong points, but there are problems. It is attached with screws. When you nail through bitumen underlayment (Grace), it seals around the spike. When you screw through it, though, the boring action of the screw causes it not to seal around the threads. Metal roofs also tend to expand and contract. You will be up there with a screw gun tightening them every 3 or 4 years unless you install a standing seam design, which is expensive, but a good investment. Eventually the screws will not tighten as they have eaten out the hole and you will have to install all new, fatter screws. Another detraction is that the color will fade on all but the most expensive metal roofs. They use high grade automobile paint on most brands. Another will actually change color as you walk around it looking from different sun angles. Others are designed to rust up to a point and then they stop, which is a nice look. Metal roofs are not fireproof. For the same cost as mid-price standing seam with hidden fasteners, you can get the best shingles which in practice are much more fireproof. You could start a campfire on a 540# shingle roof and it will eventually burn a hole through the roof and fall inside, maybe going out or at least being easier to fight. Start the same campfire on a metal roof and it will heat the metal, eventually flash firing the entire roof.

    Passive Solar textbooks all seem to present a rather confusing ‘shotgun effect’ with a laundry list of options,

    all based upon conventional residential stick-frame architectural models. but some do think outside the box.

    Malcolm Wells’ The Earth Sheltered House: an artist’s sketchbook is rich in drawings and inspiration

    Robert L. Roy’s Cordwood Series includes SBC construction details and curved wall structures

    Michael Reynold’s Earthship Series for back-to-basics rammed earth how-to-do buildings

    Daniel D. Chiras’ The Natural House and The Solar House are both excellent textbooks

    and with more focus on HTM construction. our free Passive Solar Design eBook

    We are no longer providing any HTM passive solar home design consultation services.

    While working on a new book and other projects, hourly design services will not be possible to provide.

    And architect referrals will not be available until we locate a capable, progressive firm to handle clients.

    We have our HTM Home Tour DVD to offer, but note it does not include any personal consultation time.

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