How should a built-up roof base sheet be attached to a roof deck? The decking on this roof consists of redwood boards. Should the base sheet be nailed or can it also be attached by hot mopping? If hot mopping is considered acceptable what would be the advantages and disadvantages compared to nailing?
Thanks for any advice you can give,
Scott
Replies
Base sheet on residential wood decks are normally nailed down to a specific nailing patteren.
BUR base CAN be 'strip-mopped' meaning a light sprinkling of hot on the deck adheres the base. The sprinkle/splash is enough to make contact at about 25-35% of the surface.
For pros and cons, consider the main reason for the base sheet.
It is a slip sheet
It is a point of attachment
It is a cushion.
When you are attaching with nails, the board roof deck can move with changes in temp and humidity and that motion will not play through to the main roof and crack it. But with the strip-mopped attachment, if there is too solid of an attachment, then movement in the wood can transfer thru to cause movement in the roof plies so they stress and crack. Not enough hot tar attachment and the roof raises and stresses from wind or blows off. It is also possible when strip-mopping on a boarded roof deck to have hot stuff dribble and leak through to the interior. That is not too terrible over an attic, but with an exposed surface in side such as is common in California bungalows, the interior can be ruined
Generally (IIRC) when we used strip mop attachment was on commercial work where an insulation board went down first on the wood deck. This served as cushion and skip sheet. Then the base was adheered to the insulation with the strip mop method. This kept at least one more layer of material between the roofing and the nail heads or other irregularities.
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